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Measuring Resistance with ADC & Voltage Divider, Known Resistor = R1 or R2?

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evglabs:
When measuring resistance with and ADC on a micro and voltage divider does it matter if the known resistor is connected to the positive rail or GND?

Thanks

schmitt trigger:
If I understand correctly what you are asking is:
When solving a resistive divider equation, does it matter if I select R1 and solve for R2, or do I select R2 and solve for R1?

And also arbitrarily, R1 is the leg that goes to ground, and R2 is the leg going to the voltage being measured.

Short answer, it depends. It depends what you want to optimize.

For instance, let's say you want to measure a voltage, loading it with no less than 1 Meg.
In this case, it makes sense to start with R2 = 1 Meg and solve for R1. The equivalent resistance will be larger than 1 Meg.

On the other hand, the ADC's datasheet requests that the source impedance be no larger than 10k, then you select R1 = 10k and solve for R2. The equivalent resistance will be now lower than 10k.

Like everything else, there are tradeoffs, and you will find yourself re-calculating the original divider thru several iterations.
In this instance, Excell or any other spreadsheet is your best friend.

evglabs:
Yeah, thanks.

So, basically other than something like the ADC requirements (or in an ideal situation), it wouldn't matter if I'm solving for R1 or R2 right?

ogden:

--- Quote from: schmitt trigger on November 03, 2019, 05:46:32 pm ---On the other hand, the ADC's datasheet requests that the source impedance be no larger than 10k

--- End quote ---
Not true. There are many ADC's that will happily accept higher than 10k source impedance. Especially those with built-in buffer amplifier. If you are ready to slow sampling speed down, you can increase signal source impedance. Even then you can "fix" problem by adding capacitor. Signal source impedance table for 12-bit stm32 SAR (unbuffered) ADC.

schmitt trigger:
I used 10k(max) ONLY an example.

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