How to measure Lithium battery voltage with the nRF51 ADC
I also went through the calculations for cap value. I get a value of 70.2 nF (instead of 116pF or 11.6nF). I think the error is in the initial formula for the cap voltage decay. I agree initial voltage is 0.76 and "final" voltage would be 0.61 with the ADC on. The formula for the cap voltage decay should be 0.76+(0.61-0.76)(1-e^(-t/RC)). Using R=121k, t=68us and solving for C, I get C=-68u/(121k*ln(1-1.17m/0.15)).
In short, there's a "1-" missing from the ln().
Thank you very much for your feedback Philip. With your suggestion, I will review the calculations and update accordingly.