SAADC configuration

[Tim Adu]

Hi DevZone,

Here's my setup:

1. Using NRF5 SDK 15.2.0, custom board based on NRF52840
2. Battery powered device with 3.6V battery.
3. Supply voltage stepped down to 1.8V using buck converter, meaning the MCU runs on 1.8V
4. There is a secondary I2C-based buck/boost converter in the circuit which can output voltages between 2.6V and 5V depending on I2C commands. The voltage output from this converter is VCC_AUX (see image)

Challenge:

I need to setup the ADC peripheral to sample ADC_VCC_3V8 (which sits on  AIN2) and ADC_BATT, (which sits on AIN1). There are voltage dividers on both voltage outputs.

1. I'm not quite sure how to configure the ADC channels.
2. How do I convert the ADC values back to the actual voltages?

Any suggestions are appreciated

Thanks

Tim

[Marjeris Romero]

Hi Tim,

The resistors in the voltage deviders must be big enough so the voltage on the analog input pin is lower than VDD. Also make sure you scale the input so you get the highest possible resolution on the input range, so 1/3 gain if you are using the internal voltage reference.

For firmware and conversion you can take a look at the SAADC example on the nRF5 SDK: https://infocenter.nordicsemi.com/topic/sdk_nrf5_v17.0.2/nrf_dev_saadc_example.html

Best regards,

Marjeris