As per Ohm's law, current through a conductor is proportianl to the voltage across it. If both voltage and current are known, resistance can be calculated. An unknown resistance can be measured by comparing the voltage drop with that of a known resistor. Figure shows a 1k resistor from IN1 to Ground and an unknown resistor from PVS to IN1.
Voltage applied to PVS = 4.0 volts
Voltage measured at IN1 (across 1k resistor) = 1.254
Current = V/R = 1.254/1000 = 1.254 mA . The same amount of current flows through the unknown resistor.
Voltage across the unknown resistor = 4.0 - 1.254 = 2.746 volts
value of the unknown resistor = 2.746 / 1.254 = 2190 Ohm