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5 years, 5 months ago
I recently bought a ADS1115 ADC to use with my raspberry pi. It's getting signal from an amplifier, and I've been seeing this strange 5.5 Hz sinusoid (as noise).
As seen in the picture above, its about 5.5 Hz, roughly sinusoidal. A few observations:
Noise (5.5 Hz sinusoid) is present in my signal when ADC is connected to amplifier
When ADC is disconnected and input floating (with wire), noise is present (still 5.5 Hz sinusoid, less in amplitude).
When I ground the ADC input, I don't see the waveform
Any idea on what could be causing this? It's so low frequency and I'm totally stumped... I'm trying to figure out the source and then how to get rid of it... would appreciate any ideas!
Jan 10, 2018 at 5:39
Aliasing is a distinct possibility.
If your local AC mains frequency is (say) 51.375 Hz and there are load currents in your house wiring that are harmonically rich, the 4th harmonic would be at a frequency of 205.5 Hz. If this harmonic infiltrates your signal wiring prior to the ADC then you will get 5.5 Hz content in your digital values: -
In the picture above the red waveform can be regarded as picked-up interference at 205.5 Hz. The black dots represent sample points at a rate slightly slower (200 Hz) and the blue line represents the aliased signal in your digital values (5.5 Hz).
Jan 10, 2018 at 10:54
Andy aka Andy aka
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