EC-8.3 Bridges and Instrumentation Amplifiers
You can explain why a balanced bridge measures a small change better than a direct reading does, size a bridge around a strain gauge, and say what an instrumentation amplifier gives you that a plain difference amplifier does not.
Before:S4-5. Probability Theory & Stochastic ProcessesPE2-4. Sensing Techniques and Sensor SystemsEC-1. Circuit AnalysisUnlocks:EC-13. Laboratory Practice, in Sequence
Most quantities worth measuring arrive as a very small change on top of a large steady value, and reading that directly wastes almost all of an instrument's range on the part you already knew. The bridge is the classical answer and it is still the right one, which is why nearly every load cell, pressure sensor and strain gauge in service is wired as one.
Work through these
The Wheatstone bridge, and why balance beats measurement
Four resistors in two dividers, with the reading taken between their midpoints. At balance the reading is zero, so the result depends on the ratio of known resistors rather than on the accuracy of the meter itself.
NPTEL: Electrical Measurement and Electronic Instruments (IIT Kharagpur) · CourseAC bridges, and measuring inductance and capacitance
Drive the same arrangement with a sinusoid and balance now needs the magnitude and the phase to match. That is what lets a bridge return inductance, capacitance and the loss that comes with each.
NPTEL: Electrical Measurement and Electronic Instruments (IIT Kharagpur) · CourseThe strain gauge, and the bridge built around it
A strain gauge changes resistance by a fraction of a per cent under load. Putting one, two or four gauges into the bridge arms raises the output and cancels the change temperature alone would have caused.
NPTEL: Transducers for Instrumentation (IIT Delhi) · CourseWhy a difference amplifier is not enough
A single operational amplifier difference stage loads the two inputs unequally and its rejection depends on four resistors matching exactly. Both problems get worse as the source impedance rises.
NPTEL: Transducers for Instrumentation (IIT Delhi) · CourseThe instrumentation amplifier, and common-mode rejection
Two buffer stages in front of the difference stage give very high input impedance and set the gain with one resistor. The figure that matters is common-mode rejection, meaning how much of the voltage shared by both inputs survives.
NPTEL: Transducers for Instrumentation (IIT Delhi) · CourseLoading, and the input impedance you actually need
An amplifier draws some current from whatever it measures, and that current shifts the very reading you wanted. The rule is that the input impedance should be far larger than the source, and how far depends on the accuracy you need.
NPTEL: Electrical Measurement and Electronic Instruments (IIT Kharagpur) · Course
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