Measure voltage in GOhms resistors with USB6008

[Oceano007$]

Dear all,

I cannot measure the same  voltage value with the multimeter XMM (50 MOhms)  and with the USB 6008 neither in XMM2 nor in XMM3 points (see schematics figure).

The problem is the 150 kOhms of impedance USB 6008 value.

Do you now why? How can I solve this issue?

Regards

[crossrulz]

You are basically creating a parallel resistor network.  Your DAQ in parallel with the 7GOhm will result in 144 Ohm.  The simple solution is to put an op-amp in a voltage follower configuration between your measure point and the DAQ.  The high impedence input of the op-amp should keep your circuit happy while the low output impedence will make the DAQ happier.

[Henrik_Volkers]

.. with gigaohm resistors also the tiny bias currents will have great effects 😄

and even your PCB, if not really good cleaned and using guarding techniques, will affect the actual results.

and a 6008 really needs a buffer 🙂 

[CataM]

Here is an example on how to add a buffer using an OP Amp.

[Oceano007$]

Thanks to all,

Now it works, as you say it is enough to add a follower or buffer so that we have a high input impedance and low currents can pass along the circuit.

It is very important to feed with the proper voltage the amplifier, otherwise it will be saturated.

Great, thanks again!!!!