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I'm having a hard time understand my observations. Summary: ADC reports 2660 when I expect 2570.

Main question: This discrepancy seem large enough to have a justification. I'd like to understand what the source could be. Additionally is the difference in ADC excitation voltage and reference voltage the source?

I built a simple ADC/Sensor circuit. The ADC reading appear higher than I expect, though. Voltages:

  • Reference voltage supplied by MCP1541. Measured at 4.095V
  • Excitation voltage supplied by PSU. Measured at 5.139V
  • Analog voltage supplied by sensor with no stimulus. Measured at 2.580. Implied by excitation voltage to be 5.139/2 = 2.57V

It is a 12 bit ADC. so I expect 4096/4.096 units/V. This implies my 2.57V resting voltage should give be a value around 2570. The confusing aspect is my baseline reading of 2660.

Additional possibly useful details:

Measurement: (trying to calibrate my y-axis to 0)

enter image description here

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  • \$\begingroup\$ So you are measuring 2.58V on the X, Y, and Z ADC input pins when at rest? And you see 4.095V on the Vref pin? Are the grounds connected well, i.e. is there a voltage difference between ground pins? If you tie a fixed voltage, e.g. from a pot or resistor divider off the reference voltage, do you get a more accurate reading? Do you have any filtering on the input? If not you could try an RC filter. Maybe there is some rectification going on somewhere in the ADC when sampling. \$\endgroup\$ Commented Mar 20, 2017 at 5:41
  • \$\begingroup\$ Yes to the input and reference voltages. I will check your other comments and return with more info. \$\endgroup\$ Commented Mar 20, 2017 at 17:31

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