Wawa:
I don't see how a buffer, or capacitor, or battery (without voltage divider) are any different from each other.
They all provide a solid point for the A/D to sample from. Either drawing or dumping some current.What is important is that there is no ground difference between the three above sources and analogue ground.
Maybe the tests were flawed there.A capacitor ofcourse changes ratiometric sampling into voltage sampling.
Something to watch out for if the cap is too big or Aref is too dirty.
Leo..
Many years ago somebody asked me how a deffective transistor behaves. I was quick that time: I answered that I know how a non defective transistor works, not a broken one 8)
I don't know what the problem with the pro mini ADC is, but I can assure that it exists (and I can reproduce it easily). I have identified the following sympthoms:
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When trying to read a voltage divider made with resistors bigger than 10 kOhm, not just the analog input the divider is connected to, but some of the others (some times all of them), start either giving wrong readouts and/or, apparently, block themselves (The bigger the value the most probable the problem; I like very much this english construction: "the further they are the faster they move" -the planets. I am not english native).
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Even when everything seems being working correctly, simply by disconecting one of the sensors may fool the whole ADC subsystem. The clamp diodes and the 100 nF cap keep being connected in every -of the 8- input.
I do not know how a defective ADC system works ... I just can recommend:
- Using a cap in every input.
- Do not connect sensors without disconnecting the whole system.
- Do use low impedance sensors and/or use a buffer.
Best regards.