How to compensate for mili-voltage output interaction with FieldPoint FP-AI-100 Module?

[SAFEX Engrg]

I have the white and green outputs of four Omega PX180B (100mV = 200 psig full scale) full bridge pressure transducers hooked to a FieldPoint FP-AI-100 analog input module.  I have a separate regulated 10V DC supply providing power to each transducer.  At zero pressure, FieldPoint Explorer and LabView read only small offset milivolts as expected.  When one transducer is exposed to 100 psig and the others are at zero, the exposed transducer milivolt signal is reduced to about 80 psig; while the other three read from 5 to 10 psig.  I think they are interacting?  How can I prevent this?  Can I ground the COM terminal of the FP-AI-100 module?  If so, to the 10V DC supply or to the FP-1000 module?

[Wayne.C]

Did you connect the common of the external 10VDC supply to the COM of the FP-AI-100 module?

[SAFEX Engrg]

Initially I did connect both the 10V and com of the power supply to the FP-AI-100 module.  I wound up with offset voltages of about 2.5 Volts at zero pressure.  Advice from NI tech rep was to connect the input ends of each transducer bridge to the supply independently from the FP module.  Since the FP-1000 is operating at 5 V DC, I'm also concerned about mismatch.

[Wayne.C]

Your description of the problem and the Omega PX180B data sheet confirms that this sensor you are using is just a strain gauge.  The problem is that the FP-AI-100 is a single ended input with the common of all inputs tied together.  This is what is causing the interaction.  You need differential inputs when dealing with strain gauges.  Options:

1. Purchase sensors that have either a 4-20mA output or 1-5V output.
2. Install/build some sort of a strain gauge signal conditioners that have a 4-20mA or 0-5V output that the FP-AI-100 can handle. 
3. Replace the FP-AI-100 with a FP-SG-140.

[SAFEX Engrg]

Thanks very much for the help.  It gives me some things to try.

[SAFEX Engrg]

Is there any other way, such as some programming in LabView, to deal with the interactions? Getting new sensors, signal conditioners, or purchasing the FP-SG-140 module will all blow my budget.

[Wayne.C]

Sorry, this is a hardware issue and it has to be fixed in hardware.  A couple more ideas to try:

• Take the green wire from sensor and connect to Vin of one channel.  Connect white wire to Vin of another channel.  Connect COM of AI-100 to common of 10V supply.  Measure both channels in your vi and add the two voltages together before making the conversion to PSI.  Would take twice as many AI channels but it might work.  I would read the entire module and the same time.

• You could also build your own simple signal conditioner.  Just need a good instrumentation amp set for a gain of 1.  AMP02 works well for this.

[Wayne.C]

Previous post should said to take the DIFFERENCE between the two measured values when trying the two channel method.   Duh!    Differential - Difference

[SAFEX Engrg]

Thanks for the additional suggestions.  I think I'll try connecting the white and green separately as you suggest since I have the other 4 channels of the FP-AI-100 that can be used.  What do you think of using two high value resistors (relative to the transducer resistance) to make another 5 volt reference from the 10V regulated suppy to connect to the FP-AI-100 common?  In this way I would be subtracting two milivolt signals instead of two about 5 volt signals?

[Wayne.C]

Not sure how that would work.  Can't hurt to try.