How to sample the SAADC at 200Khz?

RE: How to sample the SAADC at 200Khz?

Amanda Hsieh — Fri, 08 Apr 2022 08:55:10 GMT

Hi,

Yes, if everything should go through PPI, that would be the maximum. You may get away with lower Tacq if you have large enough buffers and can guarantee that you will not be interrupted by something else, and can trigger the START task from the application. The buffer pointer can be changed and TASKS_START can be triggered as soon as the EVENTS_STARTED is generated. May need some thorough testing though.

-Amanda

RE: How to sample the SAADC at 200Khz?

zapta — Thu, 07 Apr 2022 23:47:04 GMT

Thanks Amanda, this is very useful, especially the errata.

Let's see if we go it right. What we need to do requires scan mode (alternate samples between two inputs), ppi (for proper DMA double buffering) and tAcq should be at least 10us (I presume this is the next step after 5us) which results at a sampling speed of 1s/(10us + 2us) = 83Khz or 41Khz for each of the two scanned inputs. This is less than half of the expected 200khz.

Is it so?

RE: How to sample the SAADC at 200Khz?

Amanda Hsieh — Thu, 07 Apr 2022 06:41:48 GMT

Hi,

200ksps is the maximum sample frequency supported by the SAADC, so this will only work for 1 channel. If you need to sample two channels (e.g. in SCAN mode), 100ksps is the maximum that can be achieved. If you sample 1 channel at a time and switch to another channel sometime in the future, there should be no problems, apart from the obvious that there will be a short delay in sampling when switching channel.

Maximum sample rate requires lowest acquisition time, which limits the supported source resistance (see table 2: https://infocenter.nordicsemi.com/topic/com.nordic.infocenter.nrf52832.ps.v1.1/saadc.html?cp=4_2_0_36_8#concept_qh3_spp_qr)

ACQ=3us also enables lots of erratas in nRF52832, which they need to consider:

There are more erratas, but these are specific to TACQ=3us

You can either use PPI with the nrfx_saadc driver (preferably v2 API), or using the internal SAMPLERATE register for Continuous mode sampling. Also see this case.

Not sure if your goal can be achieved, considering the second errata I linked and the need for using PPI to connect END->START to avoid buffer swapping. If you use one channel it should not be a problem.

Regards,
Amanda