SAADC noise in Analog pin and digital data of nRF52832

[Masoumeh]

Hi, 

I am using a custom board to record 2 channels of data in differential mode, convert them to digital by AIN4,5 and saadc in nRF52832, and then transmit the converted data with ESB_ptx. when I imply an input to the analog front end or even if I short inputs to the ground to measure the noise, I see noise in the output spectrum which is somehow similar to a pulse with a frequency equals to sampleng_frequency/2/Samples_in_buffer. which is about 20000/2/32 (625Hz) for me. Also, I see its harmonics too. do you have any ideas that what is it coming from? when I change the SAMPLES_IN_BUFFER numbers in the saadc example the harmonics of the noise change. At first I thought it is a software problem, but then when I checked the input of the saadc, which is my analog input, I saw the noise on the oscilloscope.

[haakonsh]

Is this measured on the DK or a custom board?

What's the magnitude of the noise? 

I suggest you enable the HFXO when sampling, and if possible, not use the RADIO while the SAADC is sampling.