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. A circuit providing an output based on the difference between two inputs times a scale factor is given in the above figure. 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. 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 is shown here. The only advantages of making an instrumentation amplifier using 2 opamps are low cost and improved CMRR. A high gain accuracy can be achieved by using precision metal film resistors for all the resistances. Because of large negative feedback employed, the amplifier has good linearity, typically about 0.
This has been the industry standard, high performance, low cost amplifier. The user can obtain any desired gain from 1 to using a single external resistor. Figure 3 below represents the configuration of the Instrumentation Amplifier using two Op-amps where V1 and V2 are the input voltages and V01, Vo2 are the outputs of the Op-amp 1 and Op-amp 2 respectively.
R1, R2, R3 are the resistors and the output stage of the Instrumentation Amplifier is a difference amplifier, whose output V out is the amplified difference of the input signals. The inputs of the two buffer Op-amps draw no current and hence the voltage drop across Rg is proportional to the differential voltage V1 and V2. This produces a current that runs entirely through the resistors R and the voltage produced acts as the input to the differential amplifier or Subtractor circuit.
All the Resistors except Rg are equal. Rg may be an external resistor connected across two pins of the IC. If the pins are not connected, then the gain of the amplifier is 1 but preferably different gains may be obtained by connecting a resistor of relevant value.
Alternatively, a number of resistors may be fabricated on the chip to give Gains of 1, 10, and Similar to the Op-amp circuit, the input buffer amplifiers Op-amp 1 and Op-amp 2 of the Instrumentation Amplifier pass the common-mode signal through at unity gain. The signal gets amplified by both buffers. The output signals from the two buffers connect to the subtractor section of the Instrumentation amplifier.
The differential signal is amplified at low gain or unity and the common-mode voltage is attenuated. The potential at node A is the inverting input voltage V 1. From the virtual short concept the potential at node B and G is also V 1. The potential at node D is the non-inverting input voltage V 2. Hence the potential at node C and H is also V 2. The current I through the resistors R 1 , R gain and R 1 remains the same as ideally the current to the input stage Op-amps is zero.
Theoretically, this means that the end user may obtain Gain in the front end as desired without increasing the common-mode gain and error. That is, the differential signal will be increased by gain and thus CMRR is directly proportional to gain.
The biggest disadvantage of Instrumentation Amplifier is the occurrence of noise when used for long range transmission purpose. Contact us: [email protected]. Inertial Nav Do you Know Home. Thursday, January 13, Sign in. Forgot your password? Get help.
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