nRF52832 SAADC circuit R_INPUT value

[Farhang]

Hi there,

I am getting some error in my SAADC measurements and I believe it's due to the R_INPUT forming a parallel resistance with my voltage divider at the input (the voltage divider is meant to scale down the voltage of interest to under 2.4 Volts so it can be measured by the MCU). The values I have chosen for my divider are 1M and 68K.

I would like to know the typical value of R_INPUT so I can estimate what the equivalent resistance of my voltage divider is. The specification says R_INPUT is >1M but this doesn't help as it covers a huge range and theoretically it covers infinite resistance (open circuit).

infocenter.nordicsemi.com/index.jsp

I suspect there's a good reason for this but if not I would like to know the typical value so that I can scale my measurements accordingly.

[Jørn]

Hello Farhang

If you look at the bottom of the electrical specifications here you can see a more detailed schematic of the ADC input. The 1 MOhm resistance is the input resistor you see in parallel with Cpad. When the pin is configured for the ADC, PSEL closes and Rinput is in parallel with RLadder, which is, according to the SAADC electrical specification, 160 kOhm. The total input resistance seen into the analog pin will then at the very least be 1MOhm||160kOhm=137930 ohm, and at the most be 160kOhm.

Keep the acquisition times in mind when designing your input network. Take a look here

Best regards

Jørn Frøysa

[Farhang]

Thanks for further investigation Jørn. I calculated back the internal resistance seen into SAADC and it was different from NRF52 to NRF52 unit, with no hardware changes in between our boards. I looked back at my notes and I have measured it at 1.15 MOhm for one NRF52832 and another NRF52832 measured 10.26 M! (my method for measuring the resistance was 1. confirming the resistances of my voltage divider (with a meter) with the SAADC input of NRF52 NOT connected to it, 2. then I applied a DC voltage at the high end of my divider, measured the voltage at the middle point with a Fluke multimeter (re-confirming my divider and my multimeter) 3.then I connected the NRF52 to measure the middle point. And as i explained above, different units give different measurements! Hence i posted this question. Are there possibly two different revs of the chip (NRF52832) with two different resistor values?