I'm sampling 8 bioelectric signals with a embedded board which use an 8-channels ADC (AD7175-8). My sampling rate for every single channel is about 6250 Hz. When move the analysis in the frequency domain, what I obtain is the power spectral density as follow
instead of an (almost) flat one as I expect. It seems that there is a square-modulated interfering signal. I want to understand the nature of this signal and for this reason I'd like to know if you can tell me what kind of signal generate this particular power density spectrum.