A Wheatstone bridge is typically used to get measurements from resistive sensors that provide a small change of resistance. RTDs, strain gages, and load cells are typical examples.

http://en.wikipedia....eatstone_bridge

A load cell usually has a complete bridge inside, but an RTD or strain gage is typically a single resistive element, so the first step it to complete the bridge using 3 fixed resistors with the same value as the active element. Viewing the bridge as a diamond, an excitation voltage (Ve) is applied to the top and bottom, the bridge output is the voltage difference between the sides. If all resistors are exactly the same value, then the voltage on each side is Ve/2 compared to ground, and the difference (bridge output) is 0.0 volts. When the resistance of the active element (or elements) changes, you then get a small output voltage. Note that since each voltage is about Ve/2 compared to ground, the differential signal has a common-mode voltage of Ve/2.

The LJTick-InAmp (LJTIA) from LabJack is a 2-channel instrumentation amplifier that plugs into a couple analog input channels on a U3/UE9. The LJTIA has a 2.5 volt reference (Vref) that can source 25 mA. The is an excellent Ve source. Vref can supply 2 common 350 ohm or 1000 ohm bridges, or a single 120 ohm bridge. The relationship between change in the measured parameter (strain, load, temperature) and change in resistance is provided by the sensor manufacturer. Using standard bridge equations, this change in resistance can be related to bridge output voltage:

https://forums.labja...?showtopic=1273

The small differential output from a bridge is typically connected to an instrumentation amplifier or in-amp, such as the LJTIA from LabJack. An in-amp converts the differential voltage to single-ended (a single voltage referenced to ground), and amplifies the voltage. For the LJTIA, the relationship between input voltage (Vin, bridge output) and output voltage (Vout) is:

Vout = (Vin*Gain) + Voffset

Use the signal range tables in Appendix A of the LJTIA datasheet to choose Gain and Voffset. You want the highest gain you can use. Vout is the voltage measured by the U3/UE9. The relationships above can be combined to produce an equation relating measured parameter to Vout, but it is better to do a calibration by applying 2 known conditions (e.g. known loads on a load cell), noting the Vout produced at these points, an creating your own equation to relate the parameter to Vout. Here is an online calculator that takes 2 XY coordinates and provides the slope and offset of a line through those coordinates:

http://www.analyzema...Calculator.html

A good way to troubleshoot bridge circuits, is to provide a known condition (e.g. known load on a load cell), calculate the expected bridge output voltage and LJTIA output voltage, and measure those with a DMM. ****** Update: The new U6, available mid-May 2009, features high resolution and a built-in instrumentation amp, and thus can take measurements directly from a bridge.