The differential input impedance is a function of the value of

the input capacitor array and the sampling frequency The

similar to the Lambda Sensor output voltage at the stoichio-

The resulting bias voltage across the Differential Input is

defined as

5

In effect the result is the same as forcing a bias current

through the Differential Input impedance

The bias current is defined as

The Differential Input impedance is defined as

1

This bias voltage will be developed across the Differential

input has a current path to ground See

Figure 5 During

fect on accuracy

TLH12372 – 15

FIGURE 5 Equivalent Input Bias Circuit

Differential Input Filtering

Since each input is sampled independently an anti-aliasing

filter is required at the amplifier inputs to ensure that the

input signal does not exceed the Nyquist frequency

This external low-pass filter is implemented by adding a ca-

This forms an RC network across the differential inputs in

conjunction with the required external 4 kX resistors and

lected should be small enough to have minimal effect on

gain accuracy in the application yet large enough to filter

typically 500 Hz the use of a 001 mF capacitor will general-

A larger value capacitor can be used if needed but a value

larger than typically 002 mF will begin to dominate the cut-

off frequency of the application This capacitor must be a

low leakage and low ESR type so that circuit performance is

not degraded

TLH12372 – 16

FIGURE 6 Differential and Common Mode Filtering

Common Mode Filtering

The differential input sampling of the LM9040 actually re-

duces the effects of common mode input noise at low fre-

quencies The time interval between the sampling of the

inverting input and the non-inverting input is one half of a

clock period A change in the common mode voltage during

this short time interval can cause an error in the charge

voltage For a sine-wave common mode voltage the mini-

mum common mode rejection is

5