AD conversion values on the NRF52811 installed on the custom board differ from expectations.

[kkuma256]

I am using an N52811 QFN32 on a custom board.

I modified the SAADC sample as follows and ran it on this board.
(3.0V was input to AIN2.)

void saadc_init(void)
{
    ret_code_t err_code;

    nrf_drv_saadc_config_t saadc_config = NRF_DRV_SAADC_DEFAULT_CONFIG;
    saadc_config.resolution = NRF_SAADC_RESOLUTION_12BIT;

    nrf_saadc_channel_config_t channel_config =
        NRF_DRV_SAADC_DEFAULT_CHANNEL_CONFIG_SE(NRF_SAADC_INPUT_AIN2);
    channel_config.gain = NRF_SAADC_GAIN1_6;

    err_code = nrf_drv_saadc_init(&saadc_config, saadc_callback);
    APP_ERROR_CHECK(err_code);

    err_code = nrf_drv_saadc_channel_init(0, &channel_config);
    APP_ERROR_CHECK(err_code);

    err_code = nrf_drv_saadc_buffer_convert(m_buffer_pool[0], SAMPLES_IN_BUFFER);
    APP_ERROR_CHECK(err_code);

    err_code = nrf_drv_saadc_buffer_convert(m_buffer_pool[1], SAMPLES_IN_BUFFER);
    APP_ERROR_CHECK(err_code);

}

However, p_event->data.done.p_buffer[] returns a value around 1750, which is about half of what the specification would suggest.

When I run the same program on an nRF5 DK (PCA10056), it gives a value of about 3380, which I believe is correct.

When I directly measure the voltage between PIN 29 (VSS) and PIN 4 (P0.04/AIN2) on the custom board’s chip, it correctly shows 3.0V.

Is there any other configuration that might affect the SAADC?

[hmolesworth]

AIN2 is P0.04, just checking .. pin 6 on QFN48 and pin 4 on QFN32

In case there is a higher impedance (== slower signal rise time) than expected on AIN2, maybe try lengthening the sample time from 10uSecs to 40uSecs (maximum available) to see if it makes a difference:

    channel_config.gain = NRF_SAADC_GAIN1_6;
    channel_config.acq_time = NRF_SAADC_ACQTIME_40US;

"The required acquisition time depends on the source (Rsource) resistance. For high source resistance the acquisition time should be increased"

TACQ [us] Maximum source resistance [kOhm]
      3                   10
      5                   40
      10                 100
      15                 200
      20                 400
      40                 800

[kkuma256]

Thank you for your response. I don't have much hardware knowledge, so your help is appreciated.

AIN2 is P0.04, just checking .. pin 6 on QFN48 and pin 4 on QFN32

I apologize for not mentioning earlier, but the chip I am using is in a QFN32 package.

In case there is a higher impedance (== slower signal rise time) than expected on AIN2, maybe try lengthening the sample time from 10uSecs to 40uSecs (maximum available) to see if it makes a difference:

I checked, but there was no change in the values.

Additionally, for reference, the test is being conducted with a DC input.

[hmolesworth]

Can you post a snip of the schematic for the AIN2 pin and circuitry connected to the AIN2 pin? Pin 4. Maybe there is something amiss .. need to see how the DC input is connected, and where does the DC input come from?

[kkuma256]

The circuitry around the AIN2 pin is straightforward; it's directly connected. There are no other circuits, such as amplifiers, in between.

The DC input comes from a stabilized power supply.
For the 3V AD test, I used two independent stabilized power supplies: one supplied 3.3V to the chip, and the other supplied 3V to AIN2.

Additionally, I conducted another test using only one stabilized power supply, where both 3.3V to the chip and AIN2 were applied in parallel from the same source, but the results were the same.

[hmolesworth]

What value is SAMPLES_IN_BUFFER? Don't see any issues, maybe try channel 1 instead of 0:

    err_code = nrf_drv_saadc_channel_init(1, &channel_config);

and remove line setting gain (should be 1/6 by default):

    channel_config.gain = NRF_SAADC_GAIN1_6;

... and it is physical pin 4 on the QFN32, right? .. just checking ..

[kkuma256]

What value is SAMPLES_IN_BUFFER?

SAMPLES_IN_BUFFER is still set to the sample's original value of 5.

Don't see any issues, maybe try channel 1 instead of 0:

and remove line setting gain (should be 1/6 by default):

I tried it, but there was no change.

.. and it is physical pin 4 on the QFN32, right? .. just checking ..

Yes, the input is applied to the pin that is the 4th from the top or the 5th from the bottom among the 8 pins on the left side of the chip.

[kkuma256]

I resolved the issue.

Previously, pin 30 (DEC) was not connected to anything, but after adding a 1μF capacitor according to the reference circuit, I confirmed that the AD behaves as expected.