4 wire resistance measure for USB-6281 DAQ

[tzw23]

Hello.  I am trying to set up a 4-wire resistivity measurement across a small piece of doped GaAs (R~10kohm).  I am sourcing current at the outer pads through a Keithley 2400 and am then trying to measure the voltage across the inner pads using the 6281.  In the set up screen on MAX for 4-wire resistivity, it only shows two connections that need to be made to the DAQ and then asks for me to enter the Isrc.  I am not getting a good reading from this.  I have also tried to just do a voltage input in RSE and then solve the resistance manually.  When I do this, the resistance readings pick up a very strong 60Hz noise signal since the measurment is floating and the wires are quite long, after filtering this, I am left with a much lower voltage signal than I expect for what the resistance of the sample should be.  I have combed through everything that I can find on the website and have tried several different solutions.  Am I missing something simple?  Should I be using NRSE with two resistors to common ground?  I am sourcing DC 5uA.

Thanks

[AnalogKid2DigitalMan]

tzw23:

I don't have a direct answer regarding the use of the DAQ, but why can't you use the 4 wire resistance measurement that the 2400 can do itself?

-AK2DM

[tzw23]

Ultimately, I want to do noise measurements using 4-probe and the Keithley isn't capable of doing anything in the KHz regime.

[AnalogKid2DigitalMan]

I concur with the use of the DAQ then.

I would suggest going differential, try to shorten your wire lengths as musch as possible. Preferably use a sheilded twisted pair with the shield to earth ground only at one end.

The 6281 looks like it has a decent CMRR of 110dB up to 60Hz. Since your signal is only 50mV, use the +/-0.1V range for the analog input.

Hope this helps.

-AK2DM

[HeatherBaker]

Good afternoon TWZ23,

Every DAQ card measures voltage, thus when you create a resistivity task in MAX it is programmatically dividing by the current to display resistance.  Our cards are typically designed to measure pure resistive loads.  Does your GaAs have any capacitive or inductive properties? What values are you expecting versus what you are actually seeing?  How is the grounding of your setup?  Is your GaAs sample grounded?  Please look at the Field Wiring and Noise Considerations article: http://zone.ni.com/devzone/cda/tut/p/id/3344