As part of a project I’m currently working on I wanted to set a specific reference voltage (AREF) for the microcontroller’s onboard analog-to-digital converter.
Working out the analog reference voltage required so that with an analog to digital converter of 10bit accuracy, 1000/1024 will correspond to 2.5V.
Calculating the magnitude of R2 based on R1. R2 must have an impedence 104.9% than that of R1.
We still need the microcontroller AREF pin impedance, which can be found in the datasheet. We will use R3 to represent this impedance.
We can think of R2 and R3 as being in parallel, as they both sink to ground.
The total impedance across R2 and R3 (Rt) can be calculated with the above formula.
Uh oh, using resistors in the KOhm range for the voltage divider results in the current sunk by the AREF pin having a very large influence. It has pulled the voltage down to 2.48V!
Higher accuracy can be achieved by using lower impedance resistors in the voltage divider. The problem is however…
A lower impedance voltage divider consumes more current. It becomes an issue of current vs accuracy.
The solution I will be employing for this is to place a voltage-following op-amp between the voltage divider and AREF pin. The op-amp has a very high input impedance meaning that its presence on the voltage divider is negligible.
Generalized equation for the accuracy of the voltage divider.
Generalized equation for the total current passing through the voltage divider.