Cold Sensor

Typically a Lambda sensor will have an impedance of less

than 10 kX 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 MegX Any

voltage signal that may be developed is seriously attenuat-

ed During this high impedance condition the LM9040 will

provide a default output voltage

TLH12372 – 19

Each amplifier input has a bias charge applied across the

redistribution through the switched capacitor network This

typically 2025V

While the Lambda sensor is high impedance the 447 mV

da sensor is heated and the sensor impedance begins to

drop the voltage signal from the sensor will become the

dominate signal

Output Resistance

With normal operation each output has typically 25 kX of

resistance This resistance along with an external capaci-

tor form a RC low pass filter to remove any clock noise

from the output signal An external output filter capacitor

value of 001 mF is recommended Additionally the output

resistance will provide current limiting for the output stage

Any DC loading of the output will cause an error in the mea-

sured output voltage This error will be equal to the I

across the output resistance

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 LM9040 is capable of

both of the wires to the Lambda sensor open The two

inputs handle the open circuit condition differently The

For the case of an open connection of the non-inverting

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

and the LM9040 DC gain The inverting input would still be

connected to the Lambda sensor ground so any common

mode signals would still need to be allowed for in this condi-

tion See

Figure 9

For the case of an open connection of the inverting input

the device output stage switches from the amplifier output

is not dependent on the gain stage and any signal on the

non-inverting input will have no effect on the output Each

amplifier has a comparator to monitor the voltage on the

inverting input pin When the voltage on an inverting pin

goes above typically 25V the comparator will switch the

output from the amplifier output to the voltage divider stage

To fully implement this function requires external pull-up re-

sistors for each of the inverting inputs To minimize signal

errors due to DC currents through the 4 kX resistors the

pull-up resistors need to be added in the application circuit

between the 4 kX input resistor and the connection to the

Lambda sensor ground point A typical pull-up value of

effective resistance of the output stage will be 35 kX typi-

cally See

Figure 10

TLH12372 – 20

7