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?

[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.