The circuit uses a synchronous-detection scheme to measure low-level resistances. Other low-resis-tance-measuring circui
ts sometimes inject unacceptable large currents into the system-under-test. This circuit synchronously demodulates the voltage drop across the system-under-test and can hence use* extremely low currents while measuring resistance. The 10-V (pk), 1-kHz carrier generator injects a 1-mA reference current into unknown resistor, iTEST. Instrument amplifier IC1 and precision op amp IC2A amplify the voltage across iTEST by a gain of 100 000. Synchronous detector IC3 demodulates this voltage, then op amp IC2B acts a low-pass filter on the demodulated voltage. The low-pass filtering will attenuate all uncorrected disturbances (such as noise, drift, or offsets), while passing a dc voltage that is proportional to the unknown resistance.