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]

Jørn, Thanks for the reply, but i need more explanation: First of all, we're bypassing the ladder resistor with (NRF_SAADC_RESISTOR_DISABLED aka SAADC_CH_CONFIG_RESP_Bypass), so that makes things easier. Now my only question is why the exact resistance of R_Input not listed in the spec? Is it not a simple resistance similar to R_Ladder(which is listed exactly 160K without a > sign )? Having it listed as >1M doesn't make things simple to calculate the equivalent: In our circuit, we have a 1M(pull up) and 68K(pull down) resistor voltage divider at the (external to the chip) input, now if i want to calculate the equivalent resistance of the bottom side of our divider, it can be 68K||1M=63.67K or 68K||100M = 67.95K or 68K||(infinite resistance) = 68K! This means that the scale of my divider Rdown/(Rup+Rdown) can vary from 0.064 to 0.060 which is a 7% relative difference=large error