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How do I know how much load a SAR ADC puts on the analog input and is my application condition measurable?

I am designing a PCB and I would like to read the output of an external potentiometer using the SAR ADC from an ESP32S3. The challenge is the potentiometer is customer supplied and the total resistance can be as low as 150 ohms in one extreme or 20000 ohms in the other extreme. The ADC in question has a range of 0-950mV and 12bits. The power supply is 5V DC.

My first thought was to use a voltage divider. For the 20000ohm case, I would need a theoretical resistance of 85263 ohms. However, this means the current through the circuit could be as low as 59 uA.

How do I know how much load a SAR ADC puts on the analog input and is my application condition measurable?

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  • \$\begingroup\$ The ADC reads volts, not ohms, so we need to know how this external potentiometer is connected to your ADC. Is there some voltage across the ends of the pots and the wiper is connected to the ADC, or a fixed voltage with the pot acting as a variable series resistor, or... ? \$\endgroup\$
    – td127
    Jun 23, 2023 at 3:58
  • \$\begingroup\$ What's the ADC model? This is an important thing to know, before we can answer your question. \$\endgroup\$ Jun 23, 2023 at 5:58

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You need to add a voltage buffer in front of your ADC and after the potentiometer. This way you separate the ADC and the potentiometer. The buffer will drive the ADC with low impedance and it will load the potentiometer with high impedance thus not affecting the input voltage. The voltage buffer can be a unity gain opamp.

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