Instrumentation Amplifier | Instrumentation Amplifier Circuit | Instrumentation Amplifier Gain | Instrumentation Amplifier IC |Gain Of Instrumentation Amplifier| AD620 Instrumentation Amplifier | Instrumentation Amplifier Equation | Wheatstone Bridge Instrumentation Amplifier | Instrumentation Amplifier Vs Op Amp

Instrumentation amplifiers are essential in precision measurement and data acquisition systems. They amplify small differential signals in the presence of large common-mode voltages, providing accurate and stable outputs. This guide explores the working principles, circuits, gain calculations, common ICs, and key applications of instrumentation amplifiers.

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What is an Instrumentation Amplifier?

An instrumentation amplifier is a specialized amplifier that provides high input impedance, low output impedance, and excellent common-mode rejection. It is commonly used in medical, industrial, and measurement equipment where precision and low noise are critical.

Key Features:

• High Common-Mode Rejection Ratio (CMRR): Essential for amplifying signals in noisy environments.
• High Input Impedance: Prevents signal source loading.
• Low Offset Voltage: Reduces measurement errors.

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Instrumentation Amplifier Circuit

The basic circuit configuration of an instrumentation amplifier consists of three operational amplifiers (op-amps):

• Input Stage: Two op-amps configured as buffers to ensure high input impedance.
• Difference Stage: A third op-amp is used to provide the differential gain and reject common-mode signals.

Typical Circuit Diagram

      Vin+ ----|> Buffer A ----|
                                           |---- Amplification Stage ---- Vout
        Vin- ----|> Buffer B ----|

Instrumentation Amplifier Gain

The gain

G of an instrumentation amplifier is determined by a set of external resistors. The most common configuration allows for gain adjustment using a single resistor 

Rgain​.

Gain Equation:

​​Where:

• R1​ is the resistor connected to the input op-amps.
• Rgain​ is the external resistor for gain adjustment.

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Instrumentation Amplifier ICs

Several integrated circuit (IC) instrumentation amplifiers are available, each optimized for specific performance criteria such as low power, high speed, or low noise.

Common Instrumentation Amplifier ICs:

• AD620 (Analog Devices): A low-power, high-accuracy instrumentation amplifier with adjustable gain using a single external resistor.

  • Supply Voltage: ±2.3V to ±18V.
  • Gain Range: 1 to 1000.
  • CMRR: Up to 120 dB.

• INA128/INA129 (Texas Instruments): Precision instrumentation amplifiers with low offset and high CMRR.

  • Supply Voltage: ±2.25V to ±18V.
  • CMRR: Greater than 120 dB.

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Gain of Instrumentation Amplifier

The gain is the ratio of the output signal amplitude to the input signal amplitude. Instrumentation amplifiers allow easy gain adjustment by changing the value of a single resistor. The overall gain depends on the input stage configuration and resistors.

AD620 Instrumentation Amplifier

The AD620 is popular for its ease of use, low power consumption, and precision in signal amplification.

Key Specifications:

• Input Offset Voltage: Low, typically less than 50 µV.
• Power Supply: Operates on dual or single supply.
• Applications: Often used in medical instrumentation, strain gauge sensors, and transducer interfacing.

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Instrumentation Amplifier Equation

For a standard 3-op-amp instrumentation amplifier, the output voltage 

Where:

• Vin+​ and 
  Vin−​ are the input voltages.
• G is the gain set by external resistors.

Wheatstone Bridge with Instrumentation Amplifier

Instrumentation amplifiers are frequently used with Wheatstone bridge circuits to amplify small differential voltages. This combination is common in applications like strain gauge measurements, where minute changes in resistance must be accurately detected and amplified.

Key Points:

• The bridge converts small changes in resistance into a measurable voltage.
• The instrumentation amplifier boosts this small voltage with high accuracy and minimal noise.

Instrumentation Amplifier vs. Op Amp

Differences Between Instrumentation Amplifiers and Standard Op Amps:

• Input Impedance: Instrumentation amplifiers have a higher input impedance compared to standard op-amps.
• Common-Mode Rejection: Instrumentation amplifiers have superior CMRR, essential for differential signal amplification.
• Configuration: Instrumentation amplifiers are specifically designed with multiple op-amps to achieve balanced differential amplification, while op-amps are versatile building blocks for general-purpose amplification.

FAQs

1. What are the main applications of instrumentation amplifiers?

Instrumentation amplifiers are used in precision applications such as ECG machines, strain gauge sensors, and data acquisition systems.

2. Why use an instrumentation amplifier instead of a regular op-amp?

Instrumentation amplifiers provide higher CMRR and input impedance, which are essential for accurate differential signal measurement and noise rejection.

3. What is CMRR in instrumentation amplifiers?

CMRR (Common-Mode Rejection Ratio) is a measure of an amplifier's ability to reject common-mode signals, ensuring that only the differential signal is amplified.

4. How do you adjust the gain in an instrumentation amplifier?

The gain can be adjusted using an external resistor connected between specific points in the circuit. For ICs like the AD620, a single external resistor sets the desired gain.

5. What is the role of an instrumentation amplifier in a Wheatstone bridge?

It amplifies the small differential output voltage from the Wheatstone bridge, making it suitable for measurement and processing in precision applications.

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Conclusion

Instrumentation amplifiers are indispensable in scenarios where small differential signals must be amplified with high accuracy. Their high input impedance, excellent CMRR, and low offset voltage make them ideal for medical, industrial, and sensor applications. Understanding how to configure and use ICs like the AD620 enhances performance in signal conditioning circuits.