I have a handful of REF192GP reference voltage sources. They are specified to produce a relatively precise 2.5V. Measuring each device I typically get a few mV higher than 2.5V, for example 2.503V. More generally, the datasheet specifies +-10mV for the "G Grade".
The reference voltage is used on one to eight (the number changes during operation) LM324 inputs, each having a 1MΩ pull-down resistor, so on the order of >= 100kΩ total load.
I would like to shave off just a few mV from that voltage so that I can get closer to the desired 2.5V. I am looking for a "clever" solution that is ideally simple to build with a few handy components (other than simply buying the more precise "F Grade" or "E Grade").
Example: A voltage divider on the order of a kΩ.
Pro: Adds around 2.5mA of load, which is comfortably below the 30mA specified in the datasheet.
Pro: The worst-case load of 100kΩ will drop the voltage only by a tiny fraction of a mV, which is more precise than I can measure anyways.
Contra: Potentiometers (even trim potentiometers) are difficult to set reliably to such extreme ratios.
Best Answer
You can do this. The pot will be easy to set (range is only about 10mV). Output impedance is < 275 ohms. Adjust values if that's too high.
simulate this circuit – Schematic created using CircuitLab
Of course if you happen to have a reference that is < 2.5V or an offset that is in the wrong direction this won't help.