What is an Instrumentation Amplifier?

Catalog

I. General Introduction of Instrumentation Amplifier

Instrumentation amplifier, an improvement of a differential amplifier, has an input buffer and does not require input impedance matching, making the amplifier suitable for measurement and electronic instruments. Features include very low DC offset, low drift, low noise, very high open-loop gain, very large common-mode rejection ratio, and high input impedance. Instrumentation amplifiers are used in circuits that require very high accuracy and stability.

To reproduce very low-level signals, rejecting noise and interference signals, an instrumentation amplifier is used. Heartbeats, blood pressure, temperature, earthquakes, and so on can be examples. The essential features of a good instrumentation amplifier, therefore, are as follows.

There would be very low signal energy for inputs to the instrumentation amplifiers. The instrumentation amplifier should then have a high degree of gain and should be exact.

• By using a single trigger, the gain can be conveniently adjustable.

• To avoid loading, it must have High Input Impedance and Low Output Impedance.

• The instrumentation amplifier should have a high CMRR because, when broadcast over long wires, the transducer output would normally contain common-mode signals such as noise.

• To control sharp rise times of events and have a full undistorted output voltage swing, it must also provide a High Slew Rate.

With the rapid development of electronic technology, operational amplifier circuits have also been widely used. The instrumentation amplifier is a precision differential voltage amplifier, which is derived from the operational amplifier and is better than the operational amplifier. The instrumentation amplifier integrates key components inside the amplifier. Its unique structure makes it have the characteristics of high common-mode rejection ratio, high input impedance, low noise, low linearity error, low offset drift, flexible gain setting, and convenient use. Acquisition, sensor signal amplification, high-speed signal adjustment, medical instruments, and high-end audio equipment are all popular. An instrumentation amplifier is a closed-loop gain component with a differential input and a single-ended output relative to the reference end. It has a differential input and a single-ended output relative to the reference end. The difference from the operational amplifier is that the closed-loop gain of the operational amplifier is determined by the external resistance connected between the inverting input and the output, while the instrumentation amplifier uses an internal feedback resistor network isolated from the input. The input signal is applied to the two differential input terminals of the instrumentation amplifier, and its gain can be preset internally or preset by the user through the internal setting of the pin or through an external gain resistor isolated from the input signal.

Instrumentation Amplifier using Op-Amp

The use of the op-amp circuit instrumentation amplifier is seen below. The non-inverting amplifiers are op-amps 1 & 2 and op-amp 3 is a differential amplifier. Together, these three op-amps form an amplifier for instrumentation. The final value of the Vout instrumentation amplifier is the augmented variance of the input signals added to the op-amp 3 input terminals. Let the op-amp 1 and op-amp 2 outputs be Vo1 and Vo2 respectively.

Vout = (R3/R2) then (Vo1-Vo2)

As seen in the figure below, look at the input stage of the instrumentation amplifier. Below is a discussion of the instrumentation amplifier derivation.

The potential is the input voltage V1 at node A. Hence, from the virtual short definition, the capacity at node B is also V1. The capacity at node G, thus, is also V1.

The potential is the input voltage V2 at node D. The capacity at node C is therefore also V2, from the virtual short. The capacity at node H, thus, is also V2.

Ideally, the current at the input stage op-amps is zero for the operation of the instrumentation amplifier. Therefore, the current I by R1, Rgain, and R1 resistors remain the same.

Applying Ohm's law between nodes E and F,

I = (Vo1-Vo2)/(R1+Rgain+R1) ...(1)

I = (Vo1-Vo2)/(2R1+Rgain)

Since no current flows to the input of the op-amps 1 & 2, it is possible to send the current I between the nodes G and H as:

I = (VG-VH) / Rgain = (V1-V2) / Rgain...(2)

Equating equations 1 and 2,

(Vo1-Vo2)/(2R1+Rgain) = (V1-V2)/Rgain

(Vo1-Vo2) = (2R1+Rgain)(V1-V2)/Rgain ...(3)

The output of the difference amplifier is given as,

Vout = (R3/R2) (Vo1-Vo2)

Therefore, (Vo1 - Vo2) = (R2/R3)Vout

Substituting (Vo1 - Vo2) value in equation 3, we get

(R2/R3)Vout = (2R1+Rgain)(V1-V2)/Rgain

i.e. Vout = (R3/R2){(2R1+Rgain)/Rgain}(V1-V2)

The above equation gives an instrumentation amplifier's output voltage.

The expression (R3/R2){(2R1+Rgain)/Rgain} supplies the total gain of the amplifier.

By changing the value of resistor Rgain, the total voltage gain of an instrumentation amplifier can be regulated.

The differential amplifier supplies the common-mode signal attenuation for the instrumentation amplifier.

II. Features of Instrumentation Amplifier

The instrumentation amplifier is a high-gain, DC-coupled amplifier with differential input, single-ended output, high input impedance, and high common-mode rejection ratio, low noise, low linearity error, low offset voltage and offset voltage drift, and low input bias Features such as current and offset current error.

1. Advantages of Instrumentation Amplifier

(1) High common mode rejection ratio

The instrumentation amplifier has the characteristic that it can eliminate any common-mode signal (the two input terminals have the same potential) and amplify the differential mode signal (the two input terminals have different potentials). In order for the instrumentation amplifier to work properly, it is required to amplify microvolt differential mode signals and at the same time suppress common mode signals of several volts. The instrumentation amplifier that realizes this function must have a high common mode rejection capability. The typical value of the common-mode rejection ratio is 70-100dB. Generally, when the gain is high, the performance of the CMRR will be improved, that is, the CMRR is higher when the gain is high, and lower when the gain is low.

(2) Small linear error

Input offset and scale factor can be corrected by external adjustment, while linear error is an inherent defect of the device and cannot be eliminated by external adjustment. Therefore, the small linear error of the instrumentation amplifier is realized by the manufacturer through the use of advanced production technology and chip structure design. For a high-performance instrumentation amplifier, the linearity error is 0.01%, and some can even reach 0.0001%.

(3) High input impedance

In the actual application circuit, the signal source impedance may be very high or unbalanced. In order to be well-matched, the input impedance of the instrumentation amplifier is not only very high but also has good matching performance. The typical value of the input impedance is 109-1012 ohms.

(4) Low noise

Instrumentation amplifiers are often used in harsh environments to complete the pickup and preprocessing of weaker signals, so it must be a low-noise device, and the signal-to-noise ratio is too low to work. Under normal circumstances, when the frequency of the input signal is 1kHZ, the noise referred to as the input of the instrumentation amplifier should be less than 10nV/Hz. In order to improve the signal-to-noise ratio, it is generally not desirable for the instrumentation amplifier to add its own noise to the signal.

(5) Low offset voltage and low offset voltage drift

The offset voltage drift of an instrumentation amplifier is composed of two parts, input, and output. Each part has an influence on the total gain, but when the gain is increased, the offset drift of the input part will become the main error source, and the influence of the output part can be ignored. The typical values of input and output offsets are 100V and 2mV, respectively. In addition, the instrumentation amplifier has excellent stability. When operating conditions change, its key parameters remain stable. And easy to use, only need to detect the potential difference between the two input terminals. In addition, due to its high degree of integration, the main components are all made inside the chip, and there are few peripheral components.

2. Difference between Operational Amplifier and Instrumentation Amplifier

The foregoing are the main distinctions between the operating amplifier and the instrumentation amplifier.

One type of integrated circuit is an active amplifier (op-amp).

One type of differential amplifier is the instrumentation amplifier.

It is possible to construct instrumentation amplifiers with three operating amplifiers.

A single operating amplifier may be used to build the differential amplifier.

The output voltage of the differential amplifier is impacted by the mismatch of resistors

The instrumentation amplifier produces gain in its primary phase with a single resistor that does not require a corresponding resistor.

III. Applications of Instrumentation Amplifier

The following was used in the instrumentation amplifier implementations:

Such amplifiers specifically include where high differential gain accuracy is needed, power must be maintained in noisy conditions, as well as where there are big common-mode signals. Any of the apps are

Instrumentation amplifiers are used to collect data from small O/P transducers such as thermocouples, strain gauges, Wheatstone bridge scales, etc.

In navigation, pharmacy, radar, etc., these amplifiers are used.

These amplifiers are used in audio applications, such as low-amplitude audio signals, to increase the S/N ratio (signal to noise).

Such amplifiers are used in high-speed signal conditioning for imaging as well as video data acquisition.

In RF cable networks, these amplifiers are used for high-frequency signal amplification.

Reference

Analog Devices. (2019). The Designer's Guide to Instrumentation Amplifiers. https://www.analog.com/media/en/training-seminars/design-handbooks/designers-guide-instrument-amps-complete.pdf

AllecCo Elec. (2024, November 29). Instrumentation Amplifiers: Types, Applications, and Advantages. https://www.allelcoelec.com/blog/Instrumentation-Amplifiers-Types,Applications,and-Advantages.html

Electronics Hub. (n.d.). Instrumentation Amplifier Basics and Applications. https://www.electronicshub.org/instrumentation-amplifier-basics-applications/

Renesas Electronics. (n.d.). Instrumentation Amplifiers. https://www.renesas.com/en/products/analog-products/amplifiers/specialized-amplifiers/instrumentation-amplifiers

Texas Instruments. (2019). What Is an Instrumentation Amplifier?. https://www.ti.com/document-viewer/lit/html/SSZT428