Zbee radio is powered via a battery pack (4 x low temp AA) and 3.3v voltage regulator (MIC5219), the reg has the prescribed design for low noise operation from the manufacturer data sheet. I have a TMP36 temp sensor on ADC_0 and a potential divider from the battery pack to ADC_1, everything works as expected when temps are over about 1C. Under 1C there is clear noise on both ADC channels. The voltage regulator and all components are rated to -20C. Radio is configured to perfom samping in cyclic sleep mode. Graph of what is going on here : https://www.dropbox.com/s/pb6ybnyg24r6vvc/Screenshot%20from%202014-12-13%2014%3A49%3A28.png?dl=0 . . . any clues ? . . .
I have seen the same issue with a completely different set of sensors but with the same radio configuration.
Analog VREF is connected to the regulator output.
The noise looks consistent and quantized which makes me suspicious that it’s internal to the radio but surely that’s not the case ?.