LM9044VX/NOPB - NATIONAL SEMICONDUCTOR

LM9044

LM9044 Lambda Sensor Interface Amplifier

Literature Number: SNOSBP4C

LM9044

March 3, 2009

Lambda Sensor Interface Amplifier

General Description

The LM9044 is a precision differential amplifier specifically

designed for operation in the automotive environment. Gain

accuracy is guaranteed over the entire automotive tempera-

ture range (−40°C to +125°C) and is factory trimmed after

package assembly. The input circuitry has been specifically

designed to reject common-mode signals as much as 3V be-

low ground without the need for a negative voltage supply.

This facilitates the use of sensors which are grounded at the

engine block while the LM9044 itself is grounded at chassis

potential. An external capacitor on the RF pin sets the maxi-

mum operating frequency of the amplifier, thereby filtering

high frequency transients. Both inputs are protected against

accidental shorting to the battery and against load dump tran-

sients. The input impedance is typically 1.2 MΩ.

The output op amp is capable of driving capacitive loads and

is fully protected. Also, internal circuitry has been provided to

detect open circuit conditions on either or both inputs and

force the output to a “home” position (a ratio of the external

reference voltage).

Features

■Normal circuit operation guaranteed with inputs up to 3V

below ground on a single supply.

■Gain factory trimmed and guaranteed over temperature

(±3% of full-scale from −40°C to +125°C)

■Low power consumption (typically 1 mA)

■Fully protected inputs

■Input open circuit detection

■Operation guaranteed over the entire automotive

temperature range (−40°C to +125°C)

Typical Application

674415

LM9044 Lambda Sensor Interface Amplifier

Connection Diagram

Plastic Chip Carrier Package

674406

*Pins 1, 3, 4, 6, 8, 9, 10, 11, 13, 14, 16, 18, 19 are trim pins and should be left floating.

Top View

Order Number LM9044V

See NS Package Number V20A

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LM9044

Absolute Maximum Ratings (Note 1)

If Military/Aerospace specified devices are required,

please contact the National Semiconductor Sales Office/

Distributors for availability and specifications.

VCC Supply Voltage (RVCC = 15 kΩ) ±60V

VREF Supply Voltage −0.3V to +6V

DC Input Voltage (Either input)(Note 3) −3V to +16V

Input Transients (Note 2) ±60V

Power Dissipation see (Note 8) 1350 mW

Output Short Circuit Duration Indefinite

Operating Temperature Range −40°C to +125°C

Storage Temperature Range −65°C to +150°C

Soldering Information

Plastic Chip Carrier Package

Vapor Phase (60 seconds) 215°C

Infrared (15 seconds) 220°C

See AN-450 “Surface Mounting Methods and Their Effect on

Product Reliability” for other methods of soldering surface

devices.

Electrical Characteristics VCC = 12V, VREF = 5V, −40°C ≤ TA ≤ 125°C unless otherwise noted

Parameter Conditions (Note 4) (Note 5) Units

Min Typ Max Min Typ Max

Differential Voltage Gain VDIFF = 0.5, −1V ≤ VCM ≤ +1V 4.41 4.50 4.59 - - - V/V

VDIFF = 0.5, −3V ≤ VCM ≤ +1V - - - 4.36 4.50 4.64 V/V

Gain Error (Note 7) 0 ≤ VDIFF ≤ 1V, −1V ≤ VCM ≤ +1V −2 0 2 - - - %/FS

0 ≤ VDIFF ≤ 1V, −3 ≤ VCM ≤ +1V - - - −3 0 3 %/FS

Differential Input Resistance 0 ≤ VDIFF ≤ 1V, −1V ≤ VCM ≤ +1V 0.95 1.20 3.00 - - - MΩ

0 ≤ VDIFF ≤ 1V, −3 ≤ VCM ≤ +1V - - - 0.70 1.20 4.00 MΩ

Non-Inverting Input Bias Current 0 ≤ VDIFF ≤ 1V, −1 ≤ VCM ≤ +1V - ±0.38 ±0.65 - - - µA

0 ≤ VDIFF ≤ 1V, −3 ≤ VCM ≤ +1V - - - - ±0.38 ±1.5 µA

Inverting Input Bias Current 0 ≤ VDIFF ≤ 1V, −1 ≤ VCM ≤ +1V −25 −65 −100 - - - µA

0 ≤ VDIFF ≤ 1V, −3 ≤ VCM ≤ +1V - - - - −45 −150 µA

VCC Supply Current VCC = 12V, RVCC = 15k - 300 500 - - - µA

VREF Supply Current 4.75V ≤ VREF ≤ 5.5V - 0.5 1.0 - - - mA

Common-Mode Voltage Range

(Note 6) −1 - 1 −3 - 1 V

DC Common-Mode Rejection Ratio Input Referred

−1V ≤ VCM ≤ +1V, VDIFF = 0.5V 50 60 - - - - dB

Open Circuit Output Voltage

One or Both Inputs Open

−1V ≤ VCM ≤ +1V 0.371 0.397 0.423 - - - xVREF

−3V ≤ VCM ≤ +1V - - - 0.365 0.397 0.439 xVREF

Short Circuit Output Current Output Grounded 1.0 2.7 5.0 - - - mA

VCC Power Supply Rejection Ratio VCC = 12V, RVCC = 15k

VDIFF = 0.5V 50 65 - - - - dB

VREF Power Supply Rejection Ratio VREF = 5 VDC

VDIFF = 0.5V 60 74 - - - - dB

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

Note 2: This test is performed with a 1000Ω source impedance.

Note 3: With a 100Ω series resistor on each input pin.

Note 4: These parameters are guaranteed and 100% production tested.

Note 5: These parameters will be guaranteed but not 100% production tested.

Note 6: The LM9044 has been designed to common-mode to −3V, but production testing is only performed at ±1V.

Note 7: Gain error is given as a percent of full-scale. Full-scale is defined as 1V at the input and 4.5V at the output.

Note 8: For operation in ambient temperatures above 25°C the device must be derated based on a maximum junction temperature of 150°C and a thermal

resistance of 93°C/W junction to ambient.

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LM9044

Typical Performance Characteristics

Non-Inverting Input Bias

Current vs Temperature

674402

Inverting Input Bias

Current vs Temperature

674407

VREF Supply Current vs

Temperature

674408

VCC Supply Current vs

Temperature

674409

Short Circuit Output

Current vs Temperature

674410

Differential Gain vs

Temperature

674403

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LM9044

Voltage Gain vs Frequency

674411

CMRR vs Frequency

674412

VREF Power Supply

Rejection

674413

VCC Power Supply Rejection

674404

Test Circuit

674405

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LM9044

Block Diagram

674414

Application Information

CIRCUIT DESCRIPTION

The LM9044 is a single channel device intended to act as a

linear interface between a zirconium dioxide oxygen sensor

and an A-to-D convertor. The LM9044 is fabricated in Bipolar

technology and requires two supplies: a nominal 12V auto-

motive supply (i.e. VBATTERY), and a well regulated 5V supply.

The IC consists of a single channel differential input amplifier

with a nominal DC gain of 4.5 V/V. The differential inputs have

a guaranteed common mode voltage operating range of 1V

above and below ground. The circuitry also contains provi-

sions for default output voltage in the cases of cold sensor

and open sensor wiring. Additional support circuitry includes

one pin for an optional user programmed low pass filter.

COLD SENSOR

Typically, a Lambda sensor will have an impedance of less

than 10 kΩ when operating at temperatures between 300°C,

and 500°C. When a Lambda sensor is not at operating tem-

perature, its impedance can be more than 10 MegΩ. Any

voltage signal that may be developed is seriously attenuated.

During this high impedance condition the LM9044 will provide

a default output voltage.

While the Lambda sensor is high impedance the internal non-

inverting input bias current (380 nA typical) will flow through

the differential input resistance (1.2 MΩ typical) and out the

inverting input pin to ground. This will cause a voltage to be

developed across the differential inputs:

VIN(DIFF) = 380 nA x 1.2 MΩ

VIN(DIFF) = 456 mV

The 456 mV across differential input resistance will be the

dominant input signal, and the typical VOUT will be:

VOUT = VIN(DIFF) x 4.50

VOUT = 456 mV x 4.50

VOUT = 2.0V

As the Lambda sensor is heated, and the sensor impedance

begins to drop, the voltage signal from the sensor will become

the dominate signal.

The non-inverting input bias current is scaled to the VREF volt-

age. As the VREF voltage increases, or decreases, this bias

current will change proportionally.

OPEN INPUT PINS DEFAULTS

In any remote sensor application it is desirable to be able to

deal with the possibility of open connections between the

sensor and the control module. The LM9044 is capable of

providing a default output voltage should either, or both, of the

wires to the Lambda sensor open. The two inputs handle the

open circuit condition differently.

For the case of an open connection at the non-inverting input,

the device would react exactly the same as for the Cold Sen-

sor condition. The internal non-inverting input bias current

(380 nA typical) flowing through the differential input resis-

tance (1.2 MΩ typical) would cause the typical output voltage

to be at a value defined by:

VOUT = ((380 nA x 1.2MΩ) x 4.50 )

VOUT = 2.0V

The inverting input would still be connected to the Lambda

sensor ground, so common mode signals would still need to

be considered in this condition.

For the case of an open connection of the inverting input, the

device output stage switches from the amplifier output to a

resistive voltage divider. The LM9044 has a comparator to

monitor the voltage on the inverting input pin, and a 65 μA

(typical) current source that will force the pin high if the pin is

open. When the voltage on the inverting pin goes above typ-

ically 1.5V, the comparator will switch the output pin from the

amplifier output to the resistive voltage divider stage. In this

case, the default VOUT is not dependent on the gain stage,

and any signal on the non-inverting input will be ignored. In

this condition VOUT is:

VOUT = VREF x ((14k + 4k) / (26.5k + 14k + 4k))

VOUT = VREF x 0.4045

When VREF is at 5.0V, VOUT is defined as:

VOUT = 5.0V x 0.4045

VOUT = 2.0V

In the cases where both the inverting and non-inverting pins

are open, the open inverting pin condition (i.e.: a voltage di-

vider across the output) will be the dominant condition.

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LM9044

Any common mode voltage transient on the inverting input pin

which goes above the comparator threshold will immediately

cause the output to switch to the resistive voltage divider

mode. The output will return to normal operation when the

voltage on the inverting input falls below the 1.5V threshold.

OUTPUT RESISTANCE

Under normal operating conditions the output pin resistance

is typically 200Ω.

If the LM9044 is operating in a default output mode due an

open connection on the inverting input, the output resistance

will typically appear to be close to 11 kΩ.

An external output filter capacitor value of no more than

0.01 μF is generally recommended. Since the output pin volt-

age drive is basically a simple NPN emitter follower, the

output pin pull-down is done by the internal feedback resistor

string. With larger value capacitors on the output pin the effect

will be somewhat similar to a voltage peak detector where the

output capacitor is charged through the 200Ω resistor, and

discharged back through the 200Ω resistor and the 18 kΩ

feedback resistor string to ground.

The output resistance provides current limiting for the output

stage should it become shorted to Ground. Any DC loading

of the output will cause an error in the output voltage.

SUPPLY BYPASSING

For best performance the LM9044 requires a VREF supply

which is stable and noise free. The same 5V reference supply

used for the A/D converter is the recommended LM9044

VREF supply.

The LM9044 VCC pin has an internal zener shunt voltage reg-

ulator, typically 7.5V, and requires a series resistor to limit the

current. The VCC pin should be bypassed with a minimum 0.01

μF capacitor to the Ground pin, and should be located as

close to the device as possible. Some applications may re-

quire an additional bypass capacitance if the system voltage

is unusually noisy.

SETTING THE BANDWIDTH

The LM9044 bandwidth is limited by an external capacitor

(CF) on the RF pin.

This pin has an internal 175 kΩ resistor. The external capac-

itor and the internal resistor form a simple RC low-pass filter

with a corner frequency (fC) defined as:

fC = 1/ (2 x π x 175 kΩ x CF)

With a CF capacitor value of 0.001 μF, the corner frequency

is:

fC = 1/ (2 x π x 175 kΩ x 0.001 μF)

fC = 909 Hz

INPUT FILTERING

Filtering at the differential inputs is strongly recommended.

Both the differential voltage signal and the common mode

voltage signal should have low pass filters.

Input filtering is accomplished with series resistors on the in-

put pins, and appropriate bypass capacitors. Typical input pin

series resistance values are in the 100Ω to 1kΩ range. Series

resistance values larger than 1kΩ will generate offset volt-

ages that affect the accuracy of the signal voltage seen at the

differential input pins.

Simplified Internal Schematic

674401

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LM9044

Physical Dimensions inches (millimeters) unless otherwise noted

Plastic Chip Carrier Package

Order Number LM9044V

NS Package Number V20A

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LM9044

Notes

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LM9044

Notes

LM9044 Lambda Sensor Interface Amplifier

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