SAADC on NRF52832 returns strange values intermittent

RE: SAADC on NRF52832 returns strange values intermittent

Jørgen Holmefjord — Tue, 14 Nov 2017 12:53:54 GMT

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.

RE: SAADC on NRF52832 returns strange values intermittent

Mike — Tue, 07 Nov 2017 11:05:21 GMT

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.

RE: SAADC on NRF52832 returns strange values intermittent

Jørgen Holmefjord — Mon, 06 Nov 2017 15:28:42 GMT

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

RE: SAADC on NRF52832 returns strange values intermittent

Mike — Fri, 06 Oct 2017 09:00:20 GMT

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.

RE: SAADC on NRF52832 returns strange values intermittent

Jørgen Holmefjord — Fri, 06 Oct 2017 08:50:44 GMT

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?