nRF51822 ADC accuracy without an external crytsal

[Eliot Stock]

The Product Specification makes it clear that the given levels of accuracy for ADC measurements are only possible when using an external crystal for the HFCLK. We have one of those on our current prototype boards are are now looking to remove it. The cost of the part doesn't justify the power savings for BLE radio operations, in our application. I need to get an idea of the ADC accuracy possible without the external crystal.

Our battery is a lithium ion 18650 that varies in voltage up to 4.3V. Here's a schematic of the circuit we use to drop our Vbatt down to a level appropriate for the ADC.

Screen Shot 2015-11-22 at 13.53.18.png

Can we still measure to the nearest millivolt without the crystal?

[Ole Bauck]

If you are doing BLE you cannot remove the HFCLK crystal, this is mandatory for the radio to work.

[Ole Bauck]

The "Block resource requirements" in chapter 8.3 says "requires HFCLK XOSC". The problem here is that the comment is also with the ADC even though this can be run on the RC oscillator with a lower accuracy.