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I have an analogue sensor with a signal of 0-12v, I'm then using a voltage divider with R1 100k and R2 33k.

I can read this using an MCP3008 connected to a raspberry pi 3 then multiply the 10bit value by 12_and_a_bit, all good.

The analogue signal has it's own display, when not connected to the pi/MCP3008 this reads a noise of <0.1% of the signal. When I connect the internal display of the probe shows the noise goes up to ~5% of the signal value, this noisy output is also read by the pi.

  • The analogue signal ground is tied to the raspberry pi/MCP3008 ground rail.
  • I don't think I have especially long wires, 200mm is the longest.
  • I tied all the other MCP3008 channels to ground (and they read 0), nothing should be floating anywhere.
  • If I plot readings the 'noise' is loosely oscillating about 3-4 seconds.

Has anyone any experience of this? Is there a way to reduce or eliminate it? Can I just add diodes to the signal and ground between the sensor and MCP3008 or is it due to reading the signal?

Thanks for reading this :)

  • Welcome to the world of analog electronics. The Pi is largely irrelevant - your problem is layout. – Milliways Oct 20 '18 at 7:13
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I would start by adding a decoupling cap on the screen board (the source of the noise) and on the sensor supply line.

Adding diodes in the way you describe can only do harm: it's unlikely to have any impact on the noise, and may introduce range and linearity errors in addition.

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