NI 9265 cannot drive high impedance load

[kevEm]

Hi we are using a USB NI 9265 to send a DC current  with  a  noise current  superimposed, into a load.

The current is in the order of only 100's of microamps.

This works fine for loads at about 2K - 10K Ohm as long as we don't exceed the 12VDC compliance voltage.

When we attempt to drive the current into a very high impedance device such as a Grass Instruments PSIU6  stimulus isolation unit we have a problem.

Viewing the voltage at the input of this isolation unit on an oscilloscope shows that the NI 9265 is outputing something that causes the voltage to sit at 13 volts and we cannot change this.

I suspect that the very high input impedence of the isolation unit is too high for the 9265 to handle.

Has anyone else had experience with this USB NI 9265 current amplifier?

Thanks

[AnalogKid2DigitalMan · Accepted solution]

What is the input impedance of the PSIU6?

Assumming 100uA and a 12V compliance voltage, max load can be 120Kohm. Even less if driving 100's of uA.

-AK2DM

[kevEm]

Hi AK2DM,

Thanks for your reply.

This PSIU6 isolation unit has an opto coupler at its input. The actual input impedance is not given but I am guessisng that it is in the Meg Ohm range.

I can see what you mean though, at 1MOhms and a current of 100 Microamps then the voltage would need to be 100V, way above the compliance voltage of the 9265.