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Arduino ADC & Decoupling capacitors.

Hello, I have a sample Analogue to Digital Converter from Microchip.  (MCP3901). Now I want to use this ADC together with an Arduino (Uno) on a breadboard  in order to connect it to my laptop.  This works all very smoothly and all by all I am fairly happy with the result up till now. But, there is a small problem.  There is a small oscillation in the data when measuring over something with a large resistance (first 3 graphs).  The oscillation is absent (or greatly diminished) over things with small resistance. (last 2 graphs). I have only a basic knowledge about electronics and cannot get rid of (or even diminish) the oscillation. My guess is that I don't get the decoupling capacitors right. These are the symptoms: Voltage measured across the solar panel of a calculator during ~2s  (while lifting my hand). Look at the oscillation in the data . (the oscillation is about 50Hz ) And zoomed in: Secondly, in the case of a 100 kOhm potentiometer in series with a 2.2 MOhm          (There is no Omega in the special characters list?) I can see that the oscillation is scaled with the measured voltage. This means that the oscillation is probably internal.  (My guess is that the source is from the power lines which are probably not decoupled very good. And the oscillation manifests itself in the internal reference voltage) So, this goes from 0 to ≃0.2 V  (because it is a 100kOhm in series with 2.2 MOhm between 5V and ground.) increased in small steps (small turns on the potentiometer) Green = without decoupling Caps. Red = with decoupling caps (0.1µC ceramic in series with 47µf electrolytic, but it's pretty much the same with all caps.) (the oscillations here are more sinusoidal than the ones with the solar cell) And finally, there is no problem when measuring over something with a low internal resistance. Look at this graph of a (bad) potentiometer or about 100 Ohm.  And look at the near absence of noise and oscillations in the data. I can't immediately see why the ADC gives such clear data in this case. But this is definitely what I want. And a detail; Has anyone some Ideas on how to kill that oscillation? (I don't mean with fourier transforms and such)  If the decoupling capacitors are the problem, I have a limited choice in themc can you please say which ones I should search for? Thank you very much.  

Topic by Sparticles    |  last reply