right value for resistor divider input ADC

[Yacire]

Hi All,

I have a basic question, i want to know what are the right value to use with the nrf51422 chip for the resistors with ADC.

I want to use the maximum value permitted for reducing consumption, say for example two resistors of 100 MOhms each (so a resistor divider by 2).

I use a prescaler 1/3, so the ADC resistor is 389.2 kOhms for the nrf51422, and i don't use capacitor in parallel, but i sample one every 10 min.

this is correct or not if i choose those values, or i need to choose other?

Thanks !

[Carsten Wulff]

You will not get a 1/2 without a capacitor. The capacitor is required to limit the voltage change during conversion. To calculate the voltage change you first need to know the current. The current will be I = (Vin-0.6)/398k. Then the voltage change will be dV = I x conversion_time/capacitance.

Then you need to check the settling time of the RC, but with 10min between samples it should be possible with large R.