ADC channel reading values when connected to ground

[Harini Krishna]

Hi,

Hope you are doing good.

I run the SAADC signal to from the sdk and observed that  when the adc input is connected to ground, the adc reads few random negative value( reference image attached below). is this a common behavior when the analog input is connected to ground?How do i correct this so that i do not make any measurement errors?

Thank-you

Warm Regards

Harini Krisha

[Jared]

Hi,

This is due to noise and offset error. Do a calibration and see if it helps. 

regards

Jared 

[STE]

Dear Jared,

there are some particular procedure to doing that or simply collect some value, get the mean value and then use it as offset?

Thanks and best regards.

F.

[Jared]

Hi,

Call nrf_drv_saadc_calibrate_offset().

See the low power saadc example for reference.

regards

Jared 

[STE]

Thanks Jared,

actually in my application the saadc_callback() is called every 5 ms from an interrupt.

Actually my code for this part looks as below:

    saadc_sampling_event_init();
    saadc_init();
    saadc_sampling_event_enable();

it is enough insert the calibration code before call the saadc_sampling_event_enable() function and then manage the NRF_DRV_SAADC_EVT_CALIBRATEDONE event inside the saadc_callback() function or I can leave all triggered every 5 ms and then insert just some flag to start with the calibration and then when it have finished switch the flag in order to execute the default routine?

Thanks and best regards.

F.

[Jared]

Hi,

How often you should call the calibration routine depends on how often the temperature will change. If you don't expect any temperature change then it's enough to do it once at the start.

I've updated my previous answer with the correct URL. See the example for a reference on how to implement it. 

regards

Jared

[STE]

Hi Jared, thanks, I've succesfully implemented the calibration steps thanks, of course I still get negative value when the input is directly wired to the 0V and as I'm reading through the forum I'm clipping every negative value to 0.

I'm just curious how the offset calibration work, I think is done completely inside the ADC module by disconnecting the ADC input from the external pin of the chip, I'm right?

Thanks and best regards.

F.

[Jared]

Hi,

We do not share information regarding the details of the internal calibration routine. What you might be observing is ground bounce. Have you tried using differential mode, and connecting the second pin directly to the GND of the source that you're measuring? 

I think it's best that you create a new case for discussing your issues. 

regards

Jared 

[STE]

Dear Jared,

I'm not asking about the internal code used to perform the calibration, just know if this offset calibration is done internally the chip by electronically disconnect the input pin of the ADC, so is not requested any external action by me, or if to perform the calibration I've to ground externally the ADC pin. Of course, in this last scenario, this is not practicable in a real usage because the input is connected to the sensor where the signal have to be read and doing such things make the circuit more complicated, hence is mainly for this reason I've asked in what way is done the calibration, then kindly note not need to know detail about the code used to perform this task, but essentially from an hardware point of view, sorry if I've not specifiede better in my previous request.

Thanks and best regards.

F.

[Jared]

Hi,

STE said:

just know if this offset calibration is done internally the chip by electronically disconnect the input pin of the ADC, so is not requested any external action by me, or if to perform the calibration I've to ground externally the ADC pin.

No external action is needed for the calibration routine to complete. Which means that you don't have to modify the circuit externally. 

best regards

Jared

[STE]

Thanks Jared, this is what I wanted to know.

Best regards.

F.