A calibration engineer writes the following justification for skipping any check of…

PHYSICS · Multi-part · Fermi University Physics · Electronics, Instrumentation & Communication · Feedback, operational amplifiers and analogue signal processing · Sensors, detectors and experimental instrumentation

Electronics for Measurement, Ch Sensors, transducers and the measurement front end

Problem

A calibration engineer writes the following justification for skipping any check of the interface wiring: The

350\,\Omega

quarter-bridge strain gauge is fully specified by its sensitivity of

2.5\,\mu\mathrm{V}

per microstrain at

5\,\mathrm{V}

excitation. Once I know that number I can multiply it by the amplifier gain and I am done. Using this reasoning, for a measured strain of

240\,\mu\varepsilon

and a downstream difference amplifier of gain

1000

, the engineer predicts an amplifier output of

240\times2.5\,\mu\mathrm{V}\times1000 = 600\,\mathrm{mV}

. The difference amplifier has a differential input resistance of

2.1\,\mathrm{k}\Omega

, and the bridge (all four arms equal at

350\,\Omega

) presents a source resistance of

350\,\Omega

at its output terminals. ((a)) Name the entry missing from the engineer's specification of the sensor and explain why it changes the answer here. ((b)) Calculate the amplifier's actual output, and state by how many millivolts and what percentage the engineer's prediction is wrong.

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