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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.

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  • 0

    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

  • 0 in reply to Jørn

    And to me more clear i calculated back the internal resistance of the NRF52 from Fluke meter voltage measurements of the middle point of the divider. Here are the raw values of my circuit and measurements:

    1. voltage divider resistances 1M Pull up and 68K pull down (both 1% tol)
    2. voltage at the high end of the divider = 12.49V
    3. voltage at the middle point (before connecting the NRF52) = 0.790
    4. voltage at the middle point with NRF52 #1 = 0.753V! ( => Input resistance = 1.15M )
    5. voltage at the middle point with NRF52 #2 = 0.789V ( => Input resistance = 10.26M )
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  • 0 in reply to Jørn

    And to me more clear i calculated back the internal resistance of the NRF52 from Fluke meter voltage measurements of the middle point of the divider. Here are the raw values of my circuit and measurements:

    1. voltage divider resistances 1M Pull up and 68K pull down (both 1% tol)
    2. voltage at the high end of the divider = 12.49V
    3. voltage at the middle point (before connecting the NRF52) = 0.790
    4. voltage at the middle point with NRF52 #1 = 0.753V! ( => Input resistance = 1.15M )
    5. voltage at the middle point with NRF52 #2 = 0.789V ( => Input resistance = 10.26M )
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