Derive op amp offset voltage
Let’s make V2 zero by connecting the U2 input to ground, and let’s calculate Vout1 (see Figure 2). The proof of this transfer function starts with the Superposition Theorem. It is well known that the instrumentation amplifier transfer function in Figure 1 is How do we derive the instrumentation amplifier transfer function? Because of that, R1 is designed to be equal with R3. If the resistors are not equal, the voltage difference between the two generates an offset, which is amplified and transmitted at the circuit output. A small input current flows into the Op Amp inputs and is converted into voltage by the input resistors. Although, in most analysis, the input current into an Op Amp is considered zero, in reality that is not the case.
#Derive op amp offset voltage generator
(See The Differential Amplifier Common-Mode Error Part 1 and Part 2 for more on this matter.)Īnother potential error generator is the input bias current. To minimize the common-mode error and increase the CMRR (Common-Mode Rejection Ratio), the differential amplifier resistor ratios R2/R1 and R4/R3 are equal. One example of such instrumentation amplifier is Texas Instruments’ INA128/INA129. If the amplifier is integrated on a single monolithic chip, RG is usually left outside so that the user can change the gain as he wishes. Because of that, one single resistor change, RG, changes the instrumentation amplifier gain, as we will see further. Its clever design allows U1 and U2 operational amplifiers to share the current through the feedback resistors R5, R6 and RG. These qualities make the IA very useful in analog circuit design, in precision applications and in sensor signal processing.įigure 1 shows one of the most common configurations of the instrumentation amplifier. Besides that, it is designed for low DC offset, low offset drift with temperature, low input bias currents and high common-mode rejection ratio. The Instrumentation Amplifier (IA) resembles the differential amplifier, with the main difference that the inputs are buffered by two Op Amps.