How calibration SAADC Negative Value

RE: How calibration SAADC Negative Value

Empe — Mon, 20 Apr 2020 23:01:54 GMT

Hello, Håkon

i'm try to sampling IR Receive value,

minimum 130mv to max 3.05V

so i was changed the setting Gain 1/4, Ref VDD/4, 14Bit resolution.

[quote userid="2115" url="~/f/nordic-q-a/60287/how-calibration-saadc-negative-value/245563"]What value are you sampling? It is to be expected that you can get a negative number from the SAADC.[/quote]

yes, I modified it according to your advice, my saadc looks like more stable,

but as you say still negative input value is looking.

Best Regards,

Empe.

RE: How calibration SAADC Negative Value

Håkon Alseth — Mon, 20 Apr 2020 13:25:49 GMT

Hi Empe,

[quote user="Empe"][/quote]

by the way, I am using all 7 SAADC, At this time

The current method uses only CH [0] to sample one pin ADC per 1 START trigger.

It shouldn't matter wrt. the result on a specific pin, if you time-mux the sampling, or if you let the SAADC sample each input for you. SAADC will do time-multiplexing when several inputs are selected.

What value are you sampling? It is to be expected that you can get a negative number from the SAADC.

Kind regards,

Håkon

RE: How calibration SAADC Negative Value

Empe — Mon, 20 Apr 2020 00:57:48 GMT

Hello, Håkon

Thanks for help.

I need to adjust the Gain and Resolution values well.

I also checked the contents of the link you wrote down.

It helped a lot.

by the way, I am using all 7 SAADC, At this time

The current method uses only CH [0] to sample one pin ADC per 1 START trigger.

The time division method is being used.

Could this be a problem? Now i'm using "Case 1" Format

do i need to change to "Case 2" Format?

// Case 1
// Init Part
// Channel Value Change 1 to 7 
Init_SAADC
{
    ...
    ...
    NRF_SAADC->CH[0].PSELP = ((unsigned long)channel) << SAADC_CH_PSELP_PSELP_Pos;
    NRF_SAADC->CH[0].PSELN = SAADC_CH_PSELN_PSELN_NC << SAADC_CH_PSELN_PSELN_Pos;
    ...
    NRF_SAADC->TASKS_START = 1;
    while (NRF_SAADC->EVENTS_STARTED == 0);
    NRF_SAADC->EVENTS_STARTED = 0;

    // Do a SAADC sample, will put the result in the configured RAM buffer.
    NRF_SAADC->TASKS_SAMPLE = 1;
    while (NRF_SAADC->EVENTS_END == 0);
    NRF_SAADC->EVENTS_END = 0;

    return adcSample;
}


// Case 2
// Init Part
Init_SAADC
{
    ...
    ...
    NRF_SAADC->CH[channel].PSELP = ((unsigned long)channel#1) << SAADC_CH_PSELP_PSELP_Pos;
    NRF_SAADC->CH[channel].PSELN = SAADC_CH_PSELN_PSELN_NC << SAADC_CH_PSELN_PSELN_Pos;
    ...
    ...
}

Best Regards,

Empe.

RE: How calibration SAADC Negative Value

Håkon Alseth — Fri, 17 Apr 2020 14:15:23 GMT

Hi,

The effective number of bits (ENOB) is less than 12, meaning that you will get noise; thus you can get negative values when sampling a grounded pin. Please see this thread for more info:

https://devzone.nordicsemi.com/f/nordic-q-a/12287/nrf52-saadc-noise/46443#46443

PS: Be sure to request the external HFCLK when using the SAADC, then release it after you are done with the conversion. The internal 64 MHz RC oscillator (HFINT) isn't very accurate, thus you measurements might also suffer if you're running using the HFINT

Kind regards,

Håkon