Chapter 4, Problem 4.101IP

Instrumentation Amplifier with Alarm
Strain gauges measuring the deflection of a sintered metal column are connected to a Wheatstone bridge. The output of the bridge is balanced when there is no strain producing 0 V output. As the column is deflected, the bridge produces $150\text{μV/Ω}$ change caused by the strain gauges. The maximum possible defection would result in a $4\text{-Ω}$ change. Design an instrumentation amplifier that can take the voltage created in the bridge and send it to a 0—5 V ADC, where no deflection produces 0 V at the ADC and maximum deflection produces 5 V. To avoid loading, send the output of the bridge to a very high input resistance differential amplifier. The columns being tested are brittle and can shatter violently if the conditions cause the strain gauges to change by more than $3.9\Omega .$ For safety, connect the output of your instrumentation amplifier to a comparator circuit that will trigger an alarm when the strain causes the resistance to change by $3.75\Omega .$ The alarm needs to be triggered by 15 V.