nRF52832 SAADC issues (wrong values)

[juulee]

Hello, I'm developing SAADC on nRF52832.

While developing, I have problems now.

I applied sine wave on ADC input.

And the result is displayed on the figure that I attached.

It has kind of 'spikes'. (I checked using oscilloscope and saw that input signal doesn't have problem)

Does anyone had same issue like me?

It's bigger than noise, and also they appear regularly.

Thanks a lot.

[Kenneth]

Hi,

Do the spikes depend on BLE activity or SAADC calibration?  

Best regards,
Kenneth

[juulee]

I'm sending those data to phone by BLE

[Kenneth]

Assuming you wrote the firmware, then you should be able to adjust the connection interval to see if that impact the interval of the issue and/or check if the code is periodically executing an SAADC calibration at the interval seen. That may narrow down the cause.

Best regards,
Kenneth

[juulee]

Thanks for advice.

I tried several conditions, and found that those errors are not evoked periodically.

However, I found that that occured at specific range of voltage: around '0.9V, 1.8V, and 2.7V' (I tested from 0.1V to 2.9V, with Sine wave).

I'm using internal reference with gain of 1/6.

ADC activates every 12 us, and I also checked for much bigger duration. 

I also changed acquisition time: from 3 us to 10 us. Input resistance of ADC is smaller than the one that indicated in datasheet.

But it always happens like this.

Best regards,

[Kenneth]

You are sure you are not doing any processing of the SAADC? E.g. that you average several samples before they are sent?

What hardware is used here? DK?

Best regards,
Kenneth

[juulee]

Yes, I did nothing.

I'm using DK.

[Kenneth]

How does VDD look like? What is the distance between the spikes in time?

[juulee]

VDD is constant 3.1V, applied by external DC supply.

Distance between the spikes in time is different for every waves. If I apply high frequency sine waves, time interval is short and for low frequency sine waves, time interval is long.

[Kenneth]

My suggestion for debugging it to not enable BLE, but output the SAADC samples over UART or RTT to check if it's related to the radio in any way. The next suggestion is to look at the usage for the SAADC, and if you may be periodically calibrating the SAADC. Also do check if there may be any ripple on VDD on an oscilloscope.

Kenneth