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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).

Snapshot of noise, 1 second time window

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!

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    \$\begingroup\$ what's your sample rate? Is it 5.5Hz away from local mains frequency or one of its harmonics? \$\endgroup\$ – Neil_UK Jan 10 '18 at 5:55
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    \$\begingroup\$ This doesn't directly answer your question, but it may be useful. \$\endgroup\$ – Nick Alexeev Jan 10 '18 at 6:03
  • \$\begingroup\$ Sample rate is about 200 Hz. The pi is plugged into an AC to USB plug, and the ADC is about 1 meter from the pi. It's communicating via I2C. \$\endgroup\$ – joe_lou Jan 10 '18 at 6:12
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    \$\begingroup\$ What if you change your sampling frequency? To 210 Hz? 250Hz? 880 Hz? Do you see the same 5.5 Hz? If you touch internals of your amplifier, do you still have the same? Are you familiar with effects of frequency aliasing? \$\endgroup\$ – Ale..chenski Jan 10 '18 at 7:33
  • \$\begingroup\$ Actually, when I reduce sample rate to around 65 hz I do still see the same 5.5 Hz... \$\endgroup\$ – joe_lou Jan 10 '18 at 13:48
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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: -

enter image description here

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).

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  • \$\begingroup\$ Thanks -- how can I isolate this noise? Or is it something I just need to deal with and digitally filter out? \$\endgroup\$ – joe_lou Jan 10 '18 at 13:24
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    \$\begingroup\$ You cannot filter out aliased noise because the act of aliasing has already pushed it into the base band alongside (or even on top of) your desired signal. All you can do is put a low pass analogue filter in front of the ADC's input to reduce the effects. That sort of filter is called (not surprisingly) an anti-alias filter. So if circa 200 Hz is the problem then you want a filter that reduces the 200 Hz content significantly whilst allowing the desired baseband top frequency into the ADC's input. \$\endgroup\$ – Andy aka Jan 10 '18 at 13:36

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