Instrumentation Amplifier Example, Circuit Diagram, Advantages, and Applications

Among the many types of special amplifiers, instrumentation amplifiers are undoubtedly the most widely used. The application fields of instrumentation amplifiers cover many industrial and measurement fields, which place very high demands on amplifiers. In these applications, DC accuracy and gain accuracy need to be maintained in high-noise environments while dealing with interference from large amounts of common-mode signals, typically AC power line frequencies. These requirements make instrumentation amplifiers an indispensable choice.

What is an Instrumentation Amplifier

An instrumentation amplifier is a modified differential amplifier that plays a key role in many measurement and electronic instrumentation applications. This amplifier design offers a range of powerful features, including very low DC offset and drift, excellent noise performance, highly reliable open-loop gain, excellent common-mode rejection ratio, and high input impedance. The design of instrumentation amplifiers makes them suitable for circuits that require extremely high accuracy and stability.

Its primary purpose is to amplify differential signals, and it is particularly good at providing common-mode rejection (CMR). The key to CMR functionality is the elimination of the effects of signals from both input channels having the same potential. That is to say, it can effectively eliminate interference signals with the same potential in the input signal and only amplify those signals with potential differences.

Instrumentation amplifiers (In-Amp) are typically used for low frequency signals (below 1 MHz) to provide a high degree of signal gain. It can amplify the input signal and effectively suppress the common mode noise present in the input signal.

In general, a typical instrumentation amplifier configuration consists of three op amps and some resistors. To achieve the best CMRR (Common Mode Rejection Ratio), high precision resistors (0.1% tolerance or better) are usually used.

The figure shows the pin configuration and physical appearance of the AD620 In-Amp chip (instrumentation amplifier). This is a standard high performance, low cost amplifier. It is available in 8-pin DIP and SOIC packages and is fully integrated on a single chip. The user can adjust the desired gain using external resistors ranging from 1 to 1000. For designs with gains of 10 and 100, fixed resistor values are standard, and 1% metal film resistor values are used.

Instrumentation Amplifier Example

• LM324 instrumentation amplifier

LM324 is a four-op amp integrated circuit, as shown in the figure below, which is the main component. What makes it unique is the integration of four functionally independent op amps into the same chip. There are many benefits to using the LM324. In other words, it can significantly reduce device performance differences between each op amp caused by manufacturing process differences. In addition, using a unified power supply helps reduce power supply noise and improve circuit performance. At the same time, the basic working principle of the circuit remains unchanged.

• AD620 instrumentation amplifier

The circuit is primarily based on the single-chip integrated circuit AD620, as shown in the diagram below. Its characteristic feature is its simple circuit structure, consisting of a single AD620, a gain-setting resistor Rg, and a power supply. Therefore, the design efficiency is very high.

The gain calculation formula for the circuit in the diagram below is: G = 49.4K/Rg + 1.

• LM741 instrumentation amplifier

A three-operational amplifier (op-amp) instrumentation amplifier circuit is formed by three general-purpose op-amps LM741, supplemented by external resistive peripheral circuits. Simultaneously, a bridge signal input circuit is added to the in-phase input terminals A1 and A2, as shown in the diagram below.

Here are some examples of IC instrumentation amplifiers known for their performance and accuracy:

INA128: The INA128 is a well-regarded instrumentation amplifier known for its high precision and wide gain range. It's designed for applications requiring low noise and excellent CMRR, such as medical instrumentation and industrial sensors.

AD8221: The AD8221 is an instrumentation amplifier from Analog Devices, known for its versatility and robust performance. It offers low offset voltage, low noise, and exceptional CMRR, making it suitable for precision applications in industrial and medical fields.

LT1167: The LT1167, manufactured by Linear Technology (now part of Analog Devices), is another IC instrumentation amplifier recognized for its precision and low noise characteristics. It is commonly used in industrial and automotive applications where accuracy is critical.

MAX4194: The MAX4194, produced by Maxim Integrated, is an instrumentation amplifier known for its low offset voltage and low noise. It provides high CMRR and is suitable for various precision measurement tasks.

Types of Instrumentation Amplifier

Type of Instrumentation Amplifier	Description
Three-Op-Amp Instrumentation Amplifier	This is the most basic type, consisting of three operational amplifiers and external resistors to set the gain. It's widely used for its simplicity and versatility.
Two-Op-Amp Instrumentation Amplifier	A variation of the three-op-amp design, this type utilizes two operational amplifiers, offering simplicity and lower power consumption but with slightly reduced performance.
Current Feedback Instrumentation Amplifier	This type employs a current feedback configuration rather than voltage feedback, offering high bandwidth and fast response for dynamic signal applications.
Fully Differential Amplifier (FDA)	FDAs are specialized instrumentation amplifiers designed for high common-mode rejection and are often used in differential signal processing and communication systems.
Programmable Gain Instrumentation Amplifier	These amplifiers allow the user to digitally or electronically adjust the gain, making them suitable for applications where gain control needs to be dynamic or remote.
High-Speed Instrumentation Amplifier	Designed for applications that require high bandwidth and fast signal processing, these amplifiers are commonly used in data acquisition and RF systems.

Advantages

• Variable Gain Control: The gain of a three-op-amp instrumentation amplifier can be easily adjusted by changing the value of a single resistor (Rgain), making it convenient for adapting to different input signal levels and requirements.

• External Resistor Dependence: The gain of the instrumentation amplifier is primarily determined by the values of external resistors, making it straightforward to customize the amplification factor based on specific application needs.

• High Input Impedance: The use of emitter follower configurations in amplifiers 1 and 2 results in very high input impedance, which ensures that the amplifier does not load down the signal source and maintains signal integrity.

• Low Output Impedance: The presence of a difference amplifier in the instrumentation amplifier design results in a very low output impedance, allowing it to drive loads efficiently and minimizing signal degradation.

• High Common-Mode Rejection Ratio (CMRR): Op-amp 3 within the instrumentation amplifier exhibits a high CMRR, which means that common-mode signals (noise or interference that appears on both input terminals) are effectively rejected, leaving the desired differential signal largely unaffected.

Applications

Application	Description
Data Acquisition from Transducers	Instrumentation amplifiers are commonly used to collect data from small-output transducers, such as thermocouples, strain gauges, and Wheatstone bridges, ensuring accurate and high-gain measurements.
Precision Measurement	They find applications in precision fields like navigation, medical devices, and radar systems, where high accuracy and the preservation of signal strength in noisy environments are critical.
Audio Signal Enhancement	Instrumentation amplifiers are employed in audio applications to enhance the signal-to-noise ratio (S/N ratio), particularly in scenarios with low-amplitude audio signals.
Imaging and Video Data Acquisition	These amplifiers are utilized in imaging and video data acquisition for high-speed signal conditioning, ensuring the faithful representation of visual information.
RF Signal Amplification	In RF (radio frequency) cable systems, instrumentation amplifiers are used for amplifying high-frequency signals, helping boost signal strength in RF communication setups.

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