Tom,
I looked with a scope, you see glitches generated at 100uS... I assume this is the sampling rate of ADC... and each time the ADC samples, it generates an extra current on his input.
Since we cannot decrase the ADXRS impedance due to the 200uA constraint...the only solution is to add a small cap on R3... I did it and it reduced significantly (factor 5) the noise....
CCL :
- impedance as low as possible with constraint to 200uA=> R3=16K and R2=10K
- 0,1uF on R3
CU
MArc