RE: problem with ADC drawing current on nrf51822

Maipei — Tue, 19 Apr 2016 03:16:59 GMT

Ah, yes, I found the question you're referring to (posted below for any future readers). That looks like it should solve my problems. Thanks!

devzone.nordicsemi.com/.../

RE: problem with ADC drawing current on nrf51822

RK — Mon, 18 Apr 2016 15:38:15 GMT

Have you read the ADC section in the nRF51 manual? That tells you the input impedence of the ADC is between 100 and 400kOhm and that in order to get the accuracy specified, the output impedence of the connected source should be less than 1kOhm. So yes, 4.7M is too high.

The reference manual also however suggests using a capacitor to keep the voltage steady during the few microseconds of measurement, that reduces the frequency you can measure as the capacitor has to charge again, but let you have a high impedence voltage divider and still measure accurately.

If you use the search box you'll find there's a blog post somewhere about measuring lithium cells with a voltage divider of high impedence (so the voltage divider doesn't drain the battery) and a capacitor, perhaps that solution may work for you.