04-23-2007 11:38 PM
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).
04-24-2007 04:20 PM
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).
04-25-2007 12:00 AM
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
04-26-2007 05:20 PM