I am attempting to use the ADC on the XMEGA A1U Xplained Pro. It's just test code, which is supposed to make conversions on four ADC input pins every second and output the results as ASCII decimal over the UART - I am faking a datalogger for some testing. The code is working fine, but the ADC results are noisy as all get-out. I do not have an external reference (none were supplied on the Xplained Pro), so I have tried using the internal 1.1V band-gap, the VCC/1.6, and VCC/2 references. Regardless of the reference, the results are noisy - they bounce around by as much as 10 LSBs. The results are less noisy if I drop the conversion clock from 1MHz to 250kHz, but still too noisy in my thinking. The ADC is configured as signed, single-ended. I have tried setting the MUXNEG to the pad GND and internal GND, makes no difference. Impedance mode makes no difference. Checking the voltage inputs to the PCA with a DMM shows them to be rock-solid.
Has anyone out there gotten relatively noise-free ADC conversions out of an XMEGA A1U Xplained Pro? If so, could you please share your tricks? Thanks!