Instrumentation amplifier

schema novembre 12, 2011

Instrumentation amplifier is a kind of differential amplifier with additional input buffer stages. The addition of input buffer stages makes it easy to match (impedance matching) the amplifier with the preceding stage. Instrumentation are commonly used in industrial test and measurement application. The instrumentation amplifier also has some useful features like low offset voltage, high CMRR (Common mode rejection ratio), high input resistance, high gain etc. The circuit diagram of a typical instrumentation amplifier using opamp is shown below.

Instrumentation amplifier using opamp

In the circuit diagram, opamps labelled A1 and A2 are the input buffers. Anyway the gain of these buffer stages are not unity because of the presence of R1 and Rg. Op amp labelled A3 is wired as a standard differential amplifier. R3 connected from the output of A3 to its non inverting input is the feedback resistor. R2 is the input resistor. The voltage gain of the instrumentation amplifier can be expressed by using the equation below.

Voltage gain (Av) = Vo/(V2-V1) = (1 + 2R1/Rg ) x R3/R2

If need a setup for varying the gain, replace Rg with a suitable potentiometer. Instrumentation amplifiers are generally used in situations where high sensitivity, accuracy and stability are required. Instrumentation amplifiers can be also made using two opamps, but they are rarely used and the common practice is to make it using three opamps like what shown here. The only advantages of making an instrumentation amplifier using 2 opamps are low cost and improved CMRR.

A simplified instrumentation amplifier design is shown below. Here the resistances labelled R1 are shorted and Rg is removed. This results in a full series negative feedback path and the gain of A1 and A2 will be unity. The removal of R1 and Rg simplifies the equation to Av = R3/R2.