Electronics > Projects, Designs, and Technical Stuff
Crosstalk/Ghosting/offset on ADC input
ehughes:
Also, every time the mux switches changes, you have an additional dynamic load on your driving circuit. Those switches inside the ADC are not ideal and there is other "junk" in the front end. Having low impedance drive helps mitigate what Siwastaja is mentioning as you can better drive that node to the potential you eant *faster*
You gave youself the clue when you said increasing the time between channel switches makes the problem better (as well a lowereing the series R) That just means there is more time for the node where the ADC is sampling to settle to the value you expect. The RC filter circuit is not decoupled from what is going on internal to the ADC / Mux) and real world effects are what you are observing.
Like I said, draw a circuit including the mux/ switches and it will make better sense.
*Always* drive ADC inputs with a low-z source. It even tells you in the datasheet (for other reasons but is still always good practive even with "DC" inputs).
Kleinstein:
With many SAR type ADCs the MUX and input sampling is done with the same FETs. So there usually is no extra charge pulse from changing the channel. It is mainly from changing the voltage. When the voltage changes there is some charge needed. If the capacitor is really large, like C_filter > 65000 * C_sample, the voltage drop is small and everything is OK. If the filter cap is smaller the drop would be noticeable. Here C_filter / C_sampe = 100 nF/25 pF is at around 4000 - so the change would be significant and the time to recharge the capacitor is quite long, way more than the sampling phase. So if a filter cap is used, make it really large (e.g. 2 µF), so the drop would be small, or don't use a filter directly at the ADC. A class 2 ceramic cap may not have full capacitance at 5 V, so one may need a relatively bulk film cap.
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