Instrumentation amplifier and it's applications

instrumentation

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An Instrumentation amplifier is an integrated circuit (IC) used to amplify a signal, which is a type of differential amplifier because it amplifies between two input signal.

In industries, physical quantities are converted into electrical signals using transducers and the signal is amplified for signal processing. For this, an instrumentation amplifier is used instead of an Opamp.

The importance of an instrumentation amplifier is that it can reduce unwanted noise that is picked up by the circuit. The ability to reject noise or unwanted signals common to all IC pins is called the common-mode rejection ratio (CMRR)

Always the input of an instrumentation amplifier is the output from the transducers and will a small signal. Instrumentation amplifiers don’t need input impedance that makes this amplifier suits for measurement purposes.

Circuit of Instrumentational amplifier:

instrumentation amplifier

The above shown is a three-opamp instrumentation amplifier. In this circuit, a non inverting amplifier is connected to both inputs of the differential amplifier.

The two amplifiers at the left side are connected together to form a combined Non-inverting amplifier are gain buffers with Rgain is removed. The amplifier at the right side is a standard differential amplifier.The Rgain resistor increases the differention mode gain of the buffer pair amplifiers, this increase the CMRR of the amplifier. The gain of the circuit is:

The Rgain resistor increases the differention mode gain of the buffer pair amplifiers, this increase the CMRR of the amplifier.

Main features:

High CMRR: The ability of amplifier to ignore the large common signal and amplify the small signal. The CMRR should be infinity by idealy. A good instrumentation amplifier must amplify only the differential input, completely rejecting common mode inputs.

High input impedence: Idealy the input impedence should be infinity. The sensor connected to the amplifier cannot provide energy to amplifier, so the amplifier input must have a high input impedence to avoid overloading of sensor.

DC coupled: Industrial low frequency requires a DC coupled amplifier. This means no capacitor must be used at the input side.

Low output impedence: The output impedance of a good instrumentation amplifier must be very low (ideally zero), to avoid loading effect on the immediate next stage.

Differential input: The noise generated at the senors is induced into both the inverting and non-inverting terminals of differential input and it is subtracted from itselt by the op-amp.

Single ended output: The output of the instrumentation amplifier is single ended and designed to match with displays and following amplifier.

Higher slew rate: The slew rate of the instrumentation amplifier will be as high as possible to provide maximum undistorted output voltage swing.

##Instrumentation amplifier connection with transducers:

instrumentation amp

The amplifier input terminals are connected to the output of transducer connect in a bridge circuit. Sensor such as thermistor, thermocouple, strain gauge, pressure switches are connected to one leg of the bridge. By making such a bridge as a part of the circuit, it is possible to produce an electrical signal proportional to the change in the physical quantity being measured.

Such an electrical signal can be amplified and used to monitor and control the physical process. An instrumentation amplifier can be constructed with a transducer bridge connected to one of its input terminals,

Applications:

  • Strain gauge bridge interface for pressure and temperature sensing.

  • A variety of low side and high side current sensing application

  • Medical instrumentation, used in ECG connection

  • Current/voltage monitoring

  • Audio application involving weak audio signal

  • High speed signal conditioning