Vref Suppose the strain gages in the bridge in Fig. 5.21 have the value 120 2 1%. The power supplies to the op amp are +15 V, and the refer- ence voltage, Uref, is taken from the positive power supply. a) Calculate the value of Rf so that when the strain gage that is lengthening reaches its maximum length, the output voltage is 5 V. b) Suppose that we can accurately measure 50 mV changes in the output voltage. What change in strain gage resistance can be detected in milliohms? + R-AR, R + AR R + AR R- AR Rf www +Vcc -Vcc + Vo Rf Figure 5.21 An op amp circuit used for measuring the change in strain gage resistance.