SAADC readings are frequency-dependent?

RE: SAADC readings are frequency-dependent?

haakonsh — Fri, 25 Sep 2020 09:14:28 GMT

If you set SAMPLES_IN_BUFFER to 1 then you will have sampling errors when the SoftDevice is running. A SAMPLES_IN_BUFFER of 4 is still critically low. At high buffer sizes the likelihood of sampling error will be reduced, but not eliminated. The v2 driver further reduces this likelihood.

See modules\nrfx\drivers\src\nrfx_saadc.c and modules\nrfx\drivers\include\nrfx_saadc_v2.h

RE: SAADC readings are frequency-dependent?

Michael01101 — Fri, 25 Sep 2020 08:30:12 GMT

Hello,

thank you for your reply.

With your advice to take a look to the SAMPLES_IN_BUFFER I was able to figure out the problem.

SAMPLES_IN_BUFFER was set to 4, which is okey if I use four channels.

I set SAMPLES_IN_BUFFER to 1 (At the moment I am using only one channel). I receive correct readings from the saadc now.

[quote userid="13562" url="~/f/nordic-q-a/66332/saadc-readings-are-frequency-dependent/271434"]I also suggest you try out the SAADC v2 driver in nRF5 SDK17,[/quote]

Unfortunaly, I can´t find anything within this link.

Finally, thank you very much for your support.

RE: SAADC readings are frequency-dependent?

haakonsh — Fri, 25 Sep 2020 06:59:00 GMT

The SoftDevice is most likely blocking/delaying your buffer handling, but you probably also have an issue with your signal source.

I suggest you start by increasing the buffer size to a proper size, there's no point in creating multiple arrays from a single buffer if they're going to contain the same samples as if you just used one big one.

I also suggest you try out the SAADC v2 driver in nRF5 SDK17, as it has a better buffer handling than the standard one.

The SAADC is accurate up to 200ksps, 1ksps is not a problem.

RE: SAADC readings are frequency-dependent?

Michael01101 — Thu, 24 Sep 2020 14:55:27 GMT

Hello,

thank you very much for your quick response.

[quote userid="13562" url="~/f/nordic-q-a/66332/saadc-readings-are-frequency-dependent/271357"]Can you share raw samples of the two signals?[/quote]

50Hz signal (1 periode): [20 values] 42,39,38,36,35,34,34,35,37,39,41,43,45,47,48,48,48,47,46,43

5Hz signal (1 periode): [200 values] from 23 to 60

-> the raw samples, which the SAADC outputs, are equal to the pictures.

[quote userid="13562" url="~/f/nordic-q-a/66332/saadc-readings-are-frequency-dependent/271357"]What is your buffer size?[/quote]

I set SAADC_SAMPLES_IN_BUFFER to 4. But I do not use this buffer, instead I created my own array for collecting the SAADC readings for further processing. Every time the SAADC callback function gets called, the SAADC reading gets written into this array:

#define SAADC_SAMPLES_IN_BUFFER         4

static nrf_saadc_value_t       m_buffer_pool[2][SAADC_SAMPLES_IN_BUFFER];

void saadc_callback(nrf_drv_saadc_evt_t const * p_event)
{
    if (p_event->type == NRF_DRV_SAADC_EVT_DONE)
    {
        ...
        ret_code_t err_code;
        err_code = nrf_drv_saadc_buffer_convert(p_event->data.done.p_buffer, SAADC_SAMPLES_IN_BUFFER);
        APP_ERROR_CHECK(err_code);
        ...
        if (counter <= 40){
            ch_values1[counter-1] = p_event->data.done.p_buffer[0];
        }
        if (counter == 40){     
            counter = 0;
            ble_nus_data_send(...,ch_values1,...)
        }
        counter++;
    }
}

I read out the raw samples, which i mentioned earlier, from this ch_values1 array.

[quote userid="13562" url="~/f/nordic-q-a/66332/saadc-readings-are-frequency-dependent/271357"]Are you running the SoftDevice as well?[/quote]

Yes, s132. The SAADC readings were send to the central via NUS every 40ms.

In conclusion, it looks like that I am receiving wrong values from the SAADC.

1. Do you have an idea what causes this issue?

2. Do you have any suggestions how to solve this problem?

Thank you so much in advance.

RE: SAADC readings are frequency-dependent?

haakonsh — Thu, 24 Sep 2020 13:26:39 GMT

Can you share raw samples of the two signals? We need to know whether the signals are sampled correctly or not.

What is your buffer size?

Are you running the SoftDevice as well?