Max Source Resistance value of SAADC in nRF52840

RE: Max Source Resistance value of SAADC in nRF52840

Karl Ylvisaker — Tue, 01 Feb 2022 12:28:14 GMT

Thank you for your patience with this.

[quote user="Brian (Geek)"]thanks for clarification.[/quote]

No problem at all, I am happy to help!

[quote user="Brian (Geek)"]Also software side, I will be using acquisition time of around 10us and will be triggering SAADC read every 2-3 minutes. After successful conversion of SAADC samples i will be un-initializing the SAADC peripheral and cut-off voltage monitoring section using a trigger pin to save power. [/quote]

This is good - then you should only see the leakage current of the measurement pin adding into your overall current consumption.

[quote user="Brian (Geek)"]While designing our board we took reference from Thingy 52 board, where the R1 value is 1.5 M Ohms and R2 is around 180K Ohms without use of any external capacitor. So how is it possible to get proper reading in Thingy 52 board just by the use of high source resistance (1.5 M Ohm) ? Can you please explain that. [/quote]

I reached out to the Thingy52 developers about this, but they did not have any particular comment about this. Based on the calculations I would expect this level of source resistance to cause inaccurate readings of the battery level - but at the same time I have not heard about this being reported as an issue by Thingy52 users, so it is hard to say how big the potential effect of this is.

Either way, I would recommend that you go with the capacitor approach since this both theoretically and practically will cause the most accurate readings with the least possible leakage current.

Best regards,
Karl

RE: Max Source Resistance value of SAADC in nRF52840

Brian (Geek) — Thu, 27 Jan 2022 06:06:17 GMT

Hello thanks for clarification.

So i will be using the resistor and capacitor values same as the values given in the example section of

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

Based on these calculations we can for example use these values: R1 = 4MOhm, R2 = 10MOhm, Cext = 10nF. Then the leakage current will maximum be:

4.2V / (10MOhm + 4MOhm) = 300nA

Also software side, I will be using acquisition time of around 10us and will be triggering SAADC read every 2-3 minutes. After successful conversion of SAADC samples i will be un-initializing the SAADC peripheral and cut-off voltage monitoring section using a trigger pin to save power.

While designing our board we took reference from Thingy 52 board, where the R1 value is 1.5 M Ohms and R2 is around 180K Ohms without use of any external capacitor. So how is it possible to get proper reading in Thingy 52 board just by the use of high source resistance (1.5 M Ohm) ? Can you please explain that.

Thank you.

RE: Max Source Resistance value of SAADC in nRF52840

Karl Ylvisaker — Wed, 26 Jan 2022 09:17:09 GMT

[quote user="Brian (Geek)"]Okay this means if I don't use capacitor, and just use high resistance values directly for R1 and R2 the values read by ADC might not be correct. [/quote]

Yes, this is correct - because the SAADC's internal capacitor will not fill fast enough if the source resistance is too high, meaning that the SAADC readings will come out too low. Adding a capacitor to the voltage divider negates this issue through always making enough charge available (given that the SAADC sampling frequency is not too high for the new capacitor to reach its steady state) for the SAADC to sample accurately.

Best regards,
Karl

RE: Max Source Resistance value of SAADC in nRF52840

Brian (Geek) — Tue, 25 Jan 2022 16:08:44 GMT

Hello,

I have referred the link which you have mentioned. And also the nRF52840 SAADC peripheral doc, says the max source resistance that I can have is 800K ohms for acquisition time of 40us.

Okay this means if I don't use capacitor, and just use high resistance values directly for R1 and R2 the values read by ADC might not be correct.

RE: Max Source Resistance value of SAADC in nRF52840

Karl Ylvisaker — Tue, 25 Jan 2022 14:44:53 GMT

Hello,

Which guide / reference document are you referring to here - Is it the How to measure lithium ion battery's guide?
In general, when working with a very high resistance you should add a capacitor to ensure correct readings. This is also explained in the linked guide, along with an explanation of the calculations involved with determining the correct resistor and capacitor values.

Best regards,
Karl