The parts list which is not in the circuit, is listed below, the circuit is 15 years old. but you may get some idea on high resistance measurement. Use 1% MFR for all Resistors and low leakage plastic caps for low values. These instruments i used to calibrate with Victoreen Resitors and a Electrometer from Princeton Research, I faintly remember.
Theory of Operation.
The Device Under Test DUT say a transformer is placed in a Metal tray connected to the Guard SK3 terminal. The metal tray has a 3mm glass sheet on which DUT is kept. For 2 Tera ohm make sure that Humidity and Dust do not affect measurements.
Now 1000V is applied on a DUT terminal from the high voltage supply, ( in reference to Guard SK3 at earth and 0V). Then the point where the leakage is to be measured is connected to SK4 via a BNC short, scope, shielded cable. The current goes thru a shunt selected by S5 and voltage across the shunt is measured by ICL7650 a Chopper stabilized amplifier, with ultra low offset and bias. The amplified output is fed to Vref of ICL7107 which displays the Insulation Resistance. The full range is not valid in this circuit and for low values, change to a range where the reading is more number of counts. The above circuit does not include some upgrades and changes i did later using ICL7135 etc.. If i find it i will add it later.
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