If added RC-LPF on input side, the gain will decrease. I want to be able to calculate the gain when with used input LPF.
Does TI have referenceable documentation for it?
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When adding an RC filter, there will be a voltage drop across the filter resistors as they are inline with the differential input impedance of the device. For example, if your RC filter consists of 2 10ohm resistors and a 10nF differential capacitor and you are using the AMC1302 which has a typ differential impedance of 4.9kohm, the expect voltage drop would be Vin*20/4920 and the expect voltage at the inputs would be Vin*4900/4920.
Another thing to consider is the frequency of the input signal and the cutoff frequency of the RC filter. RC filters start to attenuate (decrease) gain long before their -3dB point. I've attached a TINA simulation that can be used to illustrate this point. RC_Filter.TSC