Measuring VDD with ADC

[uri danziger]

Hi,

I am running the nRF51822 from a coin-cell. I need to measure the cell voltage. Until now, I have attached VDD to an ADC pin with a voltage divider and a capacitor (as described in the reference manual para 31.1.2). However, I later saw that according to the register settings, it is possible to select a prescaled VDD with the INPSEL register, and then convert with VBG as a reference (REFSEL).

I just want to make sure that I have not overlooked anything, before I remove the voltage divider and capacitor from my design.

Help appreciated.

Thanks, Uri

[Stefan Birnir Sverrisson]

Hi Uri

You are correct. The easiest way to measure the supply voltage is to select VDD as input to the ADC, no input pin needed. This is done in this example. Choose ADC_CONFIG_INPSEL_SupplyOneThirdPrescaling option as input to ADC and VBG as reference. Then the ADC range is 0V - 3.6V which is suitable to measure supply voltage from 3V battery.

[butch]

The link is broken. Also, looking for same answer for nRF52. It seems that even though the nRF52 SAADC is not backward compatible with the nRF51 ADC, the SAADC can do the same thing: measure Vdd?

[bjorn-spockeli]

Yes, the SAADC of the nRF52 can measure VDD by setting NRF_SAADC_INPUT_VDD as the input pin. You can find examples for the nRF52 here.