DAQ card pressure sensor connections - voltage offset

[Rob T]

Hi,

I'm working in labview to create a PID control loop with a flow meter and pressure sensor for pressure control. However, I cannot get reliable results from the pressure transducer using a PCI-6024E aquistion card and labview 6.0. My problem is that when i connect the pressure transducer wheatstone bridge across differential inputs to the DAQ card, I get a voltage offset. I have checked using the MAX test panel that my input is connected to the correct channels under a diff input setup, and have even setup a test bridge from resistors but cannot get rid of the offset.

For example when applying 5V across a wheatstone bridge made up of 4 562 ohm resistors, if i connect the DAQ inputs I get a reading of 0.13V. I
have the AIGND and power supply earth connection connected to the base of the bridge so i don't think it is a grounding problem.

If I probe the bridge without the DAQ connected I get a reading of 2.5V from both sides of the bridge. however as soon as i connect the DAQ card, one side of the bridge (the one connected to the higher of the pair of diff input channels) reads 2.37V, creating this offset.

I've tested using other channels on the DAQ so its not that one pair are a little dodge.

I've spent all week frustrated by this problem, any help is much appreciated!

Rob.

[markwysong]

Rob,

I'd like to know a couple of things. First, what are you using to connect signals to your MIO board? A terminal block from NI?

Second, when reading in bridges, I've not connected the AI GND when using differential mode. I've always used A0 and A8 as the + and - respectively, and it's worked. I can't imagine that you're getting current through the input of the card--the inputs have extremely high impedances, certainly well abive a 562 ohm bridge.

Try removing the ground wire, and make sure that you have set up your MIO input as differential. Can you also see this offset in the MAX test panels?

Mark

[Rob T]

I'm using a BNC-2110 National instruments connection block to connect signals to my MIO card. If I remove the AIGND connection, it doesn't seem to affect the offset but seems to increase the noise on the input to the -ve differential connection (not sure why...).

I'm certain that the card is in differential mode, and i see exactly the same offset in the MAX test panels or if I actually probe across the bridge with a voltmeter.

Its acting as if there is a 5123 ohm resistor to earth connected to the -ve input of the diff connection which unbalances the bridge - and this is the same whichever pair of channels I use for the input.

Cheers, Rob.

[markwysong]

There is, Rob. You are correct! I read the manual for the BNC-2110. If you have the switch below the channel to the "FS" setting (floating signal), a 5K resistor is connected from the (-) signal to AIGND. If you put the switch in GS mode (ground-referenced source), this resistor is removed. Therefore, I suggest you change the switch to GS mode, removing this resistor which is causing your bridge to change.

Mark

[Rob T]

Many Thanks. I should perhaps have read the connector block manual more thoroughly. Rob.