An impedance measurement circuit and method is disclosed that uses a differential amplifier to amplify a DC voltage value developed across a device being measured. The device is biased between potentials of a source of electrical energy by first and second bias resistors. A switchable current source is connected to the device to, when actuated ON, connect the device to the source of electrical energy through first and second associated resistors. The ratio of the values of the first to second bias resistors is substantially equal to the ratio of the values of the first to second resistors associated with the switchable current source. A microcomputer calculates the resistance of the device by subtracting the value of the output signal from the differential amplifier when the switchable current source is OFF from the value when the switchable current source is ON. The difference is multiplied by a constant having a value dependent upon the gain of the amplifier and divided by a value functionally related to the value of the source of electrical energy. In this manner, the effects of common-mode errors and differential-mode errors are eliminated from a calculation of the resistance value of the device.
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