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.