SAADC on NRF52832 returns strange values intermittent

[Mike]

Hi,

I see intermittent incorrect values from the saadc. The code is described here.

The graf shows battery voltage in mV over about 4 days with one sample per minute. Note the 3 samples that are off. I'm aware that there's no calibration done in the code, but I suspect that has nothing to do with this (the periodic fluctuations are well correlated to the temperature).

Am I doing something wrong?

Thanks, Mike

[Jørgen Holmefjord]

What is VDD? Are the device doing anything else than checking the supply voltage? Have you measured battery voltage with external equipment, over the same timer period, to see that this is actually not caused by the battery?

[Mike]

Thanks for your reply Jørgen.

As described in the linked post, VDD is the supply voltage to nRF52832 from the battery:

  ch.pin_p = NRF_SAADC_INPUT_VDD;
  ch.pin_n = NRF_SAADC_INPUT_DISABLED;

The battery voltage does not dip according to an external adc that measure current and voltage to the pcb, so it's something in saadc that creates this.

I use i2c to gather data from 2 other sensors. Those sensors are disabled at the time VDD is measured (via a FET switch), and is switched on in firmware after VDD is measured.

[Jørgen Holmefjord]

Have you tried enabling oversampling, to limit the affect of noise on the readings?

[Mike]

Not yet, i want to understand the root cause first. Could this have something to do with RF rx/tx, i.e. that saadc sample at the instant RF is active? I checked VDD again, and there is no ripple on the supply rail of that magnitude during RF rx/tx (using a 100MHz scope with trigger set properly). Could it be interference /inside/ the nRF52832 chip that cause this? Note that saadc sample NRF_SAADC_INPUT_VDD which is internal.

[Jørgen Holmefjord]

It might be caused by ground bounce internally on the nRF52832, which would not be picked up by the other ADC. Could you try measuring differentially, by connecting GND to one of the AIN pins and set:

ch.pin_n = NRF_SAADC_INPUT_AINx;

Where x is the Analog input number.