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I'm building embedded system based on a Raspberry Pi with an MCP3008 for analog inputs. I have few questions regarding the PCB board design.

  1. What should I do with unused analog inputs on my MCP3008?
  2. What other countermeasures I can use for minimizing the analog noise jeopardizing my ADC measurements? I don't want to use analog ground because that requires another supply circuit for ADC board and I want to power up the MCP3008 directly from my Raspberry Pi.
  3. Any other advice?

For now I'm working with a prototype on a breadboard. The wires are long and unused ADC inputs are not connected to a ground. I can see quite a large amount of noise from time to time on my analog readings and I want to come up with good enough countermeasures in my final project in order to minimize noise observed on my prototype.

Final Project Results: http://maqister.blogspot.com/2016/08/my-homemade-raspberry-pi-expansions.html

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Normally I would say tie the unused inputs to pin 14 (analog ground). But you say you can't use that. Why not?

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  • \$\begingroup\$ Because Raspberry Pi doesn't have one and in order to create one I would have to create supply circuit for the MCP3008 with e.g. 7805 voltage regulator, some capacitors and so forth. I prefer to power up my MCP3008 from Raspberry Pi - whole system is more compact, just one power supply wire for Raspberry Pi and that's it. I know that this have disadvantage of digital noise in ADC ground but that's the way I took. \$\endgroup\$ – Krystian Sakowski Sep 1 '15 at 23:21
  • \$\begingroup\$ What are you connecting pin 14 to? \$\endgroup\$ – Kevin White Sep 1 '15 at 23:22
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    \$\begingroup\$ Just connect the unused inputs to pin14 \$\endgroup\$ – Kevin White Sep 1 '15 at 23:28
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    \$\begingroup\$ @KrystianSakowski just for your reference you should always tie your inputs high or low (some chipsets have an internal resistor to do this for you) if you don't and let the pins "float" random EMF will flip the input state and cause intermittent grief. The only other advice I have would be to include a large ground plane. however chances are that most of the noise you are seeing is coming from all those lovely hookup wire antenna and the breadboard capacitance. \$\endgroup\$ – BenG Sep 1 '15 at 23:51
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    \$\begingroup\$ analog.com/media/en/rarely_asked_questions/… \$\endgroup\$ – Houston Fortney Sep 2 '15 at 0:37

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