Question 15.4: Determine the output voltage of the instrumentation amplifie...
Determine the output voltage of the instrumentation amplifier in the RTD circuit in Figure 15–27 if the resistance of the RTD is 1320 Ω at the temperature being measured.
The bridge output voltage is
\begin{aligned}V_{\text {OUT }(\mathrm{B})} & =\left(\frac{R_{\mathrm{RTD}}}{R_3+R_{\mathrm{RTD}}}\right) 15 \mathrm{~V}-\left(\frac{R_2}{R_1+R_2}\right) 15 \mathrm{~V}=\left(\frac{1320 \Omega}{2320 \Omega}\right) 15 \mathrm{~V}-\left(\frac{10 \mathrm{k} \Omega}{20 \mathrm{k} \Omega}\right) 15 \mathrm{~V} \\& =8.53 \mathrm{~V}-7.5 \mathrm{~V}=1.03 \mathrm{~V}\end{aligned}
From Equation (12–3), the voltage gain of the AD622 instrumentation amplifier is
R_{\mathrm{G}}=\frac{50.5 \mathrm{k} \Omega}{A_v-1}
A_v=\frac{50.5 \mathrm{k} \Omega}{R_{\mathrm{G}}}+1=5.05+1=6.05
The output voltage from the amplifier is
V_{\text {OUT(A) }}=(6.05)(1.03 \mathrm{~V})=6.23 \mathrm{~V}
P R A C T I C E EXERCISE
Solve for the nominal resistance of the RTD in Figure 15–27 that will balance the bridge at 25°C. What is the amplifier output voltage when the bridge is balanced?