The Quad Picoammeter Module is designed to make fast and accurate leakage measurements.

The Quad Picoammeter Module forces voltage and measures current with up to 62,000 times more sensitivity than the Precision V/I Source (located in the test system).

The four identical current measurement circuits (channels) share a common supply voltage (±48 V from the 80-3006 power supply) To reduce overall measurement time, all channels can settle in parallel or one channel can measure while the other three are settling. A PTH can handle up to eight Quad Picoammeter modules.

Three standard ranges are provided to improve the signal-to-noise (S/N) ratio for most leakage current measurements. An additional, integrating range is provided to improve the S/N ratio for measurements of very low leakage current.

Refer to Figure 8-1 during this discussion. A Quad Picoammeter Module consists of four measurement circuits (or channels), two bias voltage DACs and an 80 MW precision calibration resistor. A PTH can be configured with up to eight Quad Picoammeter modules. A DUT can be connected to any or all of PACh1_IN, PACh2_IN, PACh3_IN or PACh4_IN via the P1 connector. See Figure 8-12, "Edge Connector Layout."

PACh1_AUX, PACh2_AUX, PACh3_AUX and PACh4_AUX are auxiliary connection paths. These paths provide a convenient method of connecting other test resources to the DUT pin without adding additional capacitance or leakage.

PACh1_GUARD, PACh2_GUARD, PACh3_GUARD and PACh4_GUARD connect to the guard traces, if present, on the Module Interface Board. This reduces the effect of printed circuit board leakage.

Each channel is a current-to-voltage converter circuit with input ranges of 500 nA, 50 nA, 10 nA and 1 nA. On the 500 nA, 50 nA and 10 nA ranges, resistive feedback is used to convert current to voltage. An optically isolated solid-state relay switches the Digitizing Voltmeter (DVM) into the circuit. The Digitizing Voltmeter (connected at PTHVMH, PTHVML) computes the leakage current based on the voltage across the feedback resistor.

On the 1 nA range the DVM computes the leakage current based on capacitive feedback. This is done by disconnecting the feedback resistor, measuring the voltage across the capacitor, then making a second measurement exactly one power line cycle later. The DVM then computes the current. (One power line cycle 16.66 msec for 60 Hz systems, 20 msec for 50 Hz systems.) The timing for the two voltage measurements comes from the Instrument Bus Interface Board (Part Number 60-1086, located in the test system) and is controlled by the Quad Picoammeter programming procedures. Specifying the 1 nA measurement range automatically integrates the measured current over one power line cycle.