hardware//electronics//Wheatstone bridge
A Wheatstone bridge is a circuit of four resistances arranged as two voltage dividers side by side, fed from the same supply, whose output is the difference between the two midpoints; it is how electronics reads a resistance that changes by a fraction of a percent, and it sits behind every strain gauge, load cell, pressure transducer and many resistive temperature sensors. When the four resistances are in the right ratio the two midpoints are at the same voltage and the output is zero; when one changes, the output moves in proportion.
A Wheatstone bridge is a circuit of four resistances arranged as two voltage dividers side by side, fed from the same supply, whose output is the difference between the two midpoints; it is how electronics reads a resistance that changes by a fraction of a percent, and it sits behind every strain gauge, load cell, pressure transducer and many resistive temperature sensors. When the four resistances are in the right ratio the two midpoints are at the same voltage and the output is zero; when one changes, the output moves in proportion.
The trick is to throw away what does not change. A 350 Ω gauge strained by 1000 microstrain changes by about 0.7 Ω, 0.2% of its value: measured directly, that change would ride on a voltage hundreds of times larger, and every drift of the supply or the converter would swamp it. In the bridge the large common part cancels between the two halves, and only the change reaches the output. With one active gauge of gauge factor GFGFGF under strain ε\varepsilonε (a quarter bridge), the output relative to the excitation voltage is
VoutVex≈GF ε4.\frac{V_{\text{out}}}{V_{\text{ex}}}\approx\frac{GF\,\varepsilon}{4}.VexVout≈4GFε.
For GF=2GF=2GF=2 and 1000 microstrain that is 0.5 mV per volt, 2.5 mV with a 5 V supply, which is why bridge outputs are quoted in mV/V and why the signal goes straight to an instrumentation amplifier and a high-resolution converter (analog-to-digital converter).
More active arms buy signal and cancel temperature at once.
A full bridge of four gauges, two stretched and two compressed, gives four times the quarter bridge's output; and because all four gauges warm together, a change of temperature moves both halves alike and largely cancels, where a quarter bridge reads it as strain. That is why a load cell is a full bridge rated around 2 mV/V.
Ratiometric reading removes the supply. Since the output is proportional to the excitation, a converter that uses the same excitation as its reference measures the ratio directly, and a supply that sags by 1% stops being an error at all.
The leads are part of the bridge. In a quarter bridge the wire resistance to a distant gauge sits in the active arm and drifts with temperature; three-wire and six-wire (remote sense) connections exist to take it out, and the cabling is where many field strain readings go wrong (measurement chain).
Millivolt signals pick up everything: mains hum, motor drive noise, thermocouple effects at connectors. Shielding, twisted pairs, filtering before the converter and a signal-to-noise ratio budget are part of using one, as the sensor note says of transducers in general.