LM9044

SNOSBP4D – FEBRUARY 1995 – REVISED MARCH 2013

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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

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 specified common mode voltage operating range of 1V above and below ground. The circuitry also

contains provisions 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 k

Ω when operating at temperatures between

300°C, and 500°C. When a Lambda sensor is not at operating temperature, its impedance can be more than 10

Meg

Ω. 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 M

Ω typical) and out the inverting input pin to ground. This will cause

a voltage to be developed across the differential inputs:

As the Lambda sensor is heated, and the sensor impedance begins to drop, the voltage signal from the sensor

will become the dominate signal.

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 Sensor condition. The internal non-inverting input bias current (380 nA typical) flowing through the

differential input resistance (1.2 M

Ω typical) would cause the typical output voltage to be at a value defined by:

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 typically 1.5V, the comparator will switch the output pin from the amplifier output to the

on the non-inverting input will be ignored.

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