Hi,
I'd like to ask about the filter to be used on a differential ADC operating on a single-ended signal.
Figure 56 of ADS8866 datasheet shows a recommended filter. This applies a capacitor across the ADC inputs and in-line resistors from the signal to the positive (non-inverting) input and from ground to the negative input.
It seems to me that this topology does not attenuate the common mode signal presented to the ADC. (I agree with the -3dB formula shown for the differential signal.) Above the -3dB frequency, the common mode signal will exceed the desired differential signal, and will need to be rejected by the ADC.
The ADS8866 datasheet doesn't show, (unless I've over-looked it), CMRR except at DC. The common mode signal may be a problem - not sure.
Would it be better to connect the negative input directly to signal ground? Or does this increase distortion due to un-matched input impedances?
Or should the negative ADC input be driven by ground through a parallel R-C network matching the positive input's impedance?
Note that the datasheet's next diagram, Figure 57, does show the negative input directly grounded, so I guess it's not fatal!
Any wisdom on this appreciated.
Thanks and regards,
Mark