ADS7818EB/250 - Texas Instruments

ADS7818

DESCRIPTION

The ADS7818 is a 12-bit sampling analog-to-digital

converter (A/D) complete with sample/hold, internal

2.5V reference, and synchronous serial interface. Typi-

cal power dissipation is 11mW at a 500kHz through-

put rate. The device can be placed into a power down

mode which reduces dissipation to just 2.5mW. The

input range is zero to two times the reference voltage,

and the internal reference can be overdriven by an

external voltage.

Low power, small size, and high-speed make the

ADS7818 ideal for battery operated systems such as

wireless communication devices, portable multi-chan-

nel data loggers, and spectrum analyzers. The serial

interface also provides low-cost isolation for remote

data acquisition. The ADS7818 is available in a plas-

tic mini-DIP-8 or an MSOP-8 package and is guaran-

teed over the –40°C to +85°C temperature range.

12-Bit High Speed Low Power Sampling

ANALOG-TO-DIGITAL CONVERTER

FEATURES

●500kHz THROUGHPUT RATE

●2.5V INTERNAL REFERENCE

●LOW POWER: 11mW

●SINGLE SUPPLY +5V OPERATION

●DIFFERENTIAL INPUT

●SERIAL INTERFACE

●GUARANTEED NO MISSING CODES

●MINI-DIP-8 AND MSOP-8

●UNIPOLAR INPUT RANGE

International Airport Industrial Park • Mailing Address: PO Box 11400, Tucson, AZ 85734 • Street Address: 6730 S. Tucson Blvd., Tucson, AZ 85706 • Tel: (520) 746-1111

Twx: 910-952-1111 • Internet: http://www.burr-brown.com/ • Cable: BBRCORP • Telex: 066-6491 • FAX: (520) 889-1510 • Immediate Product Info: (800) 548-6132

APPLICATIONS

●BATTERY OPERATED SYSTEMS

●DIGITAL SIGNAL PROCESSING

●HIGH SPEED DATA ACQUISITION

●WIRELESS COMMUNICATION SYSTEMS

SAR CLK

Serial

Interface

Comparator

S/H Amp

DATA

+In

–In CDAC

Internal

+2.5V Ref

Buffer

REF

CONV

10kΩ ±30%

For most current data sheet and other product

information, visit www.burr-brown.com

SBAS078

ADS7818

SPECIFICATIONS

At TA = –40°C to +85°C, +VCC = +5V, fSAMPLE = 500kHz, fCLK = 16 • fSAMPLE, internal reference, unless otherwise specified.

The information provided herein is believed to be reliable; however, BURR-BROWN assumes no responsibility for inaccuracies or omissions. BURR-BROWN

assumes no responsibility for the use of this information, and all use of such information shall be entirely at the user’s own risk. Prices and specifications are subject

to change without notice. No patent rights or licenses to any of the circuits described herein are implied or granted to any third party. BURR-BROWN does not

authorize or warrant any BURR-BROWN product for use in life support devices and/or systems.

ADS7818P, E ADS7818PB, EB

PARAMETER CONDITIONS MIN TYP MAX MIN TYP MAX UNITS

ANALOG INPUT

Full-Scale Input Span(1) +In – (–In) 0 5 ✻✻V

Absolute Input Range +In –0.2 VCC +0.2 ✻✻V

–In –0.2 +0.2 ✻✻V

Capacitance 15 ✻pF

Leakage Current 1✻µA

SYSTEM PERFORMANCE

Resolution 12 ✻Bits

No Missing Codes 12 ✻Bits

Integral Linearity Error ±1±2±0.5 ±1 LSB(2)

Differential Linearity Error ±0.8 ±0.5 ±1 LSB

Offset Error ±2±5±1✻LSB

Gain Error(3) 25°C±12 ±30 ±7±15 LSB

–40°C to +85°C±50 ±35 LSB

Common-Mode Rejection DC, 0.2Vp-p 70 ✻dB

1MHz, 0.2Vp-p 50 ✻dB

Noise 150 ✻µVrms

Power Supply Rejection Worst Case ∆, +VCC = 5V ±5% 1.2 ✻LSB

SAMPLING DYNAMICS

Conversion Time 1.625 ✻µs

Acquisition Time 0.350 ✻µs

Throughput Rate 500 ✻kHz

Aperture Delay 5✻ns

Aperture Jitter 30 ✻ps

Step Response 350 ✻ns

DYNAMIC CHARACTERISTICS

Signal-to-Noise Ratio VIN = 5Vp-p at 100kHz 72 ✻dB

Total Harmonic Distortion(4) VIN = 5Vp-p at 100kHz –78 –72 –82 –75 dB

Signal-to-(Noise+Distortion) VIN = 5Vp-p at 100kHz 68 70 70 72 dB

Spurious Free Dynamic Range VIN = 5Vp-p at 100kHz 72 78 75 82 dB

Usable Bandwidth SINAD > 68dB 350 ✻kHz

REFERENCE OUTPUT

Voltage IOUT = 0 2.475 2.50 2.525 2.48 ✻2.52 V

Source Current(5) Static Load 50 ✻µA

Drift IOUT = 0 20 ✻ppm/°C

Line Regulation 4.75V ≤ VCC ≤ 5.25V 0.6 ✻mV

REFERENCE INPUT

Range 2.0 2.55 ✻✻V

Resistance(6) to Internal Reference Voltage 10 ✻✻✻ kΩ

DIGITAL INPUT/OUTPUT

Logic Family CMOS ✻

Logic Levels:

VIH |IIH| ≤ +5µA 3.0 VCC +0.3 ✻✻V

VIL |IIL| ≤ +5µA –0.3 0.8 ✻✻V

VOH IOH = –500µA 3.5 ✻✻V

VOL IOL = 500µA 0.4 ✻✻V

Data Format Straight Binary ✻

POWER SUPPLY REQUIREMENT

+VCC Specified Performance 4.75 5.25 ✻✻V

Quiescent Current fSAMPLE = 500kHz 2.2 ✻mA

Power Down 0.5 ✻mA

Power Dissipation fSAMPLE = 500kHz 11 20 ✻✻ mW

Power Down 2.5 ✻mW

TEMPERATURE RANGE

Specified Performance –40 +85 ✻✻°C

✻ Specifications same as ADS7818P,E.

NOTES: (1) Ideal input span, does not include gain or offset error. (2) LSB means Least Significant Bit, with VREF equal to +2.5V, one LSB is 1.22mV. (3) Measured

relative to an ideal, full-scale input (+In – (–In)) of 4.999V. Thus, gain error includes the error of the internal voltage reference. (4) Calculated on the first nine

harmonics of the input frequency. (5) If the internal reference is required to source current to an external load, the reference voltage will change due to the internal

10kΩ resistor. (6) Can vary ±30%.

ADS7818

+VCC to GND............................................................................–0.3V to 6V

Analog Inputs to GND .............................................. –0.3V to (VCC + 0.3V)

Digital Inputs to GND ............................................... –0.3V to (VCC + 0.3V)

Power Dissipation .......................................................................... 325mW

Maximum Junction Temperature ................................................... +150°C

Operating Temperature Range ......................................... –40°C to +85°C

Storage Temperature Range .......................................... –65°C to +150°C

Lead Temperature (soldering, 10s) ............................................... +300°C

NOTE: (1) Stresses above those listed under “Absolute Maximum Ratings” may

cause permanent damage to the device. Exposure to absolute maximum condi-

tions for extended periods may affect device reliability.

ELECTROSTATIC

DISCHARGE SENSITIVITY

Electrostatic discharge can cause damage ranging from per-

formance degradation to complete device failure. Burr-

Brown Corporation recommends that all integrated circuits

be handled and stored using appropriate ESD protection

methods.

ESD damage can range from subtle performance degrada-

tion to complete device failure. Precision integrated circuits

may be more susceptible to damage because very small

parametric changes could cause the device not to meet

published specifications.

ABSOLUTE MAXIMUM RATINGS(1)

PIN CONFIGURATION

Top View

+VCC

CLK

DATA

CONV

VREF

+In

–In

GND

ADS7818

Plastic Mini-DIP-8

PIN NAME DESCRIPTION

REF Reference Output. Decouple to ground with a 0.1µF ceramic capacitor and a 2.2µF tantalum capacitor.

2 +In Non-Inverting Input.

3 –In Inverting Input. Connect to ground or to remote ground sense point.

4 GND Ground.

5 CONV Convert Input. Controls the sample/hold mode, start of conversion, start of serial data transfer, type of serial transfer, and power

down mode. See the Digital Interface section for more information.

6 DATA Serial Data Output. The 12-bit conversion result is serially transmitted most significant bit first with each bit valid on the rising edge

of CLK. By properly controlling the CONV input, it is possibly to have the data transmitted least significant bit first. See the Digital

Interface section for more information.

7 CLK Clock Input. Synchronizes the serial data transfer and determines conversion speed.

8+V

CC Power Supply. Decouple to ground with a 0.1µF ceramic capacitor and a 10µF tantalum capacitor.

PIN ASSIGNMENTS

+VCC

CLK

DATA

CONV

VREF

+In

–In

GND

ADS7818

MSOP-8

PACKAGE/ORDERING INFORMATION

MAXIMUM MAXIMUM

INTEGRAL DIFFERENTIAL

LINEARITY LINEARITY PACKAGE SPECIFICATION

ERROR ERROR DRAWING TEMPERATURE PACKAGE ORDERING TRANSPORT

PRODUCT (LSB) (LSB) PACKAGE NUMBER(1) RANGE MARKING(2) NUMBER(3) MEDIA

ADS7818E ±2 N/S(4) MSOP-8 337 –40°C to +85°C A18 ADS7818E/250 Tape and Reel

"" " " " " "ADS7818E/2K5 Tape and Reel

ADS7818EB ±1±1 MSOP-8 337 –40°C to +85°C A18 ADS7818EB/250 Tape and Reel

"" " " " " "ADS7818EB/2K5 Tape and Reel

ADS7818P ±2 N/S(4) Plastic DIP-8 006 –40°C to +85°C ADS7818P ADS7818P Rails

ADS7818PB ±1±1"" "ADS7818PB ADS7818PB Rails

NOTE: (1) For detail drawing and dimension table, please see end of data sheet or Package Drawing File on Web. (2) Performance Grade information is marked

on the reel. (3) Models with a slash(/) are available only in Tape and reel in quantities indicated (e.g. /250 indicates 250 units per reel, /2K5 indicates 2500 devices

per reel). Ordering 2500 pieces of ”ADS7818E/2K5“ will get a single 2500-piece Tape and Reel. For detailed Tape and Reel mechanical information, refer to the

www.burr-brown.com web site under Applications and Tape and Reel Orientation and Dimensions. (4) N/S = Not Specified, typical only. However, 12-Bits no missing

codes is guaranteed over temperature.

ADS7818

TYPICAL PERFORMANCE CURVES

At TA = +25°C, VCC = +5V, fSAMPLE = 500kHz, fCLK = 16 • fSAMPLE, and internal +2.5V reference, unless otherwise specified.

CHANGE IN INTEGRAL LINEARITY and

DIFFERENTIAL LINEARITY vs SAMPLE RATE

400100 600200 300 500

Sample Rate (kHz)

Delta from f

SAMPLE

= 500kHz (LSB)

–0.3

0.4

0.3

0.2

0.1

0.0

–0.1

–0.2 Change in Differential

Linearity (LSB)

Change in Integral

Linearity (LSB)

CHANGE IN OFFSET vs TEMPERATURE

20–40 100–20 0 40

Temperature (°C)

Delta from 25°C (LSB)

–0.4

2.0

1.6

1.2

0.8

0.4

0.0

60 80

POWER-DOWN SUPPLY CURRENT

vs TEMPERATURE

20–40 100–20 0 40

Temperature (°C)

Power-down Supply Current (µA)

390

470

440

430

420

410

400

450

460

60 80

SUPPLY CURRENT vs TEMPERATURE

20–40 100–20 0 40

Temperature (°C)

Supply Current (mA)

1.6

2.3

2.1

2.0

1.9

1.8

1.7

2.2

60 80

SAMPLE

= 500kHz

SAMPLE

= 125kHz

SUPPLY CURRENT vs SAMPLE RATE

400100 600200 300 500

Sample Rate (kHz)

Supply Current (mA)

1.7

2.4

2.3

2.2

2.1

2.0

1.9

1.8

CHANGE IN FULL-SCALE ERROR

vs TEMPERATURE

20–40 100–20 0 40

Temperature (°C)

Delta from 25°C (LSB)

–10

–2

–4

–6

–8

60 80

ADS7818

TYPICAL PERFORMANCE CURVES (Cont.)

At TA = +25°C, VCC = +5V, fSAMPLE = 500kHz, fCLK = 16 • fSAMPLE, and internal +2.5V reference, unless otherwise specified.

POWER SUPPLY REJECTION

vs POWER SUPPLY RIPPLE FREQUENCY

1 10 100 1k 10k 100k 1M

Power Supply Ripple Frequency (Hz)

Power Supply Rejection (mV/V)

FREQUENCY SPECTRUM

(4096 Point FFT; f

= 10.9kHz, –0.2dB)

0 25062.5 187.5125

Frequency (kHz)

Amplitude (dB)

–120

–100

–80

–60

–40

–20

FREQUENCY SPECTRUM

(4096 Point FFT; f

= 102kHz, –0.2dB)

Amplitude (dB)

–100

–80

–60

–40

–20

0 25062.5 187.5125

Frequency (kHz)

–120

CHANGE IN FULL-SCALE ERROR

vs EXTERNAL REFERENCE VOLTAGE

2.32.0 2.52.1 2.2 2.4

External Reference Voltage (V)

Delta from V

REF

= 2.5V (mV)

–1.2

0.2

–0.4

–0.6

–0.8

–1.0

–0.2

0.0

CHANGE IN OFFSET

vs EXTERNAL REFERENCE VOLTAGE

2.51.9 3.12.1 2.3 2.7 2.9

External Reference Voltage (V)

Delta from VREF = 2.5V (mV)

–0.4

0.5

0.0

–0.1

–0.2

–0.3

0.1

0.2

0.3

0.4

PEAK-TO-PEAK NOISE

vs EXTERNAL REFERENCE VOLTAGE

2.32.0 2.52.1 2.2 2.4

External Reference Voltage (LSB)

Peak-to-Peak Noise (LSB)

0.40

0.70

0.65

0.60

0.55

0.50

0.45

ADS7818

TYPICAL PERFORMANCE CURVES (Cont.)

At TA = +25°C, VCC = +5V, fSAMPLE = 500kHz, fCLK = 16 • fSAMPLE, and internal +2.5V reference, unless otherwise specified.

CHANGE IN SIGNAL-TO-NOISE RATIO

and SIGNAL-TO-(NOISE+DISTORTION)

vs TEMPERATURE

–20–40 100

Temperature (°C)

Delta from +25°C (dB)

0.2

0.1

0.0

–0.1

–0.2

–0.3

0.3

0 20 40 60 80

SINAD

SNR

= 100kHz, –0.2dB

CHANGE IN SPURIOUS FREE DYNAMIC RANGE

AND TOTAL HARMONIC DISTORTION

vs TEMPERATURE

–20–40 100

Temperature (°C)

SFDR Delta from +25°C (dB)

THD Delta from +25°C (dB)

0.5

0.0

–0.5

–1.0

–1.5

–2.0

1.0

–0.5

0.0

0.5

1.0

1.5

2.0

–1.0

0 20 40 60 80

THD

❉

SFDR

❉

First nine harmonics

= 100kHz, –0.2dB

of the input frequency

FREQUENCY SPECTRUM

(4096 Point FFT; f

= 247kHz, –0.2dB)

Amplitude (dB)

–100

–80

–60

–40

–20

0 25062.5 187.5125

Frequency (kHz)

–120

SPURIOUS FREE DYNAMIC RANGE AND

TOTAL HARMONIC DISTORTION

vs INPUT FREQUENCY

10k 100k1k 1M

Input Frequency (Hz)

SFDR (dB)

THD (dB)

–

THD

❉

First nine harmonics

of the input frequency

SFDR

SIGNAL-TO-NOISE RATIO and

SIGNAL-TO-(NOISE+DISTORTION)

vs INPUT FREQUENCY

10k 100k1k 1M

Input Frequency (Hz)

SNR and SINAD (dB)

SINAD

SNR

ADS7818

FIGURE 1. Basic Operation of the ADS7818.

THEORY OF OPERATION

The ADS7818 is a high speed successive approximation

internal 2.5V bandgap reference. The architecture is based

on capacitive redistribution which inherently includes a

sample/hold function. The converter is fabricated on a 0.6µ

CMOS process. See Figure 1 for the basic operating circuit

for the ADS7818.

The ADS7818 requires an external clock to run the conver-

sion process. This clock can vary between 200kHz (12.5Hz

throughput) and 8MHz (500kHz throughput). The duty cycle

of the clock is unimportant as long as the minimum HIGH

and LOW times are at least 50ns and the clock period is at

least 125ns. The minimum clock frequency is set by the

leakage on the capacitors internal to the ADS7818.

The analog input is provided to two input pins: +IN and –IN.

When a conversion is initiated, the differential input on these

pins is sampled on the internal capacitor array. While a

conversion is in progress, both inputs are disconnected from

any internal function.

The range of the analog input is set by the voltage on the

VREF pin. With the internal 2.5V reference, the input range

is 0 to 5V. An external reference voltage can be placed on

VREF, overdriving the internal voltage. The range for the

external voltage is 2.0V to 2.55V, giving an input voltage

range of 4.0V to 5.1V.

The digital result of the conversion is provided in a serial

manner, synchronous to the CLK input. The result is pro-

vided most significant bit first and represents the result of

the conversion currently in progress—there is no pipeline

delay. By properly controlling the CONV and CLK inputs,

it is possible to obtain the digital result least significant bit

first.

ANALOG INPUT

The +IN and –IN input pins allow for a differential input

signal to be captured on the internal hold capacitor when the

converter enters the hold mode. The voltage range on the

–IN input is limited to –0.2V to 0.2V. Because of this, the

differential input can be used to reject only small signals that

are common to both inputs. Thus, the –IN input is best used

to sense a remote ground point near the source of the +IN

signal. If the source driving the +IN signal is nearby, the

–IN should be connected directly to ground.

The input current into the analog input depends on input

voltage and sample rate. Essentially, the current into the

device must charge the internal hold capacitor during the

sample period. After this capacitance has been fully charged,

there is no further input current. The source of the analog

input voltage must be able to charge the input capacitance to

a 12-bit settling level within the sample period—which can

be as little as 350ns in some operating modes. While the

converter is in the hold mode or after the sampling capacitor

has been fully charged, the input impedance of the analog

input is greater than 1GΩ.

Care must be taken regarding the input voltage on the +In

and –IN pins. To maintain the linearity of the converter, the

+In input should remain within the range of GND – 200mV

to +VCC + 200mV. The –IN input should not drop below

GND – 200mV or exceed GND + 200mV. Outside of these

ranges, the converter’s linearity may not meet specifications.

REFERENCE

The reference voltage on the VREF pin directly sets the full-

scale range of the analog input. The ADS7818 can operate

with a reference in the range of 2.0V to 2.55V, for a full-

scale range of 4.0V to 5.1V.

The voltage at the VREF pin is internally buffered and this

buffer drives the capacitor DAC portion of the converter.

This is important because the buffer greatly reduces the

dynamic load placed on the reference source. However, the

voltage at VREF will still contain some noise and glitches

from the SAR conversion process. These can be reduced by

carefully bypassing the VREF pin to ground as outlined in the

sections that follow.

INTERNAL REFERENCE

The ADS7818 contains an on-board 2.5V reference, result-

ing in a 0V to 5V input range on the analog input. The

specification table gives the various specifications for the

CLK

DATA

CONV

REF

+In

–In

GND

ADS7818 0.1µF

+5V

0.1µF 10µF

Serial Clock from

Microcontroller

or DSP

Serial Data

Convert Start

2.2µF +

0 to 5V

Analog Input

ADS7818

internal reference. This reference can be used to supply a

small amount of source current to an external load, but the

load should be static. Due to the internal 10kΩ resistor, a

dynamic load will cause variations in the reference voltage,

and will dramatically affect the conversion result. Note that

even a static load will reduce the internal reference voltage

seen at the buffer input. The amount of reduction depends on

the load and the actual value of the internal “10kΩ” resistor.

The value of this resistor can vary by ±30%.

The VREF pin should be bypassed with a 0.1µF capacitor

placed as close as possible to the ADS7818 package. In

addition, a 2.2 µF tantalum capacitor should be used in

parallel with the ceramic capacitor. Placement of this ca-

pacitor is not as critical.

EXTERNAL REFERENCE

The internal reference is connected to the VREF pin and to the

internal buffer via a 10kΩ series resistor. Thus, the reference

voltage can easily be overdriven by an external reference

voltage. The voltage range for the external voltage is 2.0V

to 2.55V, corresponding to an analog input range of 4.0V to

5.1V.

While the external reference will not source significant

current into the VREF pin, it does have to drive the series

10kΩ resistor that is terminated into the 2.5V internal

reference (the exact value of the resistor will vary up to

±30% from part to part). In addition, the VREF pin should

still be bypassed to ground with at least a 0.1 µF ceramic

capacitor (placed as close to the ADS7818 as possible). The

reference will have to be stable with this capacitive load.

Depending on the particular reference and A/D conversion

speed, additional bypass capacitance may be required, such

as the 2.2µF tantalum capacitor shown in Figure 1.

Reasons for choosing an external reference over the internal

reference vary, but there are two main reasons. One is to

achieve a given input range. For example, a 2.048V refer-

ence provides for a 0V to 4.095V input range—or 1mV per

LSB. The other is to provide greater stability over tempera-

ture. The internal reference is typically 20ppm/°C which

translates into a full-scale drift of roughly 1 output code for

every 12°C (this does not take into account other sources of

full-scale drift). If greater stability over temperature is needed,

then an external reference with lower temperature drift will

be required.

DIGITAL INTERFACE

Figure 2 shows the serial data timing and Figure 3 shows the

basic conversion timing for the ADS7818. The specific

timing numbers are listed in Table I. There are several

important items in Figure 3 which give the converter addi-

tional capabilities over typical 8-pin converters. First, the

transition from sample mode to hold mode is synchronous to

the falling edge of CONV and is not dependent on CLK.

Second, the CLK input is not required to be continuous

during the sample mode. After the conversion is complete,

the CLK may be kept LOW or HIGH.

FIGURE 2. Serial Data and Clock Timing.

SYMBOL DESCRIPTION MIN TYP MAX UNITS

tACQ Acquisition Time 350 ns

tCONV Conversion Time 1.5 µs

tCKP Clock Period 125 5000 ns

tCKL Clock LOW 50 ns

tCKH Clock HIGH 50 ns

tCKDH Clock Falling to Current Data 5 15 ns

Bit No Longer Valid

tCKDS Clock Falling to Next Data Valid 30 50 ns

tCVL CONV LOW 40 ns

tCVH CONV HIGH 40 ns

tCKCH CONV Hold after Clock Falls(1) 10 ns

tCKCS CONV Setup to Clock Falling(1) 10 ns

tCKDE Clock Falling to DATA Enabled 20 50 ns

tCKDD Clock Falling to DATA 70 100 ns

High Impedance

tCKSP Clock Falling to Sample Mode 5 ns

tCKPD Clock Falling to Power-down Mode 50 ns

tCVHD CONV Falling to Hold Mode 5 ns

(Aperture Delay)

tCVSP CONV Rising to Sample Mode 5 ns

tCVPU CONV Rising to Full Power-up 50 ns

tCVDD CONV Changing State to DATA 70 100 ns

High Impedance

tCVPD CONV Changing State to 50 ns

Power-down Mode

tDRP CONV Falling to Start of CLK 5 µs

(for hold droop < 0.1 LSB)

Note: (1) This timing is not required under some situations. See text for more information.

TABLE I. Timing Specifications (TA = –40°C to +85°C,

CLOAD = 30pF).

The asynchronous nature of CONV to CLK raises some

interesting possibilities, but also some design consider-

ations. Figure 3 shows that CONV has timing restraints in

relation to CLK (tCKCH and tCKCS). However, if these times

are violated (which could happen if CONV is completely

asynchronous to CLK), the converter will perform a conver-

sion correctly, but the exact timing of the conversion is

indeterminate. Since the setup and hold time between CONV

and CLK has been violated in this example, the start of

conversion could vary by one clock cycle. (Note that the

start of conversion can be detected by using a pull-up

resistor on DATA. When DATA drops out of high-imped-

ance and goes LOW, the conversion has started and that

clock cycle is this first of the conversion.)

In addition if CONV is completely asynchronous to CLK

and CLK is continuous, then there is possibility that CLK

will transition just prior to CONV going LOW. If this occurs

DATA

CLK

CKH

CKP

CKDH

CKDS

CKL

ADS7818

D11

(MSB)

DATA

NOTES: (1) Clock periods 14 and 15 are shown for clarity, but are not required for proper operation of the ADS7818, provided that the

minimum tACQ time is met. The CLK input may remain HIGH or LOW during this period. (2) The transition from sample mode to hold

mode occurs on the falling edge of CONV. This transition is not dependent on CLK. (3) The device remains fully powered when

operated as shown. If the sample time is longer than 3 clock periods, power consumption can be reduced by allowing the device to

enter a power down mode. See the power down timing for more information.

HOLD

CONVERSION IN PROGRESS IDLE IDLE(3)

SAMPLESAMPLE

(2)

(1)

HOLD

CLK 1 2 3 4 11 12 13 14 1514 15 1616 1

CONV

SAMPLE/HOLD

MODE

INTERNAL

CONVERSION

STATE

D10 D9 D2 D1 D0

(LSB)

tCKDE

tCVHD

tCONV

tACQ

tCKCS

tCKCH

tCKSP

tCVL

tCKDD

tCVCK

faster than the 10ns indicated by tCKCH, then there is a

chance that some digital feedthrough may be coupled onto

the hold capacitor. This could cause a small offset error for

that particular conversion.

Thus, there are two basic ways to operate the ADS7818.

CONV can be synchronous to CLK and CLK can be con-

tinuous. This would be the typical situation when interfacing

the converter to a digital signal processor. The second

method involves having CONV asynchronous to CLK and

gating the operation of CLK (a non-continuous clock). This

method would be more typical of an SPI-like interface on a

microcontroller. This method would also allow CONV to be

generated by a trigger circuit and to initiate (after some

delay) the start of CLK. These two methods are covered

under DSP Interfacing and SPI Interfacing.

POWER-DOWN TIMING

The conversion timing shown in Figure 3 does not result in

the ADS7818 going into the power-down mode. If the

conversion rate of the device is high (approaching 500kHz),

then there is very little power that can be saved by using the

power-down mode. However, since the power-down mode

incurs no conversion penalty (the very first conversion is

valid), at lower sample rates, significant power can be saved

by allowing the device to go into power-down mode be-

tween conversions.

Figure 4 shows the typical method for placing the A/D into

the power-down mode. If CONV is kept LOW during the

conversion and is LOW at the start of the 13 clock cycle,

then the device enters the power-down mode. It remains in

this mode until the rising edge of CONV. Note that CONV

must be HIGH for at least tACQ in order to sample the signal

properly as well as to power-up the internal nodes.

There are two different methods for clocking the ADS7818.

The first involves scaling the CLK input in relation to the

conversion rate. For example, an 8MHz input clock and the

timing shown in Figure 3 results in a 500kHz conversion

rate. Likewise, a 1.6MHz clock would result in a 100kHz

conversion rate. The second method involves keeping the

clock input as close to the maximum clock rate as possible

and starting conversions as needed. This timing is similar to

that shown in Figure 4. As an example, a 50kHz conversion

rate would require 160 clock periods per conversion instead

of the 16 clock periods used at 500kHz.

The main distinction between the two is the amount of time

that the ADS7818 remains in power down. In the first mode,

the converter only remains in power down for a small

number of clock periods (depending on how many clock

periods there are per each conversion). As the conversion

rate scales, the converter always spends the same percentage

of time in power down. Since less power is drawn by the

digital logic, there is a small decrease in power consump-

tion, but it is very slight. This effect can be seen in the

typical performance curve “Supply Current vs Sample Rate.”

FIGURE 3. Basic Conversion Timing.

ADS7818

D11

(MSB)

DATA

HOLD SAMPLESAMPLE HOLD

CLK

CONV

SAMPLE/HOLD

MODE

POWER MODE FULL POWER FULL POWERLOW POWER

D10 D1 D0

(LSB)

tCKPD tCVPU

tCVSP

123 1213

tACQ

CONVERSION IN PROGRESS IDLE IDLE

INTERNAL

CONVERSION

STATE

NOTES: (1) The low power mode (“power-down”) is entered when CONV remains LOW during the conversion and is still LOW at the

start of the 13th clock cycle. (2) The low power mode is exited when CONV goes HIGH. (3) When in power-down, the transition from

hold mode to sample mode is initiated by CONV going HIGH.

(1) (2)

(3)

FIGURE 4. Power-down Timing.

D11

(MSB)

DATA

CONVERSION IN PROGRESSIDLE

LOW...

IDLE

CLK

CONV

POWER MODE FULL POWER LOW POWER

D10 D1 D0

(LSB)

1 2 3 121314 2324

D1 D10 D11

(MSB)

INTERNAL

CONVERSION

STATE

CVDD

CKCS

CKCH

CVPD

CVH

HOLDSAMPLE

SAMPLE/HOLD

MODE

NOTES: (1) The serial data can be transmitted LSB first by pulling CONV LOW during the 13th clock cycle. (2) After the MSB has been

transmitted, the DATA output pin will remain LOW until CONV goes HIGH. (3) When CONV is taken LOW to initiate the LSB first transfer,

the converter enters the power-down mode.

(1) (2)

(3)

FIGURE 5. Serial Data “LSB-First” Timing.

In contrast, the second method (clocking at a fixed rate)

means that each conversion takes X clock cycles. As the

time between conversions get longer, the converter remains

in power-down an increasing percentage of time. This re-

duces total power consumption by a considerable amount.

For example, a 50kHz conversion rate results in roughly

1/10 of the power (minus the reference) that is used at a

500kHz conversion rate.

ADS7818

Table II offers a look at the two different modes of operation

and the difference in power consumption. the conversion will terminate immediately, before all 12-bits

have been decided. This can be a very useful feature when

a resolution of 12-bits is not needed. An example would be

when the converter is being used to monitor an input voltage

until some condition is met. At that time, the full resolution

of the converter would then be used. Short-cycling the

conversion can result in a faster conversion rate or lower

power dissipation.

There are several very important items shown in Figure 6.

The conversion currently in progress is terminated when

CONV is taken HIGH during the conversion and then taken

LOW prior to tCKCH before the start of the 13th clock cycle.

Note that if CONV goes LOW during the 13th clock cycle,

then the LSB first mode will be entered (Figure 5). Also,

when CONV goes LOW, the DATA output immediately

transitions to high impedance. If the output bit that is present

during that clock period is needed, CONV must not go LOW

until the bit has been properly latched into the receiving

logic.

DATA FORMAT

The ADS7818 output data is in straight binary format as

shown in Figure 7. This figure shows the ideal output code

for the given input voltage and does not include the effects

of offset, gain, or noise.

POWER WITH POWER WITH

fSAMPLE CLK = 16 • fSAMPLE CLK = 8MHz

500kHz 11mW 11mW

250kHz 10mW 7mW

100kHz 9mW 4mW

TABLE II. Power Consumption versus CLK Input.

LSB FIRST DATA TIMING

Figure 5 shows a method to transmit the digital result in a

least-significant bit (LSB) format. This mode is entered

when CONV is pulled HIGH during the conversion (before

the end of the 12th clock) and then pulled LOW during the

13th clock (when D0, the LSB, is being transmitted). The

next 11 clocks then repeat the serial data, but in an LSB first

format. The converter enters the power-down mode during

the 13th clock and resumes normal operation when CONV

goes HIGH.

SHORT-CYCLE TIMING

The conversion currently in progress can be “short-cycled”

with the technique shown in Figure 6. This term means that

FIGURE 6. Short-cycle Timing.

D11

(MSB)

DATA

CONVERSION IN PROGRESSIDLE IDLE

CLK

CONV

POWER MODE FULL POWER LOW POWER

D10 D8D9 D7

123 5467

INTERNAL

CONVERSION

STATE

CVDD

CVL

CVH

HOLDSAMPLE

SAMPLE/HOLD

MODE

NOTE: (1) The conversion currently in progress can be stopped by pulling CONV LOW during the conversion. This must occur at

least t

CKCS

prior to the start of the 13th clock cycle. The DATA output pin will tri-state and the device will enter the power-down

mode when CONV is pulled LOW.

(1)

CVPD

ADS7818

D11

(MSB)

DATA

CLK

CONV

D10 D1 D0

(LSB)

231 4 13 14 15 16 1 2 3

D11

(MSB)

ACQ

DRP

Input Voltage

(2)

(V)

Output Code

FS = Full-Scale Voltage = 2 • V

REF

1 LSB = FS/4096

4.999V

(1)

00...010

00...001

00...000

11...101

11...110

11...111

1 LSB

NOTES: (1) For external reference, value is 2 • V

REF

– 1 LSB. (2) Voltage

at converter input: +IN–(–IN).

FIGURE 7. Ideal Input Voltages and Output Codes.

DSP INTERFACING

Figure 8 shows a timing diagram that might be used with a

typical digital signal processor such as a TI DSP. For the

buffered serial port (BSP) on the TMS320C54X family,

CONV would tied to BFSX, CLK would be tied to BCLKX,

and DATA would be tied to BDR.

SPI/QSPI INTERFACING

Figure 9 shows the timing diagram for a typical serial

peripheral interface (SPI) or queued serial peripheral inter-

face (QSPI). Such interfaces are found on a number of

microcontrollers form various manufacturers. CONV would

be tied to a general purpose I/O pin (SPI) or to a PCX pin

(QSPI), CLK would be tied to the serial clock, and DATA

would be tied to the serial input data pin such as MISO

(master in slave out).

Note the time tDRP shown in Figure 9. This represents the

maximum amount of time between CONV going LOW and

the start of the conversion clock. Since CONV going LOW

places the sample and hold in the hold mode and because the

hold capacitor looses charge over time, there is a require-

ment that time tDRP be met as well as the maximum clock

period (tCKP).

LAYOUT

For optimum performance, care should be taken with the

physical layout of the ADS7818 circuitry. This is particu-

larly true if the CLK input is approaching the maximum

input rate.

The basic SAR architecture is sensitive to glitches or sudden

changes on the power supply, reference, ground connec-

tions, and digital inputs that occur just prior to latching the

output of the analog comparator. Thus, during any single

conversion for an n-bit SAR converter, there are n “win-

dows” in which large external transient voltages can easily

affect the conversion result. Such glitches might originate

from switching power supplies, nearby digital logic, and

high power devices. The degree of error in the digital output

depends on the reference voltage, layout, and the exact

timing of the external event. The error can change if the

external event changes in time with respect to the CLK

input.

FIGURE 8. Typical DSP Interface Timing.

D11

(MSB)

DATA

CLK

CONV

D10 D1 D0

(LSB)

1215 16 3 12 13 14 15 16 1 2 3 4

D11

(MSB) D10 D9

FIGURE 9. Typical SPI/QSPI Interface Timing.

ADS7818

With this in mind, power to the ADS7818 should be clean

and well bypassed. A 0.1µF ceramic bypass capacitor should

be placed as close to the device as possible. In addition, a

1µF to 10µF capacitor is recommended. If needed, an even

larger capacitor and a 5Ω or 10Ω series resistor my be used

to lowpass filter a noisy supply.

The ADS7818 draws very little current from an external

reference on average as the reference voltage is internally

buffered. However, glitches from the conversion process

appear at the VREF input and the reference source must be

able to handle this. Whether the reference is internal or

external, the VREF pin should be bypassed with a 0.1µF

capacitor. An additional larger capacitor may also be used,

if desired. If the reference voltage is external and originates

from an op-amp, make sure that it can drive the bypass

capacitor or capacitors without oscillation.

The GND pin should be connected to a clean ground point.

In many cases, this will be the “analog” ground. Avoid

connections which are too near the grounding point of a

microcontroller or digital signal processor. If needed, run a

ground trace directly from the converter to the power supply

entry point. The ideal layout will include an analog ground

plane dedicated to the converter and associated analog

circuitry.

PACKAGE OPTION ADDENDUM

www.ti.com 16-Aug-2012

Addendum-Page 1

PACKAGING INFORMATION

Orderable Device Status (1) Package Type Package

Drawing Pins Package Qty Eco Plan (2) Lead/

Ball Finish MSL Peak Temp (3) Samples

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ADS7818E/250 ACTIVE VSSOP DGK 8 250 Green (RoHS