Measuring Lithium battery voltage with nRF51

RE: Measuring Lithium battery voltage with nRF51

Nasib — Tue, 19 Nov 2019 09:30:41 GMT

Ufff I have a couple of doubts:

1) Why don't we want the current to flow through the ADC?

Here they say that to optimize the resolution of the ADC it is recommended to choose R1 = 1M and R2 = 10Meg

https://devzone.nordicsemi.com/nordic/nordic-blog/b/blog/posts/measuring-lithium-battery-voltage-with-nrf52

2) In my case (NRF51822) the internal resistance of the ADC is 360K, so it is below R2 ... Does this fact affect the measurement?

Thank you !!

RE: Measuring Lithium battery voltage with nRF51

Mannu — Tue, 19 Feb 2019 13:34:50 GMT

What if i want to use 3.7v lipo battery. i mean what are the new parameters for me.

RE: Measuring Lithium battery voltage with nRF51

jarhead — Wed, 21 Nov 2018 18:15:05 GMT

This website is complete bullshit. Q&A page does not working. No one seems to help developers!!!

RE: Measuring Lithium battery voltage with nRF51

Hardik — Sat, 27 Oct 2018 06:03:46 GMT

Dear Stefan,

Source impedance mentioned in datasheet is less than 5K and suggested to use less than 1K but here in example we are using value in Mega ohms.

How does these matches impedance????

Please help to clear doubt.

RE: Measuring Lithium battery voltage with nRF51

Hardik — Wed, 24 Oct 2018 11:43:01 GMT

Dear Stefan,

RE: Measuring Lithium battery voltage with nRF51

Luis — Mon, 23 May 2016 05:29:30 GMT

Is this information still valid for nRF52? What changes or values should we do for nRF52?

RE: Measuring Lithium battery voltage with nRF51

Stefan Birnir Sverrisson — Tue, 09 Feb 2016 13:20:05 GMT

@prabhudurai When the ADC is not sampling, the ADC input is high impedance. When the ADC samples, the impedance of the ADC input is 130kΩ - 390kΩ, depending on your chosen ADC prescale setting. The result is that there is a voltage drop when the ADC starts sampling if the U_AIN input voltage is higher than 0.6V. However, there will be a voltage increase when the ADC starts sampling if the U_AIN input voltage is lower than 0.6V. When you have a capacitor, the voltage will remain stable during the sampling period, which is 20us for 8-bit sampling and 68us for 10-bit sampling.

Of course the voltage will drop a little bit with a mounted capacitor when the ADC starts sampling. The goal is to have the capacitor large enough so that the voltage drop is less than what corresponds to 1 LSB on the ADC output during the sampling period. If you however have the capacitor too large, the capacitor will take longer to fully charge upp after sampling completes. A large capacitor therefore puts a limit on the sampling frequency. The larger the capacitor, the slower you must sample.

RE: Measuring Lithium battery voltage with nRF51

MARTIN FR — Fri, 05 Feb 2016 10:03:40 GMT

why do we need to use the 22nf capacitor across the voltage divider circuit? whether it is used to allow the current flow in both positive and negative cycle (charging and discharging)? please verify my answer?

RE: Measuring Lithium battery voltage with nRF51

Scott Piette — Fri, 09 Jan 2015 17:18:06 GMT

This is great, thanks for providing a well documented example.