High noise (~80mV) when measuring voltages using SAADC on nRF52840 (AIN0, AIN4, AIN5)

[Hassen]

Hi,
I'm using the nRF52840 to measure voltages through the SAADC. I'm observing a measurement noise of around 80mV peak-to-peak on inputs AIN0, AIN4, and AIN5.

Here are the details of my configuration:

• SAADC reference: internal Vref = 0.6V

• Gain: 1/6

• Input voltage: around 3.5V

• Mode: single-ended

• Supply:  battery 9,6V

• the nRF52840 is powered by a MAX20075DATCA DC/DC regulator, providing a stable 3.3V.

I would like to know:

• What are the recommended ways to reduce the noise on the SAADC measurements?

Thank you in advance for your help!

[Kenneth]

Hi,

What are the recommended ways to reduce the noise on the SAADC measurements?

Some items to consider:

1. Make sure the configured acquisition time for the analog input meet the maximum source resistance for the analog source. If you are not sure, try to increase the acquisition time or place a small capacitor on the input (e.g. 10nF or more).

2. Use oversampling, higher oversampling averages multiple samples in hardware and produces a higher-resolution result.

3. Try to use internal reference if you are using external reference.

4. Use a gain setting that fit well with your analog input signal, to make sure the measured signal is well within the measurement range. If the signal is very low, adjust the gain setting.

5. Look at the layout, make sure proper ground under the analog sections of the board. Avoid running analog traces along digital traces, keep analog traces short. Measure on a different board for comparison, e.g. DK.

6. Perform calibration before sampling (especially if temperature has changed).

Hope that helps,
Kenneth