I am reading a thermistor voltage divider from Vdd using the SAADC with a reference voltage of Vdd/4 and gain of 1/4. I thought that this would track Vdd so I wouldn't need an external reference voltage. But the results vary slightly with the small ripples on Vdd. The reading is sensitive, with a 14 bit conversion, so it doesn't take much to make it jump around a bit (a degree C or two). I think part of the problem is that there is a filter cap on the divider, so the divider voltage might lag behind changes in Vdd. But it's too late to change that cap on this version of the board.
Is there an easy way to schedule SAADC reads when Vdd is more stable? I'm guessing some of the Vdd variation is due to the radio usage and the switching regulator. Is there a way I could make sure the SAADC reads are when the radio is off, for example?
