Op Amp Offset Voltage and Current

EE446 Report

To: J.A. Svoboda

From:

Date:

Subject: Effect of Op Amp Offset Voltage and Current on an Instrumentation Circuit

Introduction: The circuit shown in Figure 1 produces an output voltage, v_o, that represents the temperature T, in degree C, measured by the transducer. The nonideal op amp represents the LM 307 op amp manufactured by National Semiconductor Corporation (http://www.national.com/ds/LM/LM107.pdf#page=1&search=%22LM307%22).

Figure 1. A temperature transducer and amplifier.

Conclusion: After choosing R₃ = … to minimize the effects of the op amp offset, the maximum output offset voltage is e_os = … which is equivalent to an input temperature of T = …

Results:

R1: Accounting for the offsets introduced by the op amp, the output voltage is given by

v_o = …

R2: After choosing R₃ = … to minimize the effects of the op amp offset, the maximum output offset voltage is

e_os = …

R3: The maximum output offset voltage is equivalent to an input temperature of T = …

Findings:

F1.1 The nonideal op amp is modeled as shown in Figure 2.

Figure 2. Op Amp Model

F1.2 The specifications for a LM307 op amp at room temperature indicate that | v_os | < …, | i_B | <…, and | i_os | < ….

F1.3 Replacing the nonideal op amp by its model gives the circuit shown in Figure 3.

Figure 3.

F1.4 Using superposition (provide derivation), the output voltage of the circuit shown in Figure 3 is found to be

v_o = …

F2.1 To minimize the effects of the offsets, choose R₃ …. (Provide derivation showing how to choose resistance R₃.) Then, choose R₃.

F2.2 After choosing R₃ = … to minimize the effects of the op amp offset, the maximum output offset voltage is

e_os = …

F2.3 The PSpice simulation shown in Figure 4 confirms this value of output offset voltage.

Figure 4. Replace this Figure with your simulation.

F3.1 The maximum output offset voltage is equivalent to an input temperature of T = …