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

[Jørn]

Was the PSEL switch connected or disconnected when you did your measurements? if it was connected, could you make sure it is disconnected and redo the measurements? This would reduce any possible additional sources of error.