The circuit in Figure 1a comprises a voltage follower, IC1 , and the reference-voltage source of IC2. IC1 is an Analog Devices AD8661 op amp, which has a guaranteed input-bias current of no more than 1 pA and a typical input-bias current of 0.3 pA (Reference 1), and IC2 is an Analog Devices ADR391 precision voltage reference (Reference 2).
The manufacturer trims the input offset voltage of this op amp not to exceed 100 µV, and the typical value is 30 µV. These properties suit this amplifier for observing self-discharging of almost any type of capacitor. The leakage currents of solid-tantalum capacitors and those having high-quality plastic dielectrics are well above the input-bias current of voltage follower IC1. The CUT (capacitor under test) initially charges to the reference-voltage level of 2.5V by connecting Point A to the output of IC2. Subsequently, at some convenient time, Point A disconnects from the source of the reference voltage. A DVM (digital voltmeter) measures the output voltage of the follower at some reasonable time.
The measured voltage drop, VO, with regard to initial value, should be 0.1 to 0.5V. The leakage current, IO , is C×ΔVO /tMEAS , where C is the value of the CUT and tMEAS is the time between releasing the connection of the CUT to the 2.5V source and the instant of readout at the voltage drop of VO.
The fixture also allows determining leakage currents of reverse-polarized diodes and of various switching devices in the off state, such as JFETs, MOSFETs, BJTs (bipolar-junction transistors), SCRs (silicon-controlled rectifiers), and IGBTs (insulated-gate bipolar transistors). In this case, the parallel combination of the DUT (device under test) and the added capacitor, CADD , replaces the CUT (Figure 1b).