I have several signals with a fairly high source impedance (10k or so) from a bridge circuit that I want to measure with the ADC inputs. The voltage range of these signals exceeds that possible with the input buffers and a gain or 2 or more enabled, so the input buffers are disabled.
As I understand it, this means the ADC input will require 500nA/V, and so result in a considerable error without buffering.
So, my question is, are there any special considerations for the op-amp to drive the ADC input properly? I have come across the idea of having a small series resistor, feeding the ADC input, with a small cap to ground, the idea being that this serves as both a low pass filter (not so relevant for here) and as a reserve of charge with which to charge the ADCs internal sampling capacitor as it is rappidly switched on and off during the sampling period (the idea being it supplies short bursts of current that the buffer op-amp is unable to provide, unless you had a very fast op amp) Is this concept relevant to the ADuCM360 ADC, and how might I best drive the ADC inputs?
Thanks for any advice,
Based on your description, a external buffer in front of ADC is needed if the 500nA leakage will effect your test accuracy. I don't think the RC is necessary between external buffer and ADC, however I recommend you to add the RC filter before external buffer as a filter.