Ratiometric A/D Simplifies Digital Resistance Readings
An analog to digital (A/D) converter compares its analog input to a reference voltage. The output is proportional to Input/Reference. This can be used to simplify measuring resistance digitally.
The circuit shown passes the same current through the measured resistor and a reference resistor. The current might not be constant: it will change when R(meas) changes and if there is drift in the supply voltage. However, the ratio between the input and reference voltages will stay the same.
A precise, stable reference voltage is not required. The only precision part needed is the reference resistor.
Actual designs, of course, involve more detail than this. We recently published an article on the AllAboutCircuits.com website. To see it, click here.