nRF52832 (on a ublox NINA B112): Sampling via SAADC unreliable in certain conditions

RE: nRF52832 (on a ublox NINA B112): Sampling via SAADC unreliable in certain conditions

haakonsh — Wed, 06 Apr 2022 11:22:52 GMT

If the internal impedance is > 100kohm you need to increase the acquisition time according to Table 1 in the Acquisition time chapter.

Generally the SAADC will be less accurate during radio operation, the same is true if the HFXO is not running. That being said, I've never encountered samples that are ~25%, off as inn your case, due to the previously mentioned states.

One typical scenario I often encounter is when a signal source is derived from a battery that is also the main power source for the device, and the ADC signal is either a direct sample of the battery voltage or of an N/PTC resistor voltage divider circuit.
The issue is that the battery voltage drops as the battery cannot handle instanteneous power loads from the radio, flash erasing, CPU, external sensors, etc.

RE: nRF52832 (on a ublox NINA B112): Sampling via SAADC unreliable in certain conditions

s-teak — Tue, 05 Apr 2022 12:45:56 GMT

haakonshThank you for your suggestion. We can almost certainly exclude *floating ground*, currently verifying the *high internal impedance* point.

Will keep you posted.

RE: nRF52832 (on a ublox NINA B112): Sampling via SAADC unreliable in certain conditions

haakonsh — Tue, 05 Apr 2022 11:39:37 GMT

I've not experience the SAADC to be that inaccurate, there's something else going on here. Can you share schematics, or at least more information about the signal source?

My first guesses would be a floating ground or a too high internal impedance in the signal source.