# Multifunction DAQ

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## psuedodifferential voltage measurement for PXI 6120

Hello,

I have a PXI 6120 and want to measure rms or p-p voltage and frequency of a sine wave at around 1Khz.
I am using a PXI 5411 as the source and the voltage level is set at around 5V or less. I can see the
frequency but the voltage is very low.

How do connect the source so that I can measure the proper voltage and frequency?
Message 1 of 7
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## Re: psuedodifferential voltage measurement for PXI 6120

memco,

First of all, how are you connecting your analog signal to the PXI-6120?  If your signal source is coming from the PXI-5411, then this is a grounded signal source, which requires connection to the AI<X>+ and AI<X>- terminals of the 6120.  This is shown in Figure 4-3 of the 6120 User Manual linked below:

NI 6115/6120 User Manual

For example, if you were measuring on channel 1 of the 6120, your connections would be to pins 33 and 66, which correspond to AI1+ and AI1-.  You should first check to see if you get a valid reading in Measurement and Automation Explorer (MAX) before trying to develop an application in LabVIEW or other programming language.
Message 2 of 7
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## Re: psuedodifferential voltage measurement for PXI 6120

`I have the outputs of the 5411 connected (5V @ 2000 Hz)Center BNC -> Pin 30Shield BNC ->  Pin 63 (or Pin 29 same result)Reading Dev9/ai3 0.325 Volts.And a scope connected to Pin 30 to observe to signal which reads correctly!I have tried other AI channels with the same result.If I apply a correction factor of 11.318 = (8*sqrt(2)) to the amplitude I get goodagreement over a range of 0-7 Volts. Also I measure the correct frequency.Ernest`
Message 3 of 7
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## Re: psuedodifferential voltage measurement for PXI 6120

Try to measure a DC voltage with the 6120 and see if this gives a good reading.  Also, make sure that your input range is set to less than +/-10V.  As shown on page 4 of the NI 6115/6120 Specifications, this will ensure that the input impedance on your analog channel is 1 MΩ in parallel with 100 pF.
Message 4 of 7
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## Re: psuedodifferential voltage measurement for PXI 6120

I have tried DC voltage measurements and it seems to be different from the AC value. I am having some
unrelated problems with LabView and cannot access the hardware at this time.

Ernest
Message 5 of 7
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## Re: psuedodifferential voltage measurement for PXI 6120

I have measured DC voltages with the 6120 over the range 0-5 Volts. and I am getting
Set    Measured
0.5    0.027
1.5    0.169
2.0    0.256
2.5    0.799
5.0    4.340

Message 6 of 7
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## Re: psuedodifferential voltage measurement for PXI 6120

memco,

Well those measured voltages certainly do not look good.  One thing you can try to do as well is try a loopback type test and run one of your analog output channels directly to an input.  In this case, we know that the ground reference will be the same.  Also, going back to your original issue measuring the output of the 5411.  Did you try changing the load impedance of the 5411?  If you are using the NI-FGEN Soft Front Panel, this can be changed in Edit >> Device Configuration.  Also, how old is the 6120?  Do you know when it was last calibrated?

Regards,

LA

Message 7 of 7
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