Gigaohm Meter Circuit

To read up to a couple of hundred Gigohms is pretty easy, just as long as you use a high enough voltage.

1 gigohm and 100 volts = 100 nano amps

Modern op amps have offset / bias currents of less than a picoamp, so yes its easy.

If you want to see how easy just download the manual for the Gen Rad 1864 Gigohm meter, that just uses a fet input to drive a meter and a valve (tube) to generate up to 500 volts.

John.

I should have added that your main enemy at these high impedances and low currents is surface leakage around the input stage and contamination of the input surfaces causing low currents to flow...

Answer is simple, use PTFE connector as input and bend leg of ic or fet up and wire in the air, to avoid surface leakage currents.

John.

Thank you very much, it was very helpfull.

The LMC660 and LMC662 are very low-bias current op amps - typically femotoamp - this will reduce the voltage requirements

Analog Devices make some electrometer grade amplifiers. They have application notes on how to configure them as current to voltage converters. Then all you need is a low noise voltage source and a current limiting resistor.

http://www.analog.com/UploadedFiles/Data_Sheets/AD549.pdf#xml=http://search.analog.com/search/pdfPainter.aspx?url=http://www.analog.com/UploadedFiles/Data_Sheets/AD549.pdf&fterm=electrometer&fterm=electrometer&la=en

At such Hi-range of Insulation [Resistance measurement] the environment leakage will upset your measurements. Very high degree of sophistication neededed.

Results on 100s of MegΩ is quite uncertain for accuracy even.

Regards

Yes, its a cork resistivity meter, it has resistances in the range of 11000 Mohm. Its not going to be easy but its our job to figure out how ;)