Inconsistent ADC values - ADC conversion taking a long time

RE: Inconsistent ADC values - ADC conversion taking a long time

Kenneth — Thu, 30 Sep 2021 13:07:37 GMT

If you are only measuring remaining battery capacity, I don't expect a few % errors is any problem, but you can find the electrical parameters here:
https://infocenter.nordicsemi.com/topic/ps_nrf52840/saadc.html#unique_699150403

Kenneth

RE: Inconsistent ADC values - ADC conversion taking a long time

mrsunny — Wed, 29 Sep 2021 12:35:19 GMT

Hi Kenneth,

I believe I corrected my confusion on configuring ADC pins and assigning them to channels. However, I had to search online to find the correct gain factor, and now I read that there are ADC offsets and other nuances that I need to add/subtract to get the actual voltage reading. Can you point me to any docs or datasheets that outline these computational parameters, and if changing the ADC configurations such as NRF_SAADC_GAIN1_6, or NRF_SAADC_ACQTIME_10US modify these values?

To confirm I am doing this correctly, here's the minimum set of code to configure and read an ADC pin (in blocking mode)

// this is where the physical pin is defined. Say AINO on the board
nrf_saadc_channel_config_t channel_config0 = NRF_DRV_SAADC_DEFAULT_CHANNEL_CONFIG_SE(NRF_SAADC_INPUT_AIN0);

// in blocking mode, this callback isn't used but needs to be initialized.
// callback is not written here for brevity
err_code = nrf_drv_saadc_init(NULL, saadc_callback);
APP_ERROR_CHECK(err_code);

// this is where that pin is assigned to a ADC channel. Channels go from 0-7
ret_code_t err_code;
err_code = nrf_drv_saadc_channel_init(0, &channel_config0);
APP_ERROR_CHECK(err_code);

// this is where the ADC value is read.
// NOTE - using the same channel defined above, which is 0
nrf_saadc_value_t adc_result;
err_code = nrf_drv_saadc_sample_convert(0, &adc_result);

// math to get the equivalent voltage (mV) reading
// NOTE - gain is 3.6 for default settings and bit resolution defined in sdk_config is 12 bits.
float val = ((adc_result * 3.6 * 1000) / 4096) ;

Thank you, and hope this helps others involved.

George

RE: Inconsistent ADC values - ADC conversion taking a long time

Kenneth — Tue, 28 Sep 2021 13:50:13 GMT

I think you are mixing adc channels with analog pins, there is no dependency between the two. So if you need to sample one analog pin (e.g. AnalogInput7), then I suggest to configure adc channel 0 to sample whatever analog input you want to sample (e.g. AnalogInput7 = value 8). Also see:
https://infocenter.nordicsemi.com/topic/com.nordic.infocenter.nrf52832.ps.v1.1/saadc.html#register.CH-0.PSELP

Kenneth

RE: Inconsistent ADC values - ADC conversion taking a long time

mrsunny — Tue, 28 Sep 2021 12:24:29 GMT

Hi Kenneth,

Did some more testing and now I am a confused on how these ADC pins work. I used an ADC on an arduino and found that the resistor and cap values should be okay. I'm wondering if I configured my pins correctly, especially the pin numbering as I've read that AIN7 (pin 7, or P0.31) is NOT NRF_SAADC_INPUT_AIN7.

I am using pin 0.31, AIN7 as the voltage reading on the battery voltage divider. Is it appropriate to assign this pin as 7 and run configuration:

nrf_saadc_channel_config_t channel_config7 = NRF_DRV_SAADC_DEFAULT_CHANNEL_CONFIG_SE(7);
err_code = nrf_drv_saadc_channel_init(7, &channel_config7);
APP_ERROR_CHECK(err_code);

Then I simply read its value in blocking mode

ret_code_t err_code;
nrf_saadc_value_t adc_result;

// convert ADC value
err_code = nrf_drv_saadc_sample_convert(BATTERY_PIN, &adc_result);
APP_ERROR_CHECK(err_code);

// print
NRF_LOG_INFO("%d", adc_result);

Yet, these are giving me readings of 321 out of a 12 bit resolution (configured in sdk_config.h - #define SAADC_CONFIG_RESOLUTION 2), which means my battery is dead or not hooked up.

Any insights?

Best,

George

RE: Inconsistent ADC values - ADC conversion taking a long time

Kenneth — Mon, 27 Sep 2021 07:42:14 GMT

The 10nF should do the job here, so I don't see how this shouldn't work. But for test please try with smaller resistors just to confirm this is the issue at hand.

Kenneth

RE: Inconsistent ADC values - ADC conversion taking a long time

mrsunny — Thu, 23 Sep 2021 23:34:53 GMT

That could be right. Here's the voltage divider I am using to measure the battery level.

It is a 4MOhm by 10MOhm voltage divider with a 10nF cap. I remember I could do this with my sampling frequency from this post.

From the post

However, if you see the notes it should have been 2 100KOhm resistors. Anyways, I'm surprised it would be this slow to charge. I'm assuming a fix would be to replace the resistors for something smaller. Same for the capacitor (say 1 nF)?

Thanks,

George

RE: Inconsistent ADC values - ADC conversion taking a long time

Kenneth — Thu, 23 Sep 2021 11:12:29 GMT

Sounds to me that you are measuring a very high impedance source (e.g. >1Mohm), which each time the pin is sampled need to be "charged" before it return to the same level. Can you connect a small capacitor (e.g. >1nF) on the analog input to see if this affect your results?

Kenneth