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.

Question

Accepted Answer

The bridge output voltage is

[latex]\begin{aligned}V_{ ext {OUT }(\mathrm{B})} & =\left(\frac{R_{\mathrm{RTD}}}{R_3+R_{\mathrm{RTD}}} ight) 15 \mathrm{~V}-\left(\frac{R_2}{R_1+R_2} ight) 15 \mathrm{~V}=\left(\frac{1320 \Omega}{2320 \Omega} ight) 15 \mathrm{~V}-\left(\frac{10 \mathrm{k} \Omega}{20 \mathrm{k} \Omega} ight) 15 \mathrm{~V} \& =8.53 \mathrm{~V}-7.5 \mathrm{~V}=1.03 \mathrm{~V}\end{aligned}[/latex]

From Equation (12–3), the voltage gain of the AD622 instrumentation amplifier is

[latex]R_{\mathrm{G}}=\frac{50.5 \mathrm{k} \Omega}{A_v-1}[/latex]

[latex]A_v=\frac{50.5 \mathrm{k} \Omega}{R_{\mathrm{G}}}+1=5.05+1=6.05[/latex]

The output voltage from the amplifier is

[latex]V_{ ext {OUT(A) }}=(6.05)(1.03 \mathrm{~V})=6.23 \mathrm{~V}[/latex]

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?

Question 15.4: Determine the output voltage of the instrumentation amplifie...

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