I am using Arduino Uno for measuring sensor voltages. I am using multiplexer to switch between different sensors. The readings measured in the range of 800-900 mv are accurate, but at low millivolts (at about 20 mv) they are erratic. I read repeatedly to get average of 20 values. I get values such as 1.3, 1,4, 2,0, 3.8 etc. Can any one help on why it is happening and how to avoid it.
Additional information: I am using reference voltage of 1.25 volt for better resolution. I tried having delay between two readings, though there is some improvement, still the problem persists.
I appreciate the help by experts. Thanks