Will USB-6008 DAQ be able to power and record voltage for UMS T5 tensiometer at the same time?

Question

I would like to use my NI USB-6008 to provide power to my UMS T5 tensiometer  (http://www.ums-muc.de/en/products/tensiometer/t5.html) as well as take readings from it, but I don't know if this is possible to do properly. The supply voltage for the instrument can be as low as 5V, which I can easily get from the dedicated +5V channel. I am able to power the instrument and connect it to an analog input on the 6008 and measure a voltage in differential mode. However, while reading the support literature for the instrument, I find the following:
"Potential data acquisition pitfalls: Because the pressure transducer is configured in a Wheatstone full bridge, the input voltage and signal mV output cannot be connected to the same reference (ground). Hence, the signal mV output can only be measured using a differential voltage measurement. Therefore, do not attempt to make a single-ended measurement of the pressure transducer mV output."  (http://www.decagon.com/assets/Uploads/MeasuringUMSTensiometerswithnon-UMSControlandDataAcquisitionSystems.pdf)
My understanding is that the 6008 can take a differential measurement if I attach the signal "+" and signal "-" to the positive and negative analog input terminals. However, it seems that all of the Ground ports on the 6008 are grounded to the same reference, which would make my voltage measurement invalid according to the paragraph above. So my real questions is: if I attempt to record the voltage using one of the analog inputs on the 6008 in this way, is the measurement valid? Or do I need to find a separate power supply with a different reference ground to ensure that the measurement is accurate?

Answer 1

The specification sheet for that device is very poor. The manual is not much better. Companies which want to sell scientific equipment should publish decent data sheets or get out of business.
In section 3.4.3 General requirements the device is described as a "non-amplified bridge circuit."  This information along with the bridge impedance should be in the specifications because it is essential to applying the device under any circumstances other than the nominal behavior at 10.6 V.
The answer to your question is this:
You can use it with the USB-6008.  The 5 V power supply will result in output voltages slightly less than half (5/10.6) the voltages specified under nominal conditions. You can use the differential input mode on the USB-6008.  The absolute input voltages will be about 2.5 V with the 5 V supply. That voltage is within the working range of the device. The differences will probably be less than 100 mV. The resolution of the USB-6008 on the +/-1 V range is about 0.5 mV so your pressure resolution will be around 1 % of full scale. The input impedance and termination voltage of the USB-6008 will introduce some errors.  These may be on the order of 5-10%. I cannot predict much better without the missing bridge impedance specification.  Those errors should be systematic and relatively constant. A calibration of the whole system - sensor and DAQ device together should allow you to compensate for much of that error.
Lynn