SAADC battery measurement current consumption

[PB]

Hello,

I'm trying to measure battery voltage on nRF52832 chip using SAADC based on examples/peripheral/saadc and examples/ble_peripheral/ble_app_proximity and noticed that just enabling saadc makes my device consume ~7mA constantly.

This is the code for initialization, sampling is later done once per second.

saadc_init code:

ret_code_t                 err_code;
nrf_saadc_channel_config_t channel_config =
    NRF_DRV_SAADC_DEFAULT_CHANNEL_CONFIG_SE(NRF_SAADC_INPUT_VDD);

err_code = nrf_drv_saadc_init(NULL, _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], BATTERY_SAADC_SAMPLES_IN_BUFFER);
APP_ERROR_CHECK(err_code);

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

It doesn't matter if I later sample with nrf_drv_saadc_sample(); or not, the current is being constantly consumed. Just commenting out the code above makes the consumption drop to acceptable levels.

Does anyone had such experience before? I understand that ADC sampling comes at a cost but I didn't expect it to be a constant one.

[AmbystomaLabs]

As Jorgen mentioned 8mA is unusually high for the SAADC. Normal consumption even without the low power steps is around 1mA.

What might not be obvious in the sample code is that you need to turn off the SAADC when you are done with it. So in your handler once you are done reading the buffer, turn off the saadc and clear pending irq.

	nrf_drv_saadc_uninit();
    NRF_SAADC->INTENCLR = (SAADC_INTENCLR_END_Clear << SAADC_INTENCLR_END_Pos);
    NVIC_ClearPendingIRQ(SAADC_IRQn);

In this manner it is only on for at most a millisecond total for init, sample, read buffer and turn off.

[Luk3]

What about calibrating offset? Does saadc need recalibration after each init?

[Jørgen Holmefjord]

From the product specification:

"The ADC has a temperature dependent offset. If the ADC is to operate over a large temperature range, we recommend running CALIBRATEOFFSET at regular intervals"

You should run calibration on regular interval, or with change in temperature.

[Luk3]

My device works at room temperature. I need to perform calibration once at startup but to achieve low power consumption I am disabling ADC like AmbystomaLabs said. Does the SAADC rember calibration after disabling?

[AmbystomaLabs]

Great question, but I don't know the answer. The specs didn't provide any answers either. It would certainly save on power if it did save the cal data.

[Luk3]

Can I please someone from Nordic to answer?