PXI 6030e Voltage measurement

[EngineeringEE]

Hello All,

I have a PXI 6030E DAQ with a SCB-68 connector board.  I am setting up a simple voltage divider and connecting my Vout to pin 68 on the SCB-68.  My ground is going to pin 67.  My Vin is 5 V DC.  I used DAQ assistant to automatically generate code to read the voltage and put it in a while loop.  My R1 is a device (Copper Interdigitated Electrodes coated with Dow Corning Silicone), and my R2 is 100 Mohm resistor.  I am using an OP Amp as an isolator between the impedance on my DAQ and my voltage divider.  Unfortunately, when I run this program it will not reach a steady value or progrssively drop the voltage.  Is there something I am missing here as to why it will not allow me to get the reading I want?

[James_D]

Why such high impedance values?

[EngineeringEE]

I am expecting a insulation resistance from my devices in the giga tera and peta ohm range.  It seems that my signal is acquiring a ton of noise.  How can I reduce the noise?  I am getting an accurate reading but it is oscillating between the values .0-.3 V.

[Kimberly_Wenger_Austin]

Hi EngineeringEE,

Have you noticed if the noise changes proportionally to any changes in the Vin value? In other words, if you were to adjust your voltage divider to have a Vin of 10 V, how does it affect the noise of your system?

Also, for you reference, this KnowledgeBase is a very helpful tool when measuring analog signals. Section 6 specifically references how to solve noise issues.  Field Wiring and Noise Considerations for Analog Signals

Thanks!

Kim W.

[EngineeringEE]

Hello Kim,

I solved my voltage dividing problems and get the correct voltage (it was a very silly mistake from lack of sleep).  Now I tried a wheatstone bridge following the example provided by NI for which the link can be seen below.  I am receiving a measurement and as I increase my Vex my Vo also increases.  The problem I am having is that the reading I am receiving is 1 volt higher than what I calculate and what I measure on a multimeter.  So when I use a 5V supply I should get a 2.25V Vout but instead I get a 3.39.  I am wondering if it has to do with my ground which goes to pin 67.  Is there a better wiring schematic than in the example.  All suggestions are welcomed.

http://zone.ni.com/devzone/cda/epd/p/id/6417

[EngineeringEE]

Also,

In the example provided above for the wheatstone bridge.  How do you get that VI that takes one output from the DAQ Assistant and splits it into 2 channels?  I draged the PNG file to my block diagram and nothing happened.  I have LABVIEW 7.0.

Thank you!!!

[GovBob]

The example you linked to was written in LabVIEW 2009. You are using LabVIEW 7 which does not support the drag and drop operation of the png. Though I haven't worked with LV 7 in a long time that primitive you are looking for is called the split signals and should be located somewhere under Functions>>Express pallette.

[EngineeringEE]

Thank you!  I actually found it last night 5 minutes after I wrote the message.  I constructed the Wheatstone bridge and used a differential reading from the 2 nodes connecting each leg.  Unfortunately I received a reading that is about 10 % higher than what I calculated.  When I do a reading with respect to ground (RSE) and probe both sides of the bridge separately (with a single channel, not simultaneaously) I simply subtract the reading I receive from each side and obtain the correct value.  Ideally when using the differential method in DAQ Assistant  it is suppose to give me the right reading, but why is this giving me a different value when DAQ assistant is configured to take a differential reading.  My program is exactly like the example provided by NI.  Except I am adding the values from each signal and outputing them to a numeric indicator vi. All suggestions are welcomed!

Thank you.

[Joe_S]

Hello,

If you are taking an RSE measurement, then you will want to set the DAQ assistant for RSE rather than differential.  Hope this helps!

Regards,

Joe S.

[EngineeringEE]

Hello Joe,

I figured out the problem.  I constructed a wheatstone bridge with resistors of a known value.  I used an op amp as a buffer to isolate the impedance of the DAQ from the circuit and it solved my problems.  Now, I have another issue.  When I use Interdigitated Electrodes to replace a resistor of a known value I receive a measurement.  The measurement output continuously oscillates from 3 values going from the highest to lowest repeatedly.  I am beginning to wonder if it has to do with the my devices acting as a charging and discharging capacitor.  All suggestions are welcomed!!

Thank you!