NI USB 4431 - Not reading correct voltage

[Jimbo76]

Hi,

I have an NI USB 4431 and we're using it to measure voltage from a little op-amp circuit. The circuit has a 50 Ohm BNC output which works fine with every single other device - oscilloscopes, spectrum analysers, etc. When we attach this board to the 4431, the voltage we measure is consistently too low by about a factor of 5 compared to, say, an oscilloscope. So a 50 mV p.p signal reads as about 10 mV p.p.

Obviously I'm missing something regarding the input of this device. I've tried attaching it to a function generator and the 4431 correctly reads the voltage set on the generator. But for some reason it hates this op-amp board (although everything else seems fine with the board). The only thing I can think of is some wierd impedence issue but the impedence of the input (200 kOhm if memory serves me correctly) should be fine. Anyone experienced anything like this with the 4431 and attaching it to external electronics?

Cheers!

[IanS]

Hi Jimbo,

As you say, the imput impedence is 200kOhm on this device between + and - input, and 1kOhm between - and ground.

As you can see the device functioning correctly with FGEN input, it must be to do with the external circuitry.

Perhaps you could post a circuit diagram?

If the problem is with input impedance being too high, this can be reduced by using a unity gain buffer, but as I understand, you are measuring output from an opamp circuit, which should itself have low output impedance. 

[Jimbo76]

Hi Ian,

Thanks for that ... good idea about the unity gain buffer. I'll give that a try.