How can I accurately measure high resistances using a lock-in amplifier?

[Erikve]

Dear all,

I measured some very nice data, but it's difficult to analyze, but I try to get out of it what I can. I have diverging resistance and did measure with standard lock-teqnique: apply 0.45V over preresistor and send current over sample and measure the two-probe resistance over the same contacts as where I send the current.
At the moment that the sample reaches the border of 10MOhm we get of course problems since current starts leaking away via the 100MOhm input-impedance of the lock-in and the capacitor of 25pF. I measured the current with a second lock-in. With this data I should be able to extract still quite the resistances up to 1 GOhm, but somehow I cannot find the right way how to deal with this.

Somebody who can help me out of the spaghetti of electrical circuits?
Thanks you.

 

[0xDEADBEEF]

What Type of Lockin do you use? What frequencies? Some lockins have a current input (Those from "Signal Recovery" for example) With a cable that is patched the right way, you can measure resistances without a pre resistor. You have to be careful, because the cable capacitance will produce quite a low pass. Maybe you should think about signal guarding too, or use low frequencies. Are you sure that the second lockin was locked to the first lockins signal? It's hard to help you with spaghetti without any diagrams.