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how to measure differential DC voltage across a resistor using SCB68, NI PCI-6221 and LABview 8

Hi, I am new at using LABview and DAQ card. I have a SCB68 I/O and NI PCI-6221 (16-Bit, 250 kS/s, 16 Analog Inputs). I am using LABview 8.0.

 

   I am trying to measure a differential DC voltage across a resistor. The value of the voltage should be around 0.10V to 0.30V.  My main objective is to acquire the change in the differential voltage and the range should be around the above said values.

 

  I have the resistor connected the resistor in series with a power supply and I am trying to measure the drop in voltage across the resistor.

 

  I have the input to a DAQ channel from across the resistor. I tried using the DAQasst but I am getting a strange erroneous value which is around 2.4V always. But if I try to measure the output of a DC power supply by connecting the ends of the supply to a DAQ channel, I get the right values.

  

Could anyone please let me know what kind of connections need to be made to the SCB and also the VI to measure a differential DC volatge across the resistor( it is a floating source).

 

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Hello Techtrial,

I see that you are using a PCI-6221 with SCB-68 and programming in LabVIEW 8.0. In your application you are measuring a differential voltage.

To troubleshoot this particular issue, I would first start with the program Measurement and Automation Explorer. You can find by clicking on Start>>All Programs>>National Instruments>>Measurement and Automation Explorer (MAX). By using MAX you can eliminate the programming side of the issue. MAX allows you to configure and test your hardware before you involve the programming environment.

In MAX, expand the tree under Device and Interfaces>>NI DAQmx Devices>>PCI-6221. Highlight your device and then click on the button that says: Test Panels. Here you can select the Max and Min input ranges, input configurations, the channel you want to measure, etc. Once you have done the configuration you can now press "Start" to acquire data.

When you do it this way, do you still see the device read incorrectly? What resistor value are you using? What voltage are you applying?

In terms of connection of a resistor to the SCB-68 device, connect the terminals to the corresponding terminals on SCB-68, ai0(+) and ai8(-), if you have a floating data source you want to connected in the way it states in the M Series User Manual (page 4-14).


Give these suggestions a try.

Regards,

 Sandra T.

Applications Engineer | National Instruments

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Sandra,

 

Thanks for the reply. We followed the steps mentioned by you, but still we aren’t successful in getting the differential value. When we measure the voltage across the resistance using a multi-meter we get around 1.3 to 1.5V, whereas in Lab-view we are getting 2.4 volts irrespective of any input and it remains constant on the graph. But when we select the numeric value it gives some random value.

 

Actually we experimented by directly connecting DC supply to the AI channels and we varied the voltage and we get approximately the correct values. But the same thing when we connect across the resistor, it gives us the const 2.4V. We tried different channels but still the output is same. Below is our setup and that would make the scope clearer. Could this constant value of 2.4V be because of the CMRR?  

 

Follow this link: http://rapidshare.com/files/27790781/testbed.doc

 

Thanks

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Hello Techtrial,

Thank you for posting back with more information. I see you have a bias resistor connected between the (-) end terminal and AIGND. It is important to provide a return bath for bias currents. This additional connection will ensure your source will not float out of the maximum working range of the device.

There are a couple things to consider when making a measurement like this. What type of source are you using DC-coupled vs AC- coupled? When I say source, I am referring to the floating signal source you are trying to measure.

For a DC-coupled sources with low source impedances (<100ohm), connect the negative side (AI-) of the signal to AI GND with a wire.
For a DC-coupled sources with larger source impedance, you would connect negative line with a resistor that is about 100 times the equivalent source impedance.
If something is unclear refer to page 4-7 of the M Series User Manual for more clarification.

With regards to the rest of your circuit, what cable are you using to connect your PCI-6221 to your SCB-68?
Have you tried measuring the voltage across the resistor with a common hand held multimeter?
What if you remove the PDA from your circuit are you seeing erroneous measurements?

Also consider ohms law, V=IR with the R=1ohm and the current= 2.4A (I'm uncertain if this is a maximum spec or actual value you measured) then the expected voltage would be 2.4V. This is assuming that the current does not drop in your circuit. Could this be something to consider in your circuit?

Regards,
  Sandra T.

Applications Engineer | National Instruments
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