SAADC consistnetly reads low, regardless of source

RE: SAADC consistnetly reads low, regardless of source

CurtisHx — Tue, 07 Nov 2017 13:38:41 GMT

Looks like that's the case. The SAADC is within a mV of actual with 1V applied to one of the AIN pins on the dev kit.

RE: SAADC consistnetly reads low, regardless of source

Jørgen Holmefjord — Tue, 07 Nov 2017 13:11:19 GMT

When sampling voltages towards the maximum input voltage, your reading will be dominated by gain error, which cannot be corrected by offset calibration. How does your reading look before and after offset calibration when applying 0V to the input?

RE: SAADC consistnetly reads low, regardless of source

CurtisHx — Mon, 06 Nov 2017 16:53:46 GMT

The SAADC is reading the value 20-30mV lower than what my bench DMM says it is. For example, the SAADC will read VCC at 2.80V, when the DMM says it's at 2.82V.

RE: SAADC consistnetly reads low, regardless of source

Jørgen Holmefjord — Mon, 06 Nov 2017 15:50:23 GMT

An example would be great! Do you mean that the SAADC read the value 20-30 mV, or that the sampled value is 20-30 mV lower than you expect/read using external tools?

RE: SAADC consistnetly reads low, regardless of source

CurtisHx — Mon, 06 Nov 2017 15:24:32 GMT

@Jørgen Holmefjord By "consistently reads 20-30mV low", I mean that the ADC will read 20-30mV low regardless of channel, gain, reference, acquisition time, resolution, etc. before and after calibrating the ADC. I see the same behavior on the custom nRF52832-based board and on the dev kit. I'll reformat the code into a complete example (with "main") so it's easier to follow.

RE: SAADC consistnetly reads low, regardless of source

Jørgen Holmefjord — Mon, 06 Nov 2017 15:13:19 GMT

It's hard to debug this without having your entire code. What exactly do you mean by "consistently reads 20-30mV low"? I tried your code in this example, and it seems to work as expected. Is the issue that you do not see any difference between calibrated and non-calibrated readings?

RE: SAADC consistnetly reads low, regardless of source

Jørgen Holmefjord — Mon, 06 Nov 2017 10:41:23 GMT

I'm not sure what calibration have to do with this issue. Could you post your SAADC and channel configuration?