As shown in FIG OPA111 constituted by the input buffer leakage current measuring circuit. D1, D2 of the transistor 2N3904, which will short the base and collector contact (open
emitter). Up to 200V power is applied to the device under test, leakage current flows through 100M sampling resistor, forming a voltage drop across the sampling resistor, the voltage drop after the buffer input to the INA117 amplifier output, the output voltage eo IL 109 (1V/nA). Because you want to measure low leakage current, measurement circuit to prevent shunt caused by measurement error, so the use of high input resistance measurements constitute a buffer circuit. With an isolated power supply OPA111 op amp power supply, so the circuit allows 200V common-mode range.