ADC121C021CIMM/NOPB - NATIONAL SEMICONDUCTOR

ADC121C021,ADC121C027

ADC121C021/ADC121C027 I 2 C-Compatible, 12-Bit Analog-to-Digital Converter

with Alert Function

Literature Number: SNAS415D

ADC121C021/ADC121C027

July 27, 2010

I2C-Compatible, 12-Bit Analog-to-Digital Converter with

Alert Function

General Description

These converters are low-power, monolithic, 12-bit, analog-

to-digital converters (ADCs) that operates from a +2.7 to 5.5V

supply. The converter is based upon a successive approxi-

mation register architecture with an internal track-and-hold

circuit that can handle input frequencies up to 11MHz. These

converters operate from a single supply which also serves as

the reference. The device features an I2C-compatible serial

interface that operates in all three speed modes, including

high speed mode (3.4MHz).

The ADC121C021's Alert feature provides an interrupt that is

activated when the analog input violates a programmable up-

per or lower limit value. The device features an automatic

conversion mode, which frees up the controller and I2C inter-

face. In this mode, the ADC continuously monitors the analog

input for an "out-of-range" condition and provides an interrupt

if the measured voltage goes out-of-range.

The ADC121C021 comes in two packages: a small TSOT-6

package with an alert output, and an MSOP-8 package with

an alert output and two address selection inputs. The

ADC121C027 comes in a small TSOT-6 package with an ad-

dress selection input. The ADC121C027 provides three pin-

selectable addresses while the MSOP-8 version of the

ADC121C021 provides nine pin-selectable addresses. Pin-

compatible alternatives to the TSOT-6 options are available

with additional address options.

Normal power consumption using a +3V or +5V supply is

0.26mW or 0.78mW, respectively. The automatic power-

down feature reduces the power consumption to less than

1µW while not converting. Operation over the industrial tem-

perature range of −40°C to +105°C is guaranteed. Their low

power consumption and small packages make this family of

ADCs an excellent choice for use in battery operated equip-

ment.

The ADC121C021 and ADC121C027 are part of a family of

pin-compatible ADCs that also provide 8 and 10 bit resolution.

For 8-bit ADCs see the ADC081C021 and ADC081C027. For

10-bit ADCs see the ADC101C021 and ADC101C027.

Features

■I2C-Compatible 2-wire Interface which supports standard

(100kHz), fast (400kHz), and high speed (3.4MHz) modes

■Extended power supply range (+2.7V to +5.5V)

■Up to nine pin-selectable chip addresses (MSOP-8 only)

■Out-of-range Alert Function

■Automatic Power-down mode while not converting

■Very small TSOT-6 and MSOP-8 packages

■±8kV HBM ESD protection (SDA, SCL)

Key Specifications

■Resolution 12 bits; no missing codes

■Conversion Time 1µs (typ)

■INL & DNL ±1 LSB (max) (up to 22ksps)

■Throughput Rate 188.9ksps (max)

■Power Consumption (at 22ksps)

—3V Supply 0.26 mW (typ)

—5V Supply 0.78 mW (typ)

Applications

■System Monitoring

■Peak Detection

■Portable Instruments

■Medical Instruments

■Test Equipment

Pin-Compatible Alternatives

All devices are fully pin and function compatible across reso-

lution.

Resolution TSOT-6 (Alert only)

and MSOP-8

TSOT-6 (Addr only)

12-bit ADC121C021 ADC121C027

10-bit ADC101C021 ADC101C027

8-bit ADC081C021 ADC081C027

Connection Diagrams

30020901 30020902 30020910

I2C® is a registered trademark of Phillips Corporation.

ADC121C021/ADC121C027 I2C-Compatible, 12-Bit Analog-to-Digital Converter with Alert

Ordering Information

Order Code Option Package Top Mark

ADC121C021CIMK Alert Pin TSOT-6 X30C

ADC121C021CIMKX Alert Pin TSOT-6 Tape-and-Reel X30C

ADC121C027CIMK Address Pin TSOT-6 X31C

ADC121C027CIMKX Address Pin TSOT-6 Tape-and-Reel X31C

ADC121C021CIMM Alert and Address Pins MSOP-8 X37C

ADC121C021CIMMX Alert and Address Pins MSOP-8 Tape-and-Reel X37C

ADC121C021EB

Shipped with the ADC121C021.

Also compatible with the

ADC121C027 option.

Please order samples.

Evaluation Board

Block Diagram

30020903

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ADC121C021/ADC121C027

Pin Descriptions

Symbol Type Equivalent Circuit Description

VASupply Power and unbuffered reference voltage. VA must be free

of noise and decoupled to GND.

GND Ground Ground for all on-chip circuitry.

VIN Analog Input See Figure 4 Analog input. This signal can range from GND to VA.

ALERT Digital Output

Alert output. Can be configured as active high or active

low. This is an open drain data line that must be pulled to

the supply (VA) with an external pull-up resistor.

SCL Digital Input

Serial Clock Input. SCL is used together with SDA to

control the transfer of data in and out of the device. This is

an open drain data line that must be pulled to the supply

(VA) with an external pull-up resistor. This pin's extended

ESD tolerance( 8kV HBM) allows extension of the I2C bus

across multiple boards without extra ESD protection.

SDA Digital

Input/Output

Serial Data bi-directional connection. Data is clocked into

or out of the internal 16-bit register with SCL. This is an

open drain data line that must be pulled to the supply

(VA) with an external pull-up resistor. This pin's extended

ESD tolerance( 8kV HBM) allows extension of the I2C bus

across multiple boards without extra ESD protection.

ADR0

Digital Input,

three levels

Tri-level Address Selection Input. Sets Bits A0 & A1 of the

7-bit slave address. (see Table 1)

ADR1 Tri-level Address Selection Input. Sets Bits A2 & A3 of the

7-bit slave address. (see Table 1)

Package Pinouts

VAGND VIN ALERT SCL SDA ADR0 ADR1

ADC121C021

TSOT-6 1 2 3 4 5 6 N/A N/A

ADC121C027

TSOT-6 1 2 3 N/A 5 6 4 N/A

ADC121C021

MSOP-8 57421836

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ADC121C021/ADC121C027

Absolute Maximum Ratings

(Note 1, Note 2)

If Military/Aerospace specified devices are required,

please contact the National Semiconductor Sales Office/

Distributors for availability and specifications.

Supply Voltage, VA-0.3V to +6.5V

Voltage on any Analog Input Pin to

GND −0.3V to (VA +0.3V)

Voltage on any Digital Input Pin to

GND −0.3V to 6.5V

Input Current at Any Pin (Note 3) ±15 mA

Package Input Current (Note 3) ±20 mA

Power Dissipation at TA = 25°C See (Note 4)

ESD Susceptibility(Note 5)

VA, GND, VIN, ALERT,

ADDR pins:

Human Body Model 2500V

Machine Model 250V

Charged Device Model (CDM) 1250V

SDA, SCL pins:

Human Body Model 8000V

Machine Model 400V

Junction Temperature +150°C

Storage Temperature −65°C to +150°C

Operating Ratings (Note 1, Note 2)

Operating Temperature Range −40°C ≤ TA ≤ +105°C

Supply Voltage, VA+2.7V to 5.5V

Analog Input Voltage, VIN 0V to VA

Digital Input Voltage (Note 7) 0V to 5.5V

Sample Rate up to 188.9 ksps

Package Thermal Resistances

Package θJA

6-Lead TSOT 250°C/W

8-Lead MSOP 200°C/W

Soldering process must comply with National

Semiconductor's Reflow Temperature Profile specifications.

Refer to www.national.com/packaging. (Note 6)

Electrical Characteristics

The following specifications apply for VA = +2.7V to +5.5V, GND = 0V, fSCL up to 3.4MHz, fIN = 1kHz for fSCL up to 400kHz,

fIN = 10kHz for fSCL = 3.4MHz unless otherwise noted. Boldface limits apply for TA = TMIN to TMAX: all other limits TA = 25°C

unless otherwise noted. (Note 8)

Symbol Parameter Conditions Typical

(Note 9)

Limits

(Note 9)Units (Limits)

STATIC CONVERTER CHARACTERISTICS

Resolution with No Missing Codes 12 Bits

INL Integral Non-Linearity (End Point

Method)

VA = +2.7V to +3.6V

fSCL up to 400kHz (Note 13)±0.5 ±1 LSB (max)

VA = +2.7V to +5.5V. fSCL up to 3.4MHz +1.2 LSB

−0.9 LSB

DNL Differential Non-Linearity

VA = +2.7V to +3.6V

fSCL up to 400kHz (Note 13)

+0.5 +1 LSB (max)

−0.5 −0.9 LSB (min)

VA = +2.7V to +5.5V. fSCL up to 3.4MHz +1.3 LSB

−0.9 LSB

VOFF Offset Error

VA = +2.7V to +3.6V

fSCL up to 400kHz (Note 13)+0.1 ±1.6 LSB (max)

VA = +2.7V to +5.5V. fSCL up to 3.4MHz +1.4 LSB

GE Gain Error -0.8 ±6 LSB (max)

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ADC121C021/ADC121C027

Symbol Parameter Conditions Typical

(Note 9)

Limits

(Note 9)Units (Limits)

DYNAMIC CONVERTER CHARACTERISTICS

ENOB Effective Number of Bits VA = +2.7V to +3.6V 11.7 11.3 Bits (min)

VA = +3.6V to +5.5V 11.5 Bits (min)

SNR Signal-to-Noise Ratio VA = +2.7V to +3.6V 72.5 70.4 dB (min)

VA = +3.6V to +5.5V 71 dB (min)

THD Total Harmonic Distortion VA = +2.7V to +3.6V −92 −78 dB (max)

VA = +3.6V to +5.5V −87 dB (max)

SINAD Signal-to-Noise Plus Distortion Ratio VA = +2.7V to +3.6V 72.6 70 dB (min)

VA = +3.6V to +5.5V 71 dB (min)

SFDR Spurious-Free Dynamic Range VA = +2.7V to +3.6V 90 76 dB (min)

VA = +3.6V to +5.5V 87 dB (min)

IMD

Intermodulation Distortion, Second

Order Terms (IMD2)

VA = +3.0V,

fa = 1.035 kHz, fb = 1.135 kHz −89 dB

VA = +5.0V,

fa = 1.035 kHz, fb = 1.135 kHz −91 dB

Intermodulation Distortion, Third

Order Terms (IMD3)

VA = +3.0V,

fa = 1.035 kHz, fb = 1.135 kHz −88 dB

VA = +5.0V,

fa = 1.035 kHz, fb = 1.135 kHz −88 dB

FPBW Full Power Bandwidth (−3dB) VA = +3.0V 8 MHz

VA = +5.0V 11 MHz

ANALOG INPUT CHARACTERISTICS

VIN Input Range 0 to VAV

IDCL DC Leakage Current (Note 10) ±1 µA (max)

CINA Input Capacitance Track Mode 30 pF

Hold Mode 3 pF

SERIAL INTERFACE INPUT CHARACTERISTICS (SCL, SDA)

VIH Input High Voltage 0.7 x VAV (min)

VIL Input Low Voltage 0.3 x VAV (max)

IIN Input Current (Note 10) ±1 µA (max)

CIN Input Pin Capacitance 3 pF

VHYST Input Hysteresis 0.1 x VAV (min)

ADDRESS SELECTION INPUT CHARACTERISTICS (ADDR)

VIH Input High Voltage VA - 0.5V V (min)

VIL Input Low Voltage 0.5 V (max)

IIN Input Current (Note 10) ±1 µA (max)

LOGIC OUTPUT CHARACTERISTICS, OPEN-DRAIN (SDA, ALERT)

VOL Output Low Voltage ISINK = 3 mA 0.4 V (max)

ISINK = 6 mA 0.6 V (max)

IOZ

High Impedance Output

Leakage Current (Note 10) ±1 µA (max)

Output Coding Straight (Natural) Binary

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ADC121C021/ADC121C027

Symbol Parameter Conditions Typical

(Note 9)

Limits

(Note 9)Units (Limits)

POWER REQUIREMENTS

Supply Voltage Minimum 2.7 V (min)

Supply Voltage Maximum 5.5 V (max)

Continuous Operation Mode -- 2-wire interface active.

INSupply Current

fSCL=400kHz VA = 2.7V to 3.6V 0.08 0.14 mA (max)

VA = 4.5V to 5.5V 0.16 0.30 mA (max)

fSCL=3.4MHz VA = 2.7V to 3.6V 0.37 0.55 mA (max)

VA = 4.5V to 5.5V 0.74 0.99 mA (max)

PNPower Consumption

fSCL=400kHz VA = 3.0V 0.26 mW

VA = 5.0V 0.78 mW

fSCL=3.4MHz VA = 3.0V 1.22 mW

VA = 5.0V 3.67 mW

Automatic Conversion Mode -- 2-wire interface stopped and quiet (SCL = SDA = VA). fSAMPLE = TCONVERT * 32

IASupply Current VA = 2.7V to 3.6V 0.41 0.59 mA (max)

VA = 4.5V to 5.5V 0.78 1.2 mA (max)

PAPower Consumption VA = 3.0V 1.35 mW

VA = 5.0V 3.91 mW

Power Down Mode (PD1) -- 2-wire interface stopped and quiet. (SCL = SDA = VA).

IPD1 Supply Current (Note 10) 0.1 0.2 µA (max)

PPD1 Power Consumption (Note 10) 0.5 0.9 µW (max)

Power Down Mode (PD2) -- 2-wire interface active. Master communicating with a different device on the bus.

IPD2 Supply Current

fSCL=400kHz VA = 2.7V to 3.6V 13 45 µA (max)

VA = 4.5V to 5.5V 27 80 µA (max)

fSCL=3.4MHz VA = 2.7V to 3.6V 89 150 µA (max)

VA = 4.5V to 5.5V 168 250 µA (max)

PPD2 Power Consumption

fSCL=400kHz VA = 3.0V 0.04 mW

VA = 5.0V 0.14 mW

fSCL=3.4MHz VA = 3.0V 0.29 mW

VA = 5.0V 0.84 mW

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ADC121C021/ADC121C027

A.C. and Timing Characteristics

The following specifications apply for VA = +2.7V to +5.5V. Boldface limits apply for TMIN ≤ TA ≤ TMAX and all other limits are at

TA = 25°C, unless otherwise specified.

Symbol Parameter Conditions (Note 12)Typical

(Note 9)

Limits

(Note 9)

Units

(Limits)

CONVERSION RATE

Conversion Time 1 µs

fCONV Conversion Rate

fSCL = 100kHz 5.56 ksps

fSCL = 400kHz 22.2 ksps

fSCL = 1.7MHz 94.4 ksps

fSCL = 3.4MHz 188.9 ksps

DIGITAL TIMING SPECS (SCL, SDA)

fSCL Serial Clock Frequency

Standard Mode

Fast Mode

High Speed Mode, Cb = 100pF

High Speed Mode, Cb = 400pF

100

400

3.4

1.7

kHz (max)

MHz (max)

tLOW SCL Low Time

Standard Mode

Fast Mode

High Speed Mode, Cb = 100pF

High Speed Mode, Cb = 400pF

4.7

1.3

160

320

us (min)

ns (min)

tHIGH SCL High Time

Standard Mode

Fast Mode

High Speed Mode, Cb = 100pF

High Speed Mode, Cb = 400pF

4.0

0.6

120

us (min)

ns (min)

tSU;DAT Data Setup Time

Standard Mode

Fast Mode

High Speed Mode

250

100

ns (min)

tHD;DAT Data Hold Time

Standard Mode (Note 14) 0

3.45

us (min)

us (max)

Fast Mode (Note 14) 0

0.9

us (min)

us (max)

High Speed Mode, Cb = 100pF 0

ns (min)

ns (max)

High Speed Mode, Cb = 400pF 0

150

ns (min)

ns (max)

tSU;STA

Setup time for a start or a repeated

start condition

Standard Mode

Fast Mode

High Speed Mode

4.7

0.6

160

us (min)

ns (min)

tHD;STA

Hold time for a start or a repeated start

condition

Standard Mode

Fast Mode

High Speed Mode

4.0

0.6

160

us (min)

ns (min)

tBUF

Bus free time between a stop and start

condition

Standard Mode

Fast Mode 4.7

1.3

us (min)

tSU;STO Setup time for a stop condition

Standard Mode

Fast Mode

High Speed Mode

4.0

0.6

160

us (min)

ns (min)

trDA Rise time of SDA signal

Standard Mode 1000 ns (max)

Fast Mode 20+0.1Cb

300

ns (min)

ns (max)

High Speed Mode, Cb = 100pF 10

ns (min)

ns (max)

High Speed Mode, Cb = 400pF 20

160

ns (min)

ns (max)

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ADC121C021/ADC121C027

Symbol Parameter Conditions (Note 12)Typical

(Note 9)

Limits

(Note 9)

Units

(Limits)

tfDA Fall time of SDA signal

Standard Mode 250 ns (max)

Fast Mode 20+0.1Cb

250

ns (min)

ns (max)

High Speed Mode, Cb = 100pF 10

ns (min)

ns (max)

High Speed Mode, Cb = 400pF 20

160

ns (min)

ns (max)

trCL Rise time of SCL signal

Standard Mode 1000 ns (max)

Fast Mode 20+0.1Cb

300

ns (min)

ns (max)

High Speed Mode, Cb = 100pF 10

ns (min)

ns (max)

High Speed Mode, Cb = 400pF 20

ns (min)

ns (max)

trCL1

Rise time of SCL signal after a

repeated start condition and after an

acknowledge bit.

Standard Mode 1000 ns (max)

Fast Mode 20+0.1Cb

300

ns (min)

ns (max)

High Speed Mode, Cb = 100pF 10

ns (min)

ns (max)

High Speed Mode, Cb = 400pF 20

160

ns (min)

ns (max)

tfCL Fall time of a SCL signal

Standard Mode 300 ns (max)

Fast Mode 20+0.1Cb

300

ns (min)

ns (max)

High Speed Mode, Cb = 100pF 10

ns (min)

ns (max)

High Speed Mode, Cb = 400pF 20

ns (min)

ns (max)

Capacitive load for each bus line (SCL

and SDA) 400 pF (max)

tSP

Pulse Width of spike suppressed

(Note 11)

Fast Mode

High Speed Mode 50

ns (max)

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ADC121C021/ADC121C027

Note 1: Absolute Maximum Ratings indicate limits beyond which damage to the device may occur. Operating Ratings indicate conditions for which the device is

functional, but do not guarantee specific performance limits. For guaranteed specifications and test conditions, see the Electrical Characteristics. The guaranteed

specifications apply only for the test conditions listed. Some performance characteristics may degrade when the device is not operated under the listed test

conditions. Operation of the device beyond the maximum Operating Ratings is not recommended.

Note 2: All voltages are measured with respect to GND = 0V, unless otherwise specified.

Note 3: When the input voltage at any pin exceeds 5.5V or is less than GND, the current at that pin should be limited per the Absolute Maximum Ratings. The

maximum package input current rating limits the number of pins that can safely exceed the power supplies.

Note 4: The absolute maximum junction temperature (TJmax) for this device is 150°C. The maximum allowable power dissipation is dictated by TJmax, the

junction-to-ambient thermal resistance (θJA), and the ambient temperature (TA), and can be calculated using the formula PDMAX = (TJmax − TA) / θJA. The values

for maximum power dissipation will be reached only when the device is operated in a severe fault condition (e.g., when input or output pins are driven beyond

the operating ratings, or the power supply polarity is reversed).

Note 5: Human body model is a 100 pF capacitor discharged through a 1.5 kΩ resistor. Machine model is a 220 pF capacitor discharged through 0 Ω. Charged

device model simulates a pin slowly acquiring charge (such as from a device sliding down the feeder in an automated assembler) then rapidly being discharged.

Note 6: Reflow temperature profiles are different for lead-free packages.

Note 7: The inputs are protected as shown below. Input voltage magnitudes up to 5.5V, regardless of VA, will not cause errors in the conversion result. For

example, if VA is 3V, the digital input pins can be driven with a 5V logic device.

30020904

Note 8: To guarantee accuracy, it is required that VA be well bypassed and free of noise.

Note 9: Typical figures are at TJ = 25°C, and represent most likely parametric norms. Test limits are guaranteed to National's AOQL (Average Outgoing Quality

Level).

Note 10: This parameter is guaranteed by design and/or characterization and is not tested in production.

Note 11: Spike suppression filtering on SCL and SDA will suppress spikes that are less than the indicated width.

Note 12: Cb refers to the capacitance of one bus line. Cb is expressed in pF units.

Note 13: The ADC will meet Minimum/Maximum specifications for fSCL up to 3.4MHz and VA = 2.7V to 3.6V when operating in the Quiet Interface Mode (See

Section 1.11 QUIET INTERFACE MODE).

Note 14: The ADC121C021 will provide a minimum data hold time of 300ns to comply with the I2C Specification.

Timing Diagrams

30020960

FIGURE 1. Serial Timing Diagram

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ADC121C021/ADC121C027

Specification Definitions

ACQUISITION TIME is the time required for the ADC to ac-

quire the input voltage. During this time, the hold capacitor is

charged by the input voltage.

APERTURE DELAY is the time between the start of a con-

version and the time when the input signal is internally ac-

quired or held for conversion.

CONVERSION TIME is the time required, after the input volt-

age is acquired, for the ADC to convert the input voltage to a

digital word.

DIFFERENTIAL NON-LINEARITY (DNL) is the measure of

the maximum deviation from the ideal step size of 1 LSB.

EFFECTIVE NUMBER OF BITS (ENOB, or EFFECTIVE

BITS) is another method of specifying Signal-to-Noise and

Distortion or SINAD. ENOB is defined as (SINAD - 1.76) / 6.02

and says that the converter is equivalent to a perfect ADC of

this (ENOB) number of bits.

FULL POWER BANDWIDTH is a measure of the frequency

at which the reconstructed output fundamental drops 3 dB

below its low frequency value for a full scale input.

GAIN ERROR is the deviation of the last code transition

(111...110) to (111...111) from the ideal (VREF - 1.5 LSB), after

adjusting for offset error.

INTEGRAL NON-LINEARITY (INL) is a measure of the de-

viation of each individual code from a line drawn from negative

full scale (½ LSB below the first code transition) through pos-

itive full scale (½ LSB above the last code transition). The

deviation of any given code from this straight line is measured

from the center of that code value.

INTERMODULATION DISTORTION (IMD) is the creation of

additional spectral components as a result of two sinusoidal

frequencies being applied to an individual ADC input at the

same time. It is defined as the ratio of the power in both the

second and the third order intermodulation products to the

power in one of the original frequencies. Second order prod-

ucts are fa ± fb, where fa and fb are the two sine wave input

frequencies. Third order products are (2fa ± fb ) and

(fa ± 2fb). IMD is usually expressed in dB.

MISSING CODES are those output codes that will never ap-

pear at the ADC output. The ADC121C021 is guaranteed not

to have any missing codes.

OFFSET ERROR is the deviation of the first code transition

(000...000) to (000...001) from the ideal (i.e. GND + 0.5 LSB).

SIGNAL TO NOISE RATIO (SNR) is the ratio, expressed in

dB, of the rms value of the input signal to the rms value of the

sum of all other spectral components below one-half the sam-

pling frequency, not including harmonics or d.c.

SIGNAL TO NOISE PLUS DISTORTION (S/N+D or

SINAD) Is the ratio, expressed in dB, of the rms value of the

input signal to the rms value of all of the other spectral com-

ponents below half the clock frequency, including harmonics

but excluding d.c.

SPURIOUS FREE DYNAMIC RANGE (SFDR) is the differ-

ence, expressed in dB, between the desired signal amplitude

to the amplitude of the peak spurious spectral component,

where a spurious spectral component is any signal present in

the output spectrum that is not present at the input and may

or may not be a harmonic.

TOTAL HARMONIC DISTORTION (THD) is the ratio, ex-

pressed in dBc, of the rms total of the first n harmonic com-

ponents at the output to the rms level of the input signal

frequency as seen at the output. THD is calculated as

where Af1 is the RMS power of the input frequency at the out-

put and Af2 through Afn are the RMS power in the first n

harmonic frequencies.

THROUGHPUT TIME is the minimum time required between

the start of two successive conversions. It is the acquisition

time plus the conversion time.

LEAST SIGNIFICANT BIT (LSB) is the bit that has the small-

est value or weight of all bits in a word. This value is

LSB = VA / 2n

where VA is the supply voltage for this product, and "n" is the

resolution in bits, which is 12 for the ADC121C021.

MOST SIGNIFICANT BIT (MSB) is the bit that has the largest

value or weight of all bits in a word. Its value is 1/2 of VA.

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ADC121C021/ADC121C027

Typical Performance Characteristics fSCL = 400kHz, fSAMPLE = 22ksps, fIN = 1kHz, VA = 5.0V, TA = +25°

C, unless otherwise stated.

INL vs. Code - VA=3V

30020922

DNL vs. Code - VA=3V

30020923

INL vs. Code - VA=5V

30020924

DNL vs. Code - VA=5V

30020925

INL vs. Supply

30020926

DNL vs. Supply

30020927

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ADC121C021/ADC121C027

ENOB vs. Supply

30020928

SINAD vs. Supply

30020929

FFT Plot

30020930

FFT Plot

30020931

Offset Error vs. Temperature

30020932

Gain Error vs. Temperature

30020933

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ADC121C021/ADC121C027

Continuous Operation Supply Current vs. VA

30020934

Automatic Conversion Supply Current vs. VA

30020935

Power Down (PD1) Supply Current vs. VA

30020936

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ADC121C021/ADC121C027

1.0 Functional Description

The ADC121C021 is a successive-approximation analog-to-

digital converter designed around a charge-redistribution dig-

ital-to-analog converter. Unless otherwise stated, references

to the ADC121C021 in this section will apply to both the

ADC121C021 and the ADC121C027.

1.1 CONVERTER OPERATION

Simplified schematics of the ADC121C021 in both track and

hold operation are shown in Figure 2 and Figure 3 respec-

tively. In Figure 2, the ADC121C021 is in track mode. SW1

connects the sampling capacitor to the analog input channel,

and SW2 equalizes the comparator inputs. The ADC is in this

state for approximately 0.4µs at the beginning of every con-

version cycle, which begins at the ACK fall of SDA. Conver-

sions occur when the conversion result register is read and

when the ADC is in automatic conversion mode. (see Section

1.9 AUTOMATIC CONVERSION MODE.)

Figure 3 shows the ADC121C021 in hold mode. SW1 con-

nects the sampling capacitor to ground and SW2 unbalances

the comparator. The control logic then instructs the charge-

redistribution DAC to add or subtract fixed amounts of charge

to or from the sampling capacitor until the comparator is bal-

anced. At this time the digital word supplied to the DAC is also

the digital representation of the analog input voltage. This

digital word is stored in the conversion result register and read

via the 2-wire interface.

In the Normal (non-Automatic) Conversion mode, a new con-

version is started after the previous conversion result is read.

In the Automatic Mode, conversions are started at set inter-

vals, as determined by bits D7 through D5 of the Configuration

"watchdog" function to ensure that the input voltage remains

within the limits set in the Alert Limit Registers. The minimum

and maximum conversion results can then be read from the

Lowest Conversion Register and the Highest Conversion

TERS.

30020965

FIGURE 2. ADC121C021 in Track Mode

30020966

FIGURE 3. ADC121C021 in Hold Mode

1.2 ANALOG INPUT

An equivalent circuit for the input of the ADC121C021 is

shown in Figure 4. The diodes provide ESD protection for the

analog input. The operating range for the analog input is 0 V

to VA. Going beyond this range will cause the ESD diodes to

conduct and may result in erratic operation. For this reason,

these diodes should NOT be used to clamp the input signal.

The capacitor C1 in Figure 4 has a typical value of 3 pF and

is mainly the package pin capacitance. Resistor R1 is the on

resistance (RON) of the multiplexer and track / hold switch and

is typically 500Ω. Capacitor C2 is the ADC121C021 sampling

capacitor, and is typically 30 pF. The ADC121C021 will de-

liver best performance when driven by a low-impedance

source (less than 100Ω). This is especially important when

using the ADC121C021 to sample dynamic signals. A buffer

amplifier may be necessary to limit source impedance. Use a

precision op-amp to maximize circuit performance. Also im-

portant when sampling dynamic signals is a band-pass or low-

pass filter to reduce noise at the input.

30020967

FIGURE 4. Equivalent Input Circuit

The analog input is sampled for eight internal clock cycles, or

for typically 400 ns, after the fall of SDA for acknowledgement.

This time could be as long as about 530 ns. The sampling

switch opens and the conversion begins this time after the fall

of ACK. This time are typical at room temperature and may

vary with temperature.

1.3 ADC TRANSFER FUNCTION

The output format of the ADC121C021 is straight binary.

Code transitions occur midway between successive integer

LSB values. The LSB width for the ADC121C021 is VA / 4096.

The ideal transfer characteristic is shown in Figure 5. The

transition from an output code of 0000 0000 0000 to a code

of 0000 0000 0001 is at 1/2 LSB, or a voltage of VA / 8192.

Other code transitions occur at intervals of 1 LSB.

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ADC121C021/ADC121C027

30020968

FIGURE 5. Ideal Transfer Characteristic

1.4 REFERENCE VOLTAGE

The ADC121C021 uses the supply (VA) as the reference, so

VA must be treated as a reference. The analog-to-digital con-

version will only be as precise as the reference (VA), so the

supply voltage should be free of noise. The reference should

be driven by a low output impedance voltage source.

The Applications section provides recommended ways to

drive the ADC reference input appropriately. Refer to Section

2.1 TYPICAL APPLICATION CIRCUIT for details.

1.5 POWER-ON RESET

An internal power-on reset (POR) occurs when the supply

voltage transitions above the power-on reset threshold. Each

of the registers contains a defined value upon POR and this

data remains there until any of the following occurs:

•The first conversion is completed, causing the Conversion

Result and Status registers to be updated.

•A different data word is written to a writable register.

•The ADC is powered down.

The internal registers will lose their contents if the supply volt-

age goes below 2.4V. Should this happen, it is important that

the VA supply be lowered to a maximum of 200mV before the

supply is raised again to properly reset the device and ensure

that the ADC performs as specified.

1.6 INTERNAL REGISTERS

The ADC121C021 has 8 internal data registers and one ad-

dress pointer. The registers provide additional ADC functions

such as storing minimum and maximum conversion results,

setting alert threshold levels, and storing data to configure the

operation of the device. Figure 6 shows all of the registers and

their corresponding address pointer values. All of the regis-

ters are read/write capable except the conversion result reg-

ister, which is read-only.

15 www.national.com

ADC121C021/ADC121C027

30020969

FIGURE 6. Register Structure

1.6.1 Address Pointer Register

The address pointer determines which of the data registers is

accessed by the I2C interface. The first data byte of every

write operation is stored in the address pointer register. This

value selects the register that the following data bytes will be

written to or read from. Only the three LSBs of this register

are variable. The other bits must always be written to as zeros.

After a power-on reset, the pointer register defaults to all ze-

ros (conversion result register).

Default Value: 00h

P7 P6 P5 P4 P3 P2 P1 P0

0 0 0 0 0 Register Select

P2 P1 P0 REGISTER

0 0 0 Conversion Result (read only)

0 0 1 Alert Status (read/write)

0 1 0 Configuration (read/write)

0 1 1 Low Limit (read/write)

1 0 0 High Limit (read/write)

1 0 1 Hysteresis (read/write)

1 1 0 Lowest Conversion (read/write)

1 1 1 Highest Conversion (read/write)

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ADC121C021/ADC121C027

1.6.2 Conversion Result Register

This register holds the result of the most recent conversion. In the normal mode, a new conversion is started whenever this register

is read. The conversion result data is in straight binary format with the MSB at D11.

Pointer Address 00h (Read Only)

Default Value: 0000h

D15 D14 D13 D12 D11 D10 D9 D8

Alert Flag Reserved Conversion Result [11:8]

D7 D6 D5 D4 D3 D2 D1 D0

Conversion Result [7:0]

Bits Name Description

15 Alert Flag This bit indicates when an alert condition has occurred. When the Alert Bit Enable is set in the

Configuration Register, this bit will be high if either alert flag is set in the Alert Status Register.

Otherwise, this bit is a zero. The I2C controller will typically read the Alert Status register and

other data registers to determine the source of the alert.

14:12 Reserved Always reads zeros.

11:0 Conversion Result The Analog-to-Digital conversion result. The Conversion result data is a 12-bit data word in

straight binary format. The MSB is D11.

1.6.3 Alert Status Register

This register indicates if a high or a low threshold has been violated. The bits of this register are active high. That is, a high indicates

that the respective limit has been violated.

Pointer Address 01h (Read/Write)

Default Value: 00h

D7 D6 D5 D4 D3 D2 D1 D0

Reserved Over Range

Alert

Under Range

Alert

Bits Name Description

7:2 Reserved Always reads zeros. Zeros must be written to these bits.

1 Over Range

Alert Flag

Bit is set to 1 when the measured voltage exceeds the VHIGH limit stored in the programmable

VHIGH limit register. Flag is reset to 0 when one of the following two conditions is met: (1) The

controller writes a one to this bit. (2) The measured voltage decreases below the programmed

VHIGH limit minus the programmed VHYST value (See Figure 9) . The alert will only self-clear if the

Alert Hold bit is cleared in the Configuration register. If the Alert Hold bit is set, the only way to

clear an over range alert is to write a zero to this bit.

0 Under Range

Alert Flag

Bit is set to 1 when the measured voltage falls below the VLOW limit stored in the programmable

VLOW limit register. Flag is reset to 0 when one of the following two conditions is met: (1) The

controller writes a one to this bit. (2) The measured voltage increases above the programmed

VLOW limit plus the programmed VHYST value. The alert will only self-clear if the Alert Hold bit is

cleared in the Configuration register. If the Alert Hold bit is set, the only way to clear an under

range alert is to write a zero to this bit.

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ADC121C021/ADC121C027

1.6.4 Configuration Register

Pointer Address 02h (Read/Write)

Default Value: 00h

D7 D6 D5 D4 D3 D2 D1 D0

Cycle Time [2:0]

(See Table at

right)

Alert

Hold

Alert

Flag

Enable

Alert

Enable

0 Polarity

Cycle Time[2:0] Conversion

Interval

Typical

fconvert

(ksps)

D7 D6 D5

0 0 0 Automatic Mode Disabled 0

0 0 1 Tconvert x 32 27

0 1 0 Tconvert x 64 13.5

0 1 1 Tconvert x 128 6.7

1 0 0 Tconvert x 256 3.4

1 0 1 Tconvert x 512 1.7

1 1 0 Tconvert x 1024 0.9

1 1 1 Tconvert x 2048 0.4

Bits Name Description

7:5 Cycle Time Configures Automatic Conversion mode. When these bits are set to zeros, the automatic

conversion mode is disabled. This is the case at power-up.

When these bits are set to a non-zero value, the ADC will begin operating in automatic conversion

mode. (See Section 1.9 AUTOMATIC CONVERSION MODE). The Cycle Time table shows how

different values provide various conversion intervals.

4 Alert Hold 0: Alerts will self-clear when the measured voltage moves within the limits by more than the

hysteresis register value.

1: Alerts will not self-clear and are only cleared when a one is written to the alert high flag or the

alert low flag in the Alert Status register.

3 Alert Flag Enable 0: Disables alert status bit [D15] in the Conversion Result register.

1: Enables alert status bit [D15] in the Conversion Result register.

2 Alert Pin Enable 0: Disables the ALERT output pin. The ALERT output will be high impedance when the pin is

disabled.

1: Enables the ALERT output pin.

*This bit does not apply to and is a "don't care" for the ADC121C027.

1 Reserved Always reads zeros. Zeros must be written to this bit.

0 Polarity This bit configures the active level polarity of the ALERT output pin.

0: Sets the ALERT pin to active low.

1: Sets the ALERT pin to active high.

*This bit does not apply to and is a "don't care" for the ADC121C027.

1.6.5 VLOW -- Alert Limit Register - Under Range

This register holds the lower limit threshold used to determine the alert condition. If the conversion moves lower than this limit, a

VLOW alert is generated.

Pointer Address 03h (Read/Write)

Default Value: 0000h

D15 D14 D13 D12 D11 D10 D9 D8

Reserved VLOW Limit [11:8]

D7 D6 D5 D4 D3 D2 D1 D0

VLOW Limit [7:0]

Bits Name Description

15:12 Reserved Always reads zeros. Zeros must be written to these bits.

11:0 VLOW Limit Lower limit threshold. D11 is MSB.

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ADC121C021/ADC121C027

1.6.6 VHIGH -- Alert Limit Register - Over Range

This register holds the upper limit threshold used to determine the alert condition. If the conversion moves higher than this limit, a

VHIGH alert is generated.

Pointer Address 04h (Read/Write)

Default Value: 0FFFh

D15 D14 D13 D12 D11 D10 D9 D8

Reserved VHIGH Limit [11:8]

D7 D6 D5 D4 D3 D2 D1 D0

VHIGH Limit [7:0]

Bits Name Description

15:12 Reserved Always reads zeros. Zeros must be written to these bits.

11:0 VHIGH Limit Upper limit threshold. D11 is MSB.

1.6.7 VHYST -- Alert Hysteresis Register

This register holds the hysteresis value used to determine the alert condition. After a VHIGH or VLOW alert occurs, the conversion

result must move within the VHIGH or VLOW limit by more than this value to clear the alert condition. Note: If the Alert Hold bit is set

in the configuration register, alert conditions will not self-clear.

Pointer Address 05h (Read/Write)

Default Value: 0000h

D15 D14 D13 D12 D11 D10 D9 D8

Reserved Hysteresis [11:8]

D7 D6 D5 D4 D3 D2 D1 D0

Hysteresis [7:0]

Bits Name Description

15:12 Reserved Always reads zeros. Zeros must be written to these bits.

11:0 Hysteresis Hysteresis value. D11 is MSB.

1.6.8 VMIN -- Lowest Conversion Register

This register holds the Lowest Conversion result when in the automatic conversion mode. Each conversion result is compared

against the contents of this register. If the value is lower, it becomes the lowest conversion and replaces the current value. If the

value is higher, the register contents remain unchanged. The lowest conversion value can be cleared at any time by writing 0FFFh

to this register. The value of this register will update automatically when the automatic conversion mode is enabled, but is NOT

updated in the normal mode.

Pointer Address 06h (Read/Write)

Default Value: 0FFFh

D15 D14 D13 D12 D11 D10 D9 D8

Reserved Lowest Conversion [11:8]

D7 D6 D5 D4 D3 D2 D1 D0

Lowest Conversion [7:0]

Bits Name Description

15:12 Reserved Always reads zeros. Zeros must be written to these bits.

11:0 Lowest Conversion Lowest conversion result data. D11 is MSB.

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ADC121C021/ADC121C027

1.6.9 VMAX -- Highest Conversion Register

This register holds the Highest Conversion result when in the Automatic mode. Each conversion result is compared against the

contents of this register. If the value is higher, it replaces the previous value. If the value is lower, the register contents remain

unchanged. The highest conversion value can be cleared at any time by writing 0000h to this register. The value of this register

will update automatically when the automatic conversion mode is enabled, but is NOT updated in the normal mode.

Pointer Address 07h (Read/Write)

Default Value: 0000h

D15 D14 D13 D12 D11 D10 D9 D8

Reserved Highest Conversion [11:8]

D7 D6 D5 D4 D3 D2 D1 D0

Highest Conversion [7:0]

Bits Name Description

15:12 Reserved Always reads zeros. Zeros must be written to these bits.

11:0 Highest Conversion Highest conversion result data. D11 is MSB.

1.7 SERIAL INTERFACE

The I2C-compatible interface operates in all three speed

modes. Standard mode (100kHz) and Fast mode (400kHz)

are functionally the same and will be referred to as Standard-

Fast mode in this document. High-Speed mode (3.4MHz) is

an extension of Standard-Fast mode and will be referred to

as Hs-mode in this document.

The following diagrams describe the timing relationships of

the clock (SCL) and data (SDA) signals. Pull-up resistors or

current sources are required on the SCL and SDA busses to

pull them high when they are not being driven low. A logic zero

is transmitted by driving the output low. A logic high is trans-

mitted by releasing the output and allowing it to be pulled-up

externally. The appropriate pull-up resistor values will depend

upon the total bus capacitance and operating speed. The AD-

C121C021 offers extended ESD tolerance (8kV HBM) for the

I2C bus pins (SCL & SDA) allowing extension of the bus

across multiple boards without extra ESD protection.

1.7.1 Basic I2C Protocol

The I2C interface is bi-directional and allows multiple devices

to operate on the same bus. The bus consists of master de-

vices and slave devices which can communicate back and

forth over the I2C interface. Master devices control the bus

and are typically microcontrollers, FPGAs, DSPs, or other

digital controllers. Slave devices are controlled by a master

and are typically peripheral devices such as the

ADC121C021. To support multiple devices on the same bus,

each slave has a unique hardware address which is referred

to as the "slave address." To communicate with a particular

device on the bus, the controller (master) sends the slave ad-

dress and listens for a response from the slave. This response

is referred to as an acknowledge bit. If a slave on the bus is

addressed correctly, it Acknowledges (ACKs) the master by

driving the SDA bus low. If the address doesn't match a

device's slave address, it Not-acknowledges (NACKs) the

master by letting SDA be pulled high. ACKs also occur on the

bus when data is being transmitted. When the master is writ-

ing data, the slave ACKs after every data byte is successfully

received. When the master is reading data, the master ACKs

after every data byte is received to let the slave know it wants

to receive another data byte. When the master wants to stop

reading, it NACKs after the last data byte and creates a stop

condition on the bus.

All communication on the bus begins with either a Start con-

dition or a Repeated Start condition. The protocol for starting

the bus varies between Standard-Fast mode and Hs-mode.

In Standard-Fast mode, the master generates a

Start condition by driving SDA from high to low while SCL is

high. In Hs-mode, starting the bus is more complicated.

Please refer to Section 1.7.3 High-Speed (Hs) Mode for the

full details of a Hs-mode Start condition.

A Repeated Start is generated to address a different device

or register, or to switch between read and write modes. The

master generates a Repeated Start condition by driving SDA

low while SCL is high. Following the Repeated Start, the mas-

ter sends out the slave address and a read/write bit as shown

in Figure 7. The bus continues to operate in the same speed

mode as before the Repeated Start condition.

All communication on the bus ends with a Stop condition. In

either Standard-Fast mode or Hs-Mode, a Stop condition oc-

curs when SDA is pulled high while SCL is high. After a Stop

condition, the bus remains idle until a master generates an-

other Start condition.

Please refer to the Philips I2C® Specification (Version 2.1 Jan,

2000) for a detailed description of the serial interface.

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ADC121C021/ADC121C027

30020911

FIGURE 7. Basic Operation.

1.7.2 Standard-Fast Mode

In Standard-Fast mode, the master generates a start condi-

tion by driving SDA from high to low while SCL is high. The

start condition is always followed by a 7-bit slave address and

a Read/Write bit. After these 8 bits have been transmitted by

the master, SDA is released by the master and the

ADC121C021 either ACKs or NACKs the address. If the slave

address matches, the ADC121C021 ACKs the master. If the

address doesn't match, the ADC121C021 NACKs the master.

For a write operation, the master follows the ACK by sending

the 8-bit register address pointer to the ADC. Then the

ADC121C021 ACKs the transfer by driving SDA low. Next,

the master sends the upper 8-bits to the ADC121C021. Then

the ADC121C021 ACKs the transfer by driving SDA low. For

a single byte transfer, the master should generate a stop con-

dition at this point. For a 2-byte write operation, the lower 8-

bits are sent by the master. The ADC121C021 then ACKs the

transfer, and the master either sends another pair of data

bytes, generates a Repeated Start condition to read or write

another register, or generates a Stop condition to end com-

munication.

A read operation can take place either of two ways:

If the address pointer is pre-set before the read operation, the

desired register can be read immediately following the slave

address. In this case, the upper 8-bits of the register, set by

the pre-set address pointer, are sent out by the ADC. For a

single byte read operation, the Master sends a NACK to the

ADC and generates a Stop condition to end communication

after receiving 8-bits of data. For a 2-Byte read operation, the

Master continues the transmission by sending an ACK to the

ADC. Then the ADC sends out the lower 8-bits of the ADC

ceive more data or sends a NACK followed by a Stop or

Repeated Start. If the master sends an ACK, the ADC sends

the next data byte, and the read cycle repeats.

If the ADC121C021address pointer needs to be set, the mas-

ter needs to write to the device and set the address pointer

before reading from the desired register. This type of read

requires a start, the slave address, a write bit, the address

pointer, a Repeated Start (if appropriate), the slave address,

and a read bit (refer to Figure 12). Following this sequence,

the ADC sends out the upper 8-bits of the register. For a single

byte read operation, the Master must then send a NACK to

the ADC and generate a Stop condition to end communica-

tion. For a 2-Byte write operation, the Master sends an ACK

to the ADC. Then, the ADC sends out the lower 8-bits of the

ADC register. At this point, the master sends either an ACK

to receive more data, or a NACK followed by a Stop or Re-

peated Start. If the master sends an ACK, the ADC sends

another pair of data bytes, and the read cycle will repeat. The

number of data words that can be read is unlimited.

1.7.3 High-Speed (Hs) Mode

For Hs-mode, the sequence of events to begin communica-

tion differs slightly from Standard-Fast mode. Figure 8 de-

scribes this in further detail. Initially, the bus begins running

in Standard-Fast mode. The master generates a

Start condition and sends the 8-bit Hs master code

(00001XXX) to the ADC121C021. Next, the ADC121C021

responds with a NACK. Once the SCL line has been pulled to

a high level, the master switches to Hs-mode by increasing

the bus speed and generating a second Repeated Start con-

dition (driving SDA low while SCL is pulled high). At this point,

the master sends the slave address to the ADC121C021, and

communication continues as shown above in the "Basic Op-

eration" Diagram (see Figure 7).

When the master generates a Repeated Start condition while

in Hs-mode, the bus stays in Hs-mode awaiting the slave ad-

dress from the master. The bus continues to run in Hs-mode

until a Stop condition is generated by the master. When the

master generates a Stop condition on the bus, the bus must

be started in Standard-Fast mode again before increasing the

bus speed and switching to Hs-mode.

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ADC121C021/ADC121C027

30020912

FIGURE 8. Beginning Hs-Mode Communication

1.7.4 I2C Slave (Hardware) Address

The ADC has a seven-bit hardware address which is also referred to as a slave address. For the MSOP-8 version of the AD-

C121C021, this address is configured by the ADR0 and ADR1 addres selection inputs. For the ADC121C027, the address is

configured by the ADR0 address selection input. ADR0 and ADR1 can be grounded, left floating, or tied to VA. If desired, ADR0

and ADR1 can be set to VA/2 rather than left floating. The state of these inputs sets the hardware address that the ADC responds

to on the I2C bus (see Table 1). For the ADC121C021, the hardware address is not pin-configurable and is set to 1010100. The

diagrams in Section 1.10 COMMUNICATING WITH THE ADC121C021 describes how the I2C controller should address the ADC

via the I2C interface.

TABLE 1. Slave Addresses

Slave Address

[A6 - A0]

ADC121C027

(TSOT-6)

ADC121C021

(TSOT-6)

ADC121C021

(MSOP-8)

ADR0 ALERT ADR1 ADR0

1010000 Floating ----------------- Floating Floating

1010001 GND ----------------- Floating GND

1010010 VA----------------- Floating VA

1010100 ----------------- Single Address GND Floating

1010101 ----------------- ----------------- GND GND

1010110 ----------------- ----------------- GND VA

1011000 ----------------- ----------------- VAFloating

1011001 ----------------- ----------------- VAGND

1011010 ----------------- ----------------- VAVA

1.8 ALERT FUNCTION

The ALERT function is an "out-of-range" indicator. At the end

of every conversion, the measured voltage is compared to the

values in the VHIGH and VLOW registers. If the measured volt-

age exceeds the value stored in VHIGH or falls below the value

stored in VLOW, an alert condition occurs. The Alert condition

is indicated in up to three places. First, the alert condition al-

ways causes either or both of the alert flags in the Alert Status

VHIGH limit, the Over Range Alert Flag is set. If the measured

voltage falls below the VLOW limit, the Under Range Alert Flag

is set. Second, if the Alert Flag Enable bit is set in the Con-

figuration register, the alert condition also sets the MSB of the

Conversion Result register. Third, if the Alert Pin Enable bit is

set in the Configuration register, the ALERT output becomes

active (see Figure 9). The ALERT output (ADC121C021 only)

can be configured as an active high or active low output via

the Polarity bit in the Configuration register. If the Polarity bit

is cleared, the ALERT output is configured as active low. If

the Polarity bit is set, the ALERT output is configured as active

high.

The Over Range Alert condition is cleared when one of the

following two conditions is met:

1. The controller writes a one to the Over Range Alert Flag

bit.

2. The measured voltage goes below the programmed

VHIGH limit minus the programmed VHYST value and the

Alert Hold bit is cleared in the Configuration register. (see

Figure 9). If the Alert Hold bit is set, the alert condition

persists and only clears when a one is written to the Over

Range Alert Flag bit.

The Under Range Alert condition is cleared when one of the

following two conditions is met:

1. The controller writes a one to the Under Range Alert Flag

bit.

2. The measured voltage goes above the programmed

VLOW limit plus the programmed VHYST value and the

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ADC121C021/ADC121C027

Alert Hold bit is cleared in the Configuration register. If

the Alert Hold bit is set, the alert condition persists and

only clears when a one is written to the Under Range

Alert Flag bit.

If the alert condition has been cleared by writing a one to the

alert flag while the measured voltage still violates the VHIGH

or VLOW limits, an alert condition will occur again after the

completion of the next conversion (see Figure 10).

Alert conditions only occur if the input voltage exceeds the

VHIGH limit or falls below the VLOW limit at the sample-hold

instant. The input voltage can exceed the VHIGH limit or fall

below the VLOW limit briefly between conversions without

causing an alert condition.

30020974

FIGURE 9. Alert condition cleared when measured

voltage crosses VHIGH - VHYST

30020975

FIGURE 10. Alert condition cleared by writing a "1" to the

Alert Flag.

1.9 AUTOMATIC CONVERSION MODE

The automatic conversion mode configures the ADC to con-

tinually perform conversions without receiving "read" instruc-

tions from the controller over the I2C interface. The mode is

activated by writing a non-zero value into the Cycle Time bits

- D[7:5] - of the Configuration register (see Section 1.6.4 Con-

figuration Register). Once the ADC121C021 enters this

mode, the internal oscillator is always enabled. The ADC's

control logic samples the input at the sample rate set by the

cycle time bits. Although the conversion result is not trans-

mitted by the 2-wire interface, it is stored in the conversion

result register and updates the various status registers of the

device.

In automatic conversion mode, the out-of-range alert function

is active and updates after every conversion. The ADC can

operate independently of the controller in automatic conver-

sion mode. When the input signal goes "out-of-range", an

alert signal is sent to the controller. The controller can then

read the status registers and determine the source of the alert

condition. Also, comparison and updating of the VMIN and

VMAX registers occurs after every conversion in automatic

conversion mode. The controller can occasionally read the

VMIN and/or VMAX registers to determine the sampled input

extremes. These register values persist until the user resets

the VMIN and VMAX registers. These two features are useful in

system monitoring, peak detection, and sensing applications.

1.10 COMMUNICATING WITH THE ADC121C021

The ADC121C021's data registers are selected by the ad-

dress pointer (see Section 1.6.1 Address Pointer Register).

To read/write a specific data register, the pointer must be set

to that register's address. The pointer is always written at the

beginning of a write operation. When the pointer needs to be

updated for a read cycle, a write operation must precede the

read operation to set the pointer address correctly. On the

other hand, if the pointer is preset correctly, a read operation

can occur without writing the address pointer register. The

following timing diagrams describe the various read and write

operations supported by the ADC.

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ADC121C021/ADC121C027

1.10.1 Reading from a 2-Byte ADC Register

The following diagrams indicate the sequence of actions re-

quired for a 2-Byte read from an ADC121C021 Register.

30020963

FIGURE 11. (a) Typical Read from a 2-Byte ADC Register with Preset Pointer

30020970

FIGURE 12. (b) Typical Pointer Set Followed by Immediate Read of a 2-Byte ADC Register

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ADC121C021/ADC121C027

1.10.2 Reading from a 1-Byte ADC Register

The following diagrams indicate the sequence of actions required for a single Byte read from an ADC121C021 Register.

30020971

FIGURE 13. (a) Typical Read from a 1-Byte ADC Register with Preset Pointer

30020972

FIGURE 14. (b) Typical Pointer Set Followed by Immediate Read of a 1-Byte ADC Register

1.10.3 Writing to an ADC Register

The following diagrams indicate the sequence of actions required for writing to an ADC121C021 Register.

30020964

FIGURE 15. (a) Typical Write to a 1-Byte ADC Register

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ADC121C021/ADC121C027

30020973

FIGURE 16. (b) Typical Write to a 2-Byte ADC Register

1.11 QUIET INTERFACE MODE

To improve performance at High Speed, operate the ADC in

Quiet Interface Mode. This mode provides improved INL and

DNL performance in I2C Hs-Mode (3.4MHz). The Quiet Inter-

face mode provides a maximum throughput rate of 162ksps.

Figure 17 describes how to read the conversion result register

in this mode. Basically, the Master needs to release SCL for

at least 1µs before the MSB of every upper data byte. The

diagram assumes that the address pointer register is set to

its default value.

Quiet Interface mode will only improve INL and DNL perfor-

mance in Hs-Mode. Standard and Fast mode performance is

unaffected by the Quiet Interface mode.

30020976

FIGURE 17. Reading in Quiet Interface Mode

www.national.com 26

ADC121C021/ADC121C027

2.0 Applications Information

2.1 TYPICAL APPLICATION CIRCUIT

A typical application circuit is shown in Figure 18. The analog

supply is bypassed with a capacitor network located close to

the ADC121C021. The ADC uses the analog supply (VA) as

its reference voltage, so it is very important that VA be kept as

clean as possible. Due to the low power requirements of the

ADC121C021, it is possible to use a precision reference as a

power supply.

The bus pull-up resistors (RP) should be powered by the

controller's supply. It is important that the pull-up resistors are

pulled to the same voltage potential as VA. This will ensure

that the logic levels of all devices on the bus are compatible.

If the controller's supply is noisy, an appropriate bypass ca-

pacitor should be added between the controller's supply pin

and the pull-up resistors. For Hs-mode applications, this by-

pass capacitance will improve the accuracy of the ADC.

The value of the pull-up resistors (RP) depends upon the

characteristics of each particular I2C bus. The I2C specifica-

tion describes how to choose an appropriate value. As a

general rule-of-thumb, we suggest using a 1kΩ resistor for

Hs-mode bus configurations and a 5kΩ resistor for Standard

or Fast Mode bus configurations. Depending upon the bus

capacitance, these values may or may not be sufficient to

meet the timing requirements of the I2C bus specification.

Please see the I2C specification for further information.

30020920

FIGURE 18. Typical Application Circuit

2.2 BUFFERED INPUT

A buffered input application circuit is shown in Figure 19. The

analog input is buffered by a National Semiconductor

LMP7731. The non-inverting amplifier configuration provides

a buffered gain stage for a single ended source. This appli-

cation circuit is good for single-ended sensor interface. The

input must have a DC bias level that keeps the ADC input

signal from swinging below GND or above the supply (+5V in

this case).

The LM4132, with its 0.05% accuracy over temperature, is an

excellent choice as a reference source for the ADC121C021.

30020921

FIGURE 19. Buffered Input Circuit

27 www.national.com

ADC121C021/ADC121C027

2.3 INTELLIGENT BATTERY MONITOR

The ADC121C021 is easily used as an intelligent battery

monitor. The simple circuit shown in Figure 20, uses the

ADC121C021, the LP2980 fixed reference, and a resistor di-

vider to implement an intelligent battery monitor with a window

supervisory feature. The window supervisory feature is im-

plemented by the "out of range" alert function. When the

battery is recharging, the Over Range Alert will indicate that

the charging cycle is complete (see Figure 21). When the

battery is nearing depletion, the Under Range Alert will indi-

cate that the battery is low (see Figure 22).

30020977

FIGURE 20. Intelligent Battery Monitor Circuit

30020978

FIGURE 21. Recharge Cycle

30020979

FIGURE 22. Discharge Cycle

In addition to the window supervisory feature, the

ADC121C021 will allow the controller to read the battery volt-

age at any time during operation.

The accurate voltage reading and the alert feature will allow

a controller to improve the efficiency of a battery-powered

device. During the discharge cycle, the controller can switch

to a low-battery mode, safely suspend operation, or report a

precise battery level to the user. During the recharge cycle,

the controller can implement an intelligent recharge cycle,

decreasing the charge rate when the battery charge nears

capacity.

2.3.1 Trickle Charge Controller

While a battery is discharging, the ADC121C021 can be used

to control a trickle charge to keep the battery near full capacity

(see Figure 23). When the alert output is active, the battery

will recharge. An intelligent recharge cycle will prevent over-

charging and damaging the battery. With a trickle charge, the

battery powered device can be disconnected from the charger

at any time with a full charge.

30020980

FIGURE 23. Trickle Charge

2.4 LAYOUT, GROUNDING, AND BYPASSING

For best accuracy and minimum noise, the printed circuit

board containing the ADC121C021 should have separate

analog and digital areas. The areas are defined by the loca-

tions of the analog and digital power planes. Both of these

planes should be located on the same board layer. A single,

solid ground plane is preferred if digital return current does

not flow through the analog ground area. Frequently a single

ground plane design will utilize a "fencing" technique to pre-

vent the mixing of analog and digital ground current. Separate

ground planes should only be utilized when the fencing tech-

nique is inadequate. The separate ground planes must be

connected in one place, preferably near the ADC121C021.

Special care is required to guarantee that signals do not pass

over power plane boundaries. Return currents must always

have a continuous return path below their traces.

The ADC121C021 power supply should be bypassed with a

4.7µF and a 0.1µF capacitor as close as possible to the device

with the 0.1µF right at the device supply pin. The 4.7µF ca-

pacitor should be a tantalum type and the 0.1µF capacitor

should be a low ESL type. The power supply for the

ADC121C021 should only be used for analog circuits.

Avoid crossover of analog and digital signals and keep the

clock and data lines on the component side of the board. The

clock and data lines should have controlled impedances.

www.national.com 28

ADC121C021/ADC121C027

Physical Dimensions inches (millimeters) unless otherwise noted

6-Lead TSOT

Order Numbers ADC121C021CIMK & ADC121C027CIMK

NS Package Number MK06A

8-Lead MSOP

Order Numbers ADC121C021CIMM

NS Package Number MUA08A

29 www.national.com

ADC121C021/ADC121C027

Notes

ADC121C021/ADC121C027 I2C-Compatible, 12-Bit Analog-to-Digital Converter with Alert

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