SAADC reading problem?

[Ricardo Nunes]

I'm using SAADC on a NRF52832 DK to read a analog channel between 0-3.3V. I wanted to supply current with the board VDD but it's value is 2.8V and they're not event stable, so I used a LM1117 3.3V regulator between the board 5V and m module for it.

Regarding my measures, I'm configuring SAADC to use 10 bit with a 1KHz sampling frequency (10uS acquisition time, no oversampling). I wasn't getting a good value out of my module output (it's an ECG module) so I tried connecting GND, the digital output of the module I'm using and internal VCC (NRF_SAADC_INPUT_VDD) with 1/6 GAIN and using internal reference.

Also I run nrf_drv_saadc_calibrate_offset() at initialization time.

GND shows a weird wave between 0 and -5, not sure if that's normal I'm used to see it at stable 0 in every ADC I've used until now.

Internal VCC shows me similar noise problems, reading a value between 795 and 815.

Reading the module digital output and confirming with an oscilloscope that the signal is stable at both 0 and 3.3V.

Any ideas on what am I doing wrong? I'm using SDK 12.1.

UPDATE

Due to possible noise from USB I've powered it with a USB power bank and using a bluetooth to serial port bridge to transmit the ADC values. It gives me the same results

[Jørgen Holmefjord]

Hi, can you try enabling oversampling? This should improve the signal-to-noise ratio. Also, what source do you power your board with? If using USB, this can introduce a lot of noise.

[Ricardo Nunes]

I tried it but it doesn't hide the problem and even if it does it's not normal that a stable line gets reading like those

[Jørgen Holmefjord]

I'm getting similar readings to you for GND, a little better for VDD. As described here and here, you should expect some noise in the range of up to a few LSB. Notice that you should rund offset calibration regularly to correct for temperature drift. This is described in this post. What are you showing in the last picture?

[Ricardo Nunes]

Updated the post after testing with a USB power bank and the result is the same

[Jørgen Holmefjord]

I have spoken to the designer, and the first and second image (VCC and GND) shows normal readings. Your third image does not show normal readings for the ADC. What are you reading in this image?