measuring current with usb6008

[oryan]

Hi,

I'm using a PMT detector whose output signal varies from 0 microamps to 100 microamps but must remain under 100 microamps. I've put a 100 kilo-ohm resistor into the signal line and, using a voltmeter, I can measure the voltage across the resistor and calculate the current. I'd like to replace the voltmeter with my USB6008. However, when using a differential pair of channels to measure the voltage on either side of the resistor, the USB6008 sinks current and affects the signal.

Is there a way to measure the current without affecting the signal?

Regards,

Oliver.

[Wayne.C · Accepted solution]

Oliver,

Specs for the 6008 indicate that input impedance of AI is 144K ohms.  Much too low for what you are trying to do.  If you are stuck with using the 6008, I would consider using an instrumentation amp ( AMP02 comes to mind http://www.analog.com/en/amplifiers-and-comparators/instrumentation-amplifiers/amp02/products/produc...) 

to build up a single stage amp with a gain of 1 to act as a buffer.  High impedance input that won't load your microamp source, plenty of drive for the 6008 input.

[oryan]

Brilliant, thanks.