High Input Impedance op amp

Here's a tutorial on chopper stabilised opamps :

http://www.analog.com/media/en/training-seminars/tutorials/MT-055.pdf

Particularly note figure1 ,  if you were to hang a few gigaohms on Vin , there would be no bias current flowing into the source (as there is no DC connection).  To adapt this circuit to your 4wire measurement, connect capacitors to ground from the top of your gigohm resistor, and the bottom. Move node Z on the input switch from ground  to the bottom of your gigohm resistor, and Vin to the top, (R1 and R2 can be zero ohms)

With this mod,  the peak to peak voltage at the amp input is exactly equal to the DC voltage across the gigohm resistor.

This arrangement is open loop, so you will get some second order error terms sneaking in. (so you will measure ~99.9% of the actual voltage)

I can't find any diagrams on the web to show how to add the servo function, but you basically have the capacitor measuring the difference between the input (from the gigohm resistor) and the output from your amplifier, so current only flows briefly when something changes, otherwise the loop is balanced.

I was going to suggest googling " autobalance AC bridge "  but it seems some clueless person has used "autobalance" to describe a mundane virtual earth transresistance opamp circuit.

Hmm maybe something like  the figure in this https://www.google.com/patents/US3454850?dq=servo+ac+bridge+chart+recorder+chopper&hl=en&sa=X&ei=GBVoVf-PIoSI8QX8h4GgAg&…

(but use another chopper instead of the motor and pot, and you get the whole idea of the motor adjusting the pot, until E1 and E2 are at the same potential)

Here's a tutorial on chopper stabilised opamps :

http://www.analog.com/media/en/training-seminars/tutorials/MT-055.pdf

Particularly note figure1 ,  if you were to hang a few gigaohms on Vin , there would be no bias current flowing into the source (as there is no DC connection).  To adapt this circuit to your 4wire measurement, connect capacitors to ground from the top of your gigohm resistor, and the bottom. Move node Z on the input switch from ground  to the bottom of your gigohm resistor, and Vin to the top, (R1 and R2 can be zero ohms)

With this mod,  the peak to peak voltage at the amp input is exactly equal to the DC voltage across the gigohm resistor.

This arrangement is open loop, so you will get some second order error terms sneaking in. (so you will measure ~99.9% of the actual voltage)

I can't find any diagrams on the web to show how to add the servo function, but you basically have the capacitor measuring the difference between the input (from the gigohm resistor) and the output from your amplifier, so current only flows briefly when something changes, otherwise the loop is balanced.

I was going to suggest googling " autobalance AC bridge "  but it seems some clueless person has used "autobalance" to describe a mundane virtual earth transresistance opamp circuit.

Hmm maybe something like  the figure in this https://www.google.com/patents/US3454850?dq=servo+ac+bridge+chart+recorder+chopper&hl=en&sa=X&ei=GBVoVf-PIoSI8QX8h4GgAg&…

(but use another chopper instead of the motor and pot, and you get the whole idea of the motor adjusting the pot, until E1 and E2 are at the same potential)