nRF52832 - Analog read with higher impedance divisor

[Abomin3v3l]

Hello.

The MCU of our new design will be the nRF52832.

I would like to know if it is ok to read the voltage of the battery that powers the system, nominal voltage of 3.6V, type Li-SoCl2, using a divisor with these higher values of resistors.

One advantage is the low cost.

3.6V / (0.82M + 0.82M) is just around 2.2uA. Another advantage.

Five time constants:
820K // 820K = 410K
5 * 1uF * 410K
5 * (1*10^-6) * (410*10^3) is around 2 seconds.

The battery voltage would be read each 3 seconds, or more, more than 3 seconds is also ok for the application.

And there is the 1uF capacitor, in order when the internal capacitor of the ADC needs to sample the voltage on the pin, there will be such 1uF capacitor charged on the analog pin. I could also try to place, in the PCB layout, this capacitor nearby the nRF52832 chip.

We would like to know if this circuit is fine for the nRF52832 analog reads.

[Jared]

Hi there,

I think this will be fine, but the only way to know is to actually test it. 

Higher resistors in the divisor can lead to more noise susceptibility and the longer increased acquisition time. I think the resistor values that you've chosen can work but you might need to adjust the capacitor as well. How did you choose that capacitor size, have you already tested it?

Also, we have written a blogpost regarding this topic here: 

However, often the easiest way to determine a suitable capacitor si+ze for a custom voltage divider or any connected external circuit with impedance higher than 1kΩ, is to use trial and error method. Start with mounting e.g. 100 nF capacitor between the ADC input pin and ground. If you do not get adequate ADC accuracy with low sample rate (<1Hz), increase the size until error disappears. The trade-off is that if you have large capacitor, the maximum sampling frequency is lower then when you have a small capacitor. Typically you are limited to 1Hz - 10Hz maximum sampling frequency when using a capacitor.

regards

Jared