For a common mode sine wave signal having a frequency

mon mode rejection would be

If the common mode sine wave has a peak to peak value of

2V the maximum voltage error at the output would be

to the frequency of the CMR signal If the frequency is dou-

to ground will help counter this problem See

Figure 6 How-

ever the use of this bypass capacitor creates a new prob-

lem in that the differential input is no longer balanced While

is little difference in CMR performance As the Lambda sen-

sor heats to the operating temperature and the sensor re-

sistance decreases the common mode signal is no longer

applied to both inputs equally

This imbalance causes

inverting input will see the full common mode signal while

the non-inverting input will see an attenuated common

mode signal

The selection of the value of the CMR bypass capacitor

needs to be balanced with the need for reasonable reduc-

tion or elimination of common mode signals with both cold

and hot sensors Since normal operation will need to in-

clude consideration of the entire impedance range of the

sensor a trade off in overall application performance may

be needed

Generally the value of the CMR bypass capacitor should be

kept as low as possible and should not be larger than the

differential input filter capacitor Values in the range of

0001 mF to 001 mF will usually provide reasonable CMR

results but optimum results will need to be determined em-

pirically as the source of common mode signals will be

unique to each application

Gain and Filter Stage

The signal gain and filter stage is designed to have a DC

gain of 453 VV with a cut-off frequency of typically

500 Hz The external 4 kX resistors on each input pin are in

series with the differential input impedance Together they

form a voltage divider circuit across the input such that the

net DC gain of the application circuit is 450 VV

TLH12372 – 17

FIGURE 7 Simplified Gain and Filter Circuit

74213 pF

16383 pF

e

453 VV

2

1E5

2

e

500 Hz

TLH12372 – 18

FIGURE 8 Equivalent Gain and Filter Circuit

6