Whats the best way to measure high impedance ?

High impedance measure

I need to measure impedance (only real part) between 1MOhm to 60MOhm, passing through them a 1khz sine wave, and I can't use more than 20nA pp. I was working in it some time ago, I tested a lot of VCCS combinations to sustent this small current. But today I still have troubles to understand or to decide the best way to solve this problem.
I need help; theory, papers, suggestions, applications note, schematics or advice are welcome.

Thanks in advance

Re: High impedance measure

Have you considered a bridge circuit with an electrometer type amplifier?

Re: High impedance measure

Yes, but I have a lot of doubt about implementation of it. I know that the measure impedance have a considerable capacitance component, and I don't how to configure the opposite impedance in the bridge, and fix the peak to peak current. When I started, I read an application note from Agilent Technology (somebody posted to me in elktroda forum), and I decided to use current source and four point topology.
But now I'm using a current source with the simplest OP, and the measure is made in the loop, precarious (in my point of view), but is working fine, with the problem that I can't down the current, lees than 100nA the signal noise ratio become poor, and the high measure point is very sensitive to the noise.
Could you post me a link or recommend me a e-book about using bridge with an electrometer op.

Thank you very much flatulent

Re: High impedance measure

I do not have any links. One way to reduce the noise in your circuit is to use a narrow band filter in the receiving part of the system.

If you synchronously detected with I and Q channels, you could get the real and reactive components of your sample. Following these I and Q detectors is where to put the narrow filters.