# Instrumentation amplifier

An instrumentation amplifier (INA, or in-amp) is a differential-input amplifier designed to amplify the voltage difference between two inputs while rejecting any voltage common to both, and it combines high input impedance, low output impedance, and high common-mode rejection ratio (CMRR).<sup>[1](https://www.ti.com/lit/ta/sszt428/sszt428.pdf?ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252Fes-mx%252FINA818&ts=1774241968195)</sup> [Texas Instruments](https://www.edgepedia.org/texas-instruments) offers a broad definition: an instrumentation amplifier is any circuit or device containing between one and three internal op amps, built to enhance signal quality and to raise common-mode rejection.<sup>[1](https://www.ti.com/lit/ta/sszt428/sszt428.pdf?ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252Fes-mx%252FINA818&ts=1774241968195)</sup> These properties make INAs suited to applications with small error budgets, such as motor controllers, battery test equipment, analog input modules, LCD test equipment, and patient monitoring systems.<sup>[1](https://www.ti.com/lit/ta/sszt428/sszt428.pdf?ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252Fes-mx%252FINA818&ts=1774241968195)</sup>

| Key fact | Detail |
|---|---|
| Defining traits | Differential gain with high CMRR, high input impedance, low output impedance; newer devices add low offset and low noise<sup>[1](https://www.ti.com/lit/ta/sszt428/sszt428.pdf?ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252Fes-mx%252FINA818&ts=1774241968195)</sup> |
| Classic topology | Three op amps: two buffer/pre-amp amplifiers plus a subtractor (difference amplifier) output stage; gain set by one external resistor \( R_G \)<sup>[2](https://www.analog.com/en/resources/technical-articles/choosing-precision-amplifier-topology.html?gated=1753386568838)</sup> |
| Transfer function | \( V_{\mathrm{OUT}} = G \cdot V_{\mathrm{DIFF}} + V_{\mathrm{REF}} \), where \( V_{\mathrm{REF}} \) level-shifts the output<sup>[2](https://www.analog.com/en/resources/technical-articles/choosing-precision-amplifier-topology.html?gated=1753386568838)</sup> |
| Typical CMRR | Integrated in-amps commonly specify 130 dB to 140 dB or more, against 90 dB to 100 dB for integrated difference amplifiers<sup>[2](https://www.analog.com/en/resources/technical-articles/choosing-precision-amplifier-topology.html?gated=1753386568838)</sup> |
| Example part (AD620, 1991) | Gain 1 to 10,000 with one resistor; 9 nV/√Hz input noise at 1 kHz; 50 µV max offset; 1.0 nA max bias current<sup>[3](https://www.analog.com/en/timeline/timeline.html)</sup><sup> • </sup><sup>[4](https://www.analog.com/en/products/ad620.html)</sup> |
| Example part (INA823) | 2.7 V to 36 V supply; CMRR 120 dB min at G ≥ 100; 21 nV/√Hz noise; 180 µA supply current; ±60 V input overvoltage protection<sup>[5](https://www.ti.com/product/INA823)</sup> |
| Practical requirement | A DC return path for input bias current must be provided, or the inputs float and the amplifier saturates<sup>[6](https://www.ti.com/lit/pdf/sboa503)</sup> |

## The three-op-amp topology

The most common INA configuration uses three op amps.<sup>[1](https://www.ti.com/lit/ta/sszt428/sszt428.pdf?ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252Fes-mx%252FINA818&ts=1774241968195)</sup> In the classic arrangement described by [Analog Devices](https://www.edgepedia.org/analog-devices), amplifiers A1 and A2 form the first, pre-amp stage, and a third amplifier A3 forms the second stage, a subtractor recognizable as the familiar difference amplifier topology.<sup>[2](https://www.analog.com/en/resources/technical-articles/choosing-precision-amplifier-topology.html?gated=1753386568838)</sup> A1 and A2 provide a balanced, extremely high input impedance, amplify the differential input voltage, and pass the common-mode voltage through without amplification; the subtractor at A3 then rejects the common-mode voltage and passes the amplified differential voltage to the output.<sup>[2](https://www.analog.com/en/resources/technical-articles/choosing-precision-amplifier-topology.html?gated=1753386568838)</sup> Texas Instruments' application literature describes the same two-stage structure: a high-input-impedance first stage that amplifies the differential signal, followed by a difference amplifier that largely removes the common-mode signal while referencing the output to a specific reference voltage.<sup>[6](https://www.ti.com/lit/pdf/sboa503)</sup>

**Single-resistor gain setting.** A defining convenience of the topology is that the system designer sets the gain with a single external resistor \( R_G \), eliminating the need to match discrete resistor ratios.<sup>[2](https://www.analog.com/en/resources/technical-articles/choosing-precision-amplifier-topology.html?gated=1753386568838)</sup> In the AD620, feedback through the input loops impresses the input voltage across \( R_G \), giving a first-stage differential gain of \( G = (R_1 + R_2)/R_G + 1 \); the internal resistors \( R_1 \) and \( R_2 \) are trimmed to 24.7 kΩ, which yields the part's gain equation \( G = 49.4\,\mathrm{k\Omega}/R_G + 1 \).<sup>[7](https://www.analog.com/media/en/technical-documentation/data-sheets/AD620.pdf)</sup> Reducing \( R_G \) for larger gains also raises the preamp transconductance, which boosts open-loop gain, increases the gain-bandwidth product, and reduces input voltage noise.<sup>[7](https://www.analog.com/media/en/technical-documentation/data-sheets/AD620.pdf)</sup>

## Why monolithic parts outperform discrete builds

The three-op-amp circuit can be assembled from discrete op amps, but an integrated INA saves board space and reduces CMRR errors.<sup>[1](https://www.ti.com/lit/ta/sszt428/sszt428.pdf?ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252Fes-mx%252FINA818&ts=1774241968195)</sup> The reason lies in resistor matching. Any mismatch among the four gain-setting resistors of the output difference stage has a direct impact on CMRR, and mismatch in their temperature drift degrades rejection further; the absolute values and absolute drifts do not matter, only the matching.<sup>[8](https://www.analog.com/en/resources/technical-articles/common-mode-instrumentation-amplifiers.html)</sup> Thin-film resistors fabricated on silicon have initial tolerances of up to ±20%, but laser trimming during production reduces the ratio error between resistors to 0.01% (100 ppm), and their temperature coefficients track to typically under 3 ppm/°C.<sup>[8](https://www.analog.com/en/resources/technical-articles/common-mode-instrumentation-amplifiers.html)</sup>

The consequences are measurable. With a 0.1% mismatch among four gain-setting resistors at a gain of 11, the measured DC CMRR was about 84 dB against a theoretical 85 dB, and it degrades quickly as frequency rises.<sup>[8](https://www.analog.com/en/resources/technical-articles/common-mode-instrumentation-amplifiers.html)</sup> In that circuit, a 200 mV peak-to-peak mains harmonic at 180 Hz produced about 800 µV at the output; a 12-bit data acquisition system with a 0 V to 2.5 V input range has an LSB weighting of 610 µV, so the interference exceeds one code step.<sup>[8](https://www.analog.com/en/resources/technical-articles/common-mode-instrumentation-amplifiers.html)</sup> Integrated in-amps, with precisely matched monolithic resistors, commonly reach CMRR of 130 dB to 140 dB or more.<sup>[2](https://www.analog.com/en/resources/technical-articles/choosing-precision-amplifier-topology.html?gated=1753386568838)</sup>

## Alternative architectures

**Indirect current feedback.** The indirect current feedback (ICF) architecture replaces the resistor-matched subtractor with two matched transconductance amplifiers, \( G_{M1} \) and \( G_{M2} \), and a high-gain transimpedance amplifier.<sup>[9](https://www.microchip.com/en-us/about/media-center/blog/2022/two-common-architectures)</sup> Because the design is insensitive to resistor matching, internally trimmed resistors are not required, which reduces manufacturing cost; \( G_{M1} \) rejects common-mode signals to provide high DC CMRR, and unlike the three-op-amp configuration the AC CMRR does not decrease significantly with frequency.<sup>[9](https://www.microchip.com/en-us/about/media-center/blog/2022/two-common-architectures)</sup> ICF parts also decouple output swing from input common-mode voltage, giving a wider operating range, though the temperature coefficients of the external \( R_F \) and \( R_G \) resistors must be matched to minimize gain drift.<sup>[9](https://www.microchip.com/en-us/about/media-center/blog/2022/two-common-architectures)</sup> TI's INA630 is an ICF device specifying 126 dB CMRR and an input impedance of approximately 100 GΩ with a typical 3 nA input bias current.<sup>[10](https://www.ti.com/lit/ds/symlink/ina630.pdf)</sup>

**Limitations of the three-op-amp design.** While it provides high DC CMRR through close matching of on-chip resistors, the feedback architecture can substantially degrade AC CMRR, and unmatched parasitic capacitances reduce CMRR at higher frequencies.<sup>[9](https://www.microchip.com/en-us/about/media-center/blog/2022/two-common-architectures)</sup> The common-mode voltage input range is limited to prevent saturation of internal nodes, and the \( V_{\mathrm{REF}} \) pin needs a [buffer amplifier](https://www.edgepedia.org/buffer-amplifier) for optimal performance because its impedance otherwise affects CMRR.<sup>[9](https://www.microchip.com/en-us/about/media-center/blog/2022/two-common-architectures)</sup>

## Bias current return paths

An instrumentation amplifier's high input impedance means its input bias current, though small, must have somewhere to flow. TI's application report warns that ac-coupling an INA by placing capacitors in series with the inputs, without a DC path, lets the bias current charge the capacitors until the output is driven to one of the supply rails.<sup>[6](https://www.ti.com/lit/pdf/sboa503)</sup> The standard fix is a resistor from each input to system ground, or to a mid-supply bias voltage in single-supply designs; typical values run from 100 kΩ to 1 MΩ, chosen large compared with the differential source resistance, with the trade-off of added thermal noise and DC offset from bias current flowing through them.<sup>[6](https://www.ti.com/lit/pdf/sboa503)</sup> The INA630 datasheet gives the same rule: without a bias current path the inputs float beyond the common-mode range and the input amplifiers saturate; with a low-impedance source such as a thermocouple one return resistor suffices, while higher-impedance sources favor two equal resistors for a balanced input.<sup>[10](https://www.ti.com/lit/ds/symlink/ina630.pdf)</sup>

## Representative devices and specifications

The AD620, released by Analog Devices in 1991,<sup>[3](https://www.analog.com/en/timeline/timeline.html)</sup> is a monolithic part based on a modification of the classic three-op-amp approach, with Superβeta processing in the input transistors giving 10 times lower input bias current than a standard bipolar input.<sup>[7](https://www.analog.com/media/en/technical-documentation/data-sheets/AD620.pdf)</sup> Its headline specifications are gains of 1 to 10,000 from one external resistor, 9 nV/√Hz input voltage noise at 1 kHz, 50 µV maximum offset voltage with 0.6 µV/°C maximum drift, 1.0 nA maximum bias current, 100 dB minimum CMRR at G = 10, and 120 kHz bandwidth at G = 100, with a maximum supply current of 1.3 mA.<sup>[4](https://www.analog.com/en/products/ad620.html)</sup>

Texas Instruments' INA823 illustrates a modern low-power part: supply from 2.7 V to 36 V, CMRR of 84 dB minimum at G = 1 rising to 120 dB minimum at G ≥ 100, 21 nV/√Hz input noise, 100 µV maximum offset with 1.2 µV/°C maximum drift, 8 nA maximum bias current, 1.9 MHz bandwidth at G = 1, 180 µA typical supply current, and input overvoltage protection up to ±60 V.<sup>[5](https://www.ti.com/product/INA823)</sup>

At the extreme of input impedance, the INA116, a dielectrically isolated FET-input three-op-amp instrumentation amplifier, specifies a typical bias current of 3 fA (3 × 10<sup>−15</sup> A typical, 2.5 × 10<sup>−14</sup> A maximum) and a 10<sup>15</sup> Ω input impedance, making it suitable for pH electrodes whose source impedance can reach a few gigohms; TI's handbook notes such a part could be called a differential electrometer.<sup>[11](https://e2e.ti.com/cfs-file/__key/communityserver-discussions-components-files/14/Burr_2D00_Brown-The-Instrumentation-Amplifier-Handbook_2D00_1.pdf)</sup>

## Applications

Instrumentation amplifiers are frequently used to amplify differential voltages in the presence of common-mode noise; TI lists microphone preamplifiers and thermocouple amplifiers as common applications, noting that a thermocouple's low output impedance means a single biasing resistor can provide the return path without a large offset error.<sup>[6](https://www.ti.com/lit/pdf/sboa503)</sup> Analog Devices positions the AD620 for precision data acquisition such as weigh scales and transducer interfaces, and for medical uses including ECG and noninvasive blood pressure monitors.<sup>[4](https://www.analog.com/en/products/ad620.html)</sup> Programmable-gain instrumentation amplifiers (PGIAs) extend the concept to data acquisition systems that must interface sensors of very different sensitivities, for example a load sensor with 10 mV maximum output and submicrovolt sensitivity alongside a sensor preconditioned for 10 V; integrated PGIAs such as the AD825x, AD8231, and LTC6915 achieve digitally adjustable gain through internal precision resistor arrays trimmed on-chip to optimize gain, CMRR, and offsets.<sup>[12](https://www.analog.com/en/resources/analog-dialogue/articles/programmable-gain-instrumentation-amplifiers-finding-one-that-works-for-you.html)</sup>

## References

1. [Brendan Hess, "What Is an Instrumentation Amplifier?" Texas Instruments, SSZT428, August 2019](https://www.ti.com/lit/ta/sszt428/sszt428.pdf?ref_url=https%253A%252F%252Fwww.ti.com%252Fproduct%252Fes-mx%252FINA818&ts=1774241968195)
2. ["Choosing a Precision Amplifier Topology," Analog Devices](https://www.analog.com/en/resources/technical-articles/choosing-precision-amplifier-topology.html?gated=1753386568838)
3. [Analog Devices timeline (AD620 release, 1991)](https://www.analog.com/en/timeline/timeline.html)
4. [AD620 product page and datasheet overview, Analog Devices](https://www.analog.com/en/products/ad620.html)
5. [INA823 product page, Texas Instruments](https://www.ti.com/product/INA823)
6. ["Importance of Input Bias Current Return Paths in Instrumentation Amplifier Applications," Texas Instruments, SBOA503, July 2021](https://www.ti.com/lit/pdf/sboa503)
7. [AD620 datasheet (Rev. H), Analog Devices](https://www.analog.com/media/en/technical-documentation/data-sheets/AD620.pdf)
8. ["A Practical Review of Common Mode and Instrumentation Amplifiers," Analog Devices](https://www.analog.com/en/resources/technical-articles/common-mode-instrumentation-amplifiers.html)
9. [Kevin Tretter, "Using Instrumentation Amplifiers: Two Common Architectures," Microchip Technology, 2022](https://www.microchip.com/en-us/about/media-center/blog/2022/two-common-architectures)
10. [INA630 datasheet (Rev. A), Texas Instruments](https://www.ti.com/lit/ds/symlink/ina630.pdf)
11. [Burr-Brown, The Instrumentation Amplifier Handbook](https://e2e.ti.com/cfs-file/__key/communityserver-discussions-components-files/14/Burr_2D00_Brown-The-Instrumentation-Amplifier-Handbook_2D00_1.pdf)
12. [Kristina Fortunado, "Programmable Gain Instrumentation Amplifiers: Finding One that Works for You," Analog Devices](https://www.analog.com/en/resources/analog-dialogue/articles/programmable-gain-instrumentation-amplifiers-finding-one-that-works-for-you.html)

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*Topic: Encyclopedia › Physical world and mathematics › Measurement and time › Metrology, instrumentation, and applied measurement › Calibration and instrumentation › Sensors, transducers, and instrumentation systems*

*Initially written Sep 17, 2026 · Reviewed: Oct 15, 2026 · Edited: Oct 15, 2026 · Last review: Oct 15, 2026*

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