B ridge circuits have long been popu-lar for conditioning signals from re-sistive sensors. These circuits are sensitive to small changes in resistance, and they provide a differential output from a sin-gle current or voltage source. But the sensors you connect to a passive bridge with one measuring branch don’t pro-duce linear outputs. Temperature sensors such as RTDs produce small resistance changes as a function of temperature. You can linear-ize a bridge circuit’s output by adding external linearizing circuits. But adding op amps to linearize the output means you’ll need a bipolar power supply. The circuit in Figure 1 represents an active bridge providing a linear voltage output using a unipolar power supply. The circuit uses the popular Pt100 RTD, which has a resistance of 100 V at 0°C. Its temperature coefficient of 0.00385 V / V /ºC produces a 38.5-V in-crease in resistance from 0°C to 100°C. Thus, the resistance is 138.5 V at 100°C. In Figure 1, resistors R3 and R4 convert the output from
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