Chapter 6, Problem 47E

A common application for instrumentation amplifiers is to measure voltages in resistive strain gauge circuits. These strain sensors work by exploiting the changes in resistance that result from geometric distortions, as in Eq. [6] of Chap. 2. They are often part of a bridge circuit, as shown in Fig. 6.61a, where the strain gauge is identified as R_G. (a) Show that $V_{\text{out}}=V_{\text{in}}\left[\frac{R_2}{R_1+R_2}-\frac{R_3}{R_3+R_{\text{Gauge}}}\right]$. (b) Verify that V_out = 0 when the three fixed-value resistors R1, R2, and R3 are all chosen to be equal to the unstrained gauge resistance R_Gauge. (c) For the intended application, the gauge selected has an unstrained resistance of 5 kΩ, and a maximum resistance increase of 50 mΩ is expected. Only ±12 V supplies are available. Using the instrumentation amplifier of Fig. 6.61b, design a circuit that will provide a voltage signal of +1 V when the strain gauge is at its maximum loading.

AD622 Specifications

Amplifier gain G can be varied from 2 to 1000 by connecting a resistor Between pins 1 and 8 with a value calculated by $R\frac{50.5}{G-1}\text{k}\Omega$.

FIGURE 6.61