DAQ board cannot read data correctly

[lz250616]

I want to get current value from potentiostat current output and read it in Labview. However, I found the value change a lot when I pinned the wires in SCC-68 AI3 and AI ground.

If I measure voltage from potentiostat current output directly with voltmeter, the value is 9.8mV.

When the wire is connected to DAQ board, the voltmeter measured value is 16.8mV.

If I just disconnect the wire in AI ground, the value is 9.4mV which should be right one.

If I just disconnect the wire in AI 3, the reading is 16.8mV again.

(See attachment pictures)

I also tried other channels of SCC-68 and NI-USB-6009 board but got the same result. How could I solve the problem?

Thank you very much.

[crossrulz]

Since your potentiostat is outputting a current, measuring the voltage is useless unless you have a known load.  Throw in a 500 Ohm resistor (I'm assuming a 4-20mA signal here) between the signal and ground and measure the voltage drop across that resistor.

[lz250616]

Thanks for your answer. Actually I know the ratio of current to voltage of that potentiostat is 10μA/V, which means I can do the conversion in Labview program if I could get the voltage value from potentiostat current output. But my question is why voltage changed to different number(16.8mV) when I connected the wires to DAQ board. I think it should show me the same value (9.8mV) as the one I measured directly from potentiostat.  

Thanks.

[Daniel_Eduardo]

Hello Iz250616,

Could you please specify which DAQ board are you using?

Thanks! 

[lz250616]

I tried both SCC-68 and NI-USB-6009 board.

[Dennis_Knutson]

The scc-68 is not a DAQ board. It is just a dumb terminal block. It has already been mentioned that you cannot simply connect a current output to a DAQ board. You can't simply connect it to a voltmeter either. You must use that known load as mentioned.

[Daniel_Eduardo]

You can also take a look to this white paper on some considerations to read analog signals.

http://www.ni.com/white-paper/3344/en/