Instrumentation Amplifier Symbol
Definition: The Instrumentation Amplifier symbol represents a precision differential-input amplifier with high common-mode rejection ratio (CMRR), very high input impedance, and a single external resistor-programmable gain, shown in schematics as a five-terminal block with IN+ (non-inverting input), IN− (inverting input), OUT (output), V+ (positive supply), and V− (negative supply), designated A or U in ANSI/IEEE 315-1975 circuits.
Also known as: INA, in-amp, instrumentation amp, INA128, INA114, AD620, precision differential amplifier, bridge amplifier.
What the Instrumentation Amplifier symbol means
The Instrumentation Amplifier (in-amp) symbol denotes a closed-loop precision amplifier specifically designed to amplify the differential voltage between its two input terminals (IN+ and IN−) while rejecting any signal common to both inputs (common-mode voltage). The gain is set by a single external resistor (R_G) connected across dedicated pins on the device, making gain adjustment simple and precise without disturbing the input or output impedance. In a schematic, the instrumentation amplifier symbol appears at the input stage of measurement circuits where small differential signals (millivolt-range) must be extracted from noisy environments with large common-mode voltages.
Instrumentation amplifiers differ from standard op-amps in that their input stage consists of two matched buffer amplifiers that provide very high input impedance (typically >10 GΩ) and low input bias current, followed by a difference amplifier output stage that amplifies only the differential component. This architecture gives the instrumentation amplifier a CMRR typically exceeding 80 dB (and up to 130 dB for precision devices), making it the preferred choice for sensor signal conditioning.
How to identify the Instrumentation Amplifier symbol
The instrumentation amplifier symbol is drawn as a triangular or rectangular amplifier block with two differential inputs on the left side — IN+ (non-inverting, upper) and IN− (inverting, lower) — and a single output on the right (OUT). Supply voltage terminals V+ and V− appear at the top and bottom of the block respectively. Some schematic representations include a small 'INA' label or gain resistor indication inside or adjacent to the block.
Function in a circuit
The instrumentation amplifier amplifies the differential input voltage V_diff = (V_IN+ − V_IN−) by a gain G determined by the external gain-setting resistor R_G: for an INA128, G = 1 + (50 kΩ / R_G); for an AD620, G = 1 + (49.4 kΩ / R_G). The output is V_out = G × V_diff, referenced to the reference voltage at the REF pin (typically ground). The supply terminals (V+ and V−) power the internal amplifier stages, typically at ±2.25 V to ±18 V for dual-supply devices or 2.7–36 V for single-supply variants.
Standards: IEC vs ANSI
| IEC 60617 | IEC 60617-02 represents amplifiers as triangular blocks; the instrumentation amplifier is identified by labelled differential input terminals (+ and −) and the label 'INA' or 'InAmp'. Differential amplifiers are covered under functional block conventions in IEC 60617. |
|---|---|
| ANSI/IEEE 315 | ANSI/IEEE 315-1975 uses a triangular amplifier symbol with differential inputs; the instrumentation amplifier designator is A (amplifier) or U (integrated circuit) with a type qualifier. IEEE 1057 (digitising waveform recorders) and IEEE 1241 (ADC testing) reference instrumentation amplifier performance parameters. |
| Key difference | IEC and ANSI/IEEE use identical triangular or rectangular block symbols for amplifiers including instrumentation amplifiers. The internal architecture (three-op-amp topology) is not depicted in the symbol; only the terminal labels and gain annotation distinguish the in-amp symbol from a generic op-amp symbol. |
Terminals / pins
| Pin | Name |
|---|---|
| in_pos | IN+ |
| in_neg | IN- |
| out | OUT |
| vcc | V+ |
| vee | V- |
Typical values
Gain range: 1 to 10,000 (set by R_G). CMRR: 80–130 dB (DC, at minimum gain). Input offset voltage: 25 µV to 500 µV. Input bias current: 1–25 nA. Input impedance: >10 GΩ differential. Bandwidth (−3 dB): 1 kHz–1 MHz (gain-dependent). Supply voltage: ±2.25 V to ±18 V (dual) or 2.7–36 V (single). Common devices: INA128, INA129, AD620, AD8221, INA333 (micropower), INA826.
Where the Instrumentation Amplifier symbol is used
- Wheatstone bridge and strain gauge amplification: instrumentation amplifier conditions the millivolt differential output of a bridge sensor (load cell, pressure sensor, torque sensor) in the presence of large supply common-mode voltages
- ECG and biomedical instrumentation: INA amplifies the ≤1 mV cardiac electrical signal from chest electrodes while rejecting the 50/60 Hz mains interference common to both electrode leads (CMRR >80 dB required)
- Thermocouple signal conditioning: instrumentation amplifier amplifies the microvolt-range thermoelectric EMF between thermocouple junction and cold-junction reference, rejecting cable-induced noise
- Industrial 4–20 mA current loop receiver: in-amp converts the differential voltage across a shunt resistor in a 4–20 mA process control loop into a single-ended signal for ADC input
- Data acquisition systems: instrumentation amplifier provides a high-impedance, high-CMRR front-end for multi-channel ADC inputs receiving signals from remote sensors over long cables
- Motor current sensing: in-amp measures the differential voltage across a shunt resistor in a high-side current sense position, rejecting the motor supply rail common-mode voltage
Example
In a load cell signal conditioning circuit, a Wheatstone bridge (350 Ω load cell) produces a differential output of ±2 mV/V (±10 mV full scale at 5 V excitation) with a common-mode voltage of 2.5 V. An INA128 instrumentation amplifier with R_G = 499 Ω (gain ≈ 101) amplifies this differential signal to ±1.01 V full scale at its OUT terminal, while rejecting the 2.5 V common-mode signal by >90 dB. The V+ and V− pins are supplied at ±5 V from a precision low-noise supply.
Key facts
- The Instrumentation Amplifier symbol has five terminals: IN+ (non-inverting input), IN− (inverting input), OUT (output), V+ (positive supply), and V− (negative supply); gain is set by an external R_G resistor not shown in the block symbol.
- The gain equation for a three-op-amp in-amp is G = 1 + (2R / R_G), where R is an internal fixed resistor specified per device (e.g., 50 kΩ for INA128, 49.4 kΩ for AD620).
- CMRR (common-mode rejection ratio) is the key specification of an instrumentation amplifier; precision devices achieve 100–130 dB at DC, meaning a 10 V common-mode signal produces less than 30 µV of differential error.
- The input impedance of an instrumentation amplifier is typically >10 GΩ differential and >10 GΩ common-mode, avoiding loading of high-impedance sensor sources such as pH electrodes and photodiodes.
- The INA128 (Texas Instruments) is one of the most widely used instrumentation amplifiers: G = 1 + 50 kΩ/R_G, CMRR min 80 dB, input offset 25 µV, supply ±2.25 V to ±18 V.
- Instrumentation amplifiers differ from standard differential op-amp configurations in that they provide constant, high input impedance regardless of gain setting — a standard op-amp difference configuration's input impedance decreases at higher gain.
- In ANSI/IEEE 315 schematics, the instrumentation amplifier designator is A (amplifier) or U (integrated circuit); in IEC-based designs it may be designated A or AV (voltage amplifier).
- Rail-to-rail instrumentation amplifiers (e.g., INA333, AD8221) operate from single 3.3 V or 5 V supplies, enabling use in portable, battery-powered measurement devices.
Frequently asked questions
What does the instrumentation amplifier symbol mean in a schematic?
The instrumentation amplifier symbol represents a precision differential amplifier that amplifies the voltage difference between its IN+ and IN− input terminals while rejecting signals common to both inputs. It appears as a five-terminal block (IN+, IN−, OUT, V+, V−) and is used to condition small signals from sensors in the presence of electrical noise and common-mode interference.
What does the instrumentation amplifier symbol look like?
The instrumentation amplifier symbol is a rectangular or triangular amplifier block with two input terminals on the left (IN+ and IN−), one output on the right (OUT), and supply terminals at the top (V+) and bottom (V−). It resembles an op-amp symbol but with two explicitly labelled differential inputs. A gain annotation or R_G notation may appear near the symbol.
How is the gain of an instrumentation amplifier set?
The gain of a three-op-amp instrumentation amplifier is set by a single external resistor R_G connected across two dedicated pins on the device: G = 1 + (2R / R_G), where R is a fixed internal resistor (e.g., 25 kΩ per side = 50 kΩ for INA128). A smaller R_G gives higher gain. This single-resistor gain setting is a key advantage over standard op-amp configurations that require matched resistor networks.
What is CMRR and why does it matter for an instrumentation amplifier?
CMRR (common-mode rejection ratio) is the ratio of the instrumentation amplifier's differential gain to its common-mode gain, expressed in decibels. A high CMRR (80–130 dB) means the amplifier amplifies the desired differential signal while suppressing noise and interference that appears equally on both input terminals. This is critical for sensor signal conditioning where ground loops, 50/60 Hz mains interference, and long cable noise are common-mode disturbances.
What is the difference between an instrumentation amplifier and an op-amp?
An op-amp is a general-purpose amplifier requiring external feedback and input resistors to set gain, with input impedance that changes with gain configuration. An instrumentation amplifier has internal matched resistors that provide fixed high input impedance regardless of gain, a single external resistor gain setting, and much higher CMRR. In-amps are specifically designed for precision sensor signal conditioning; op-amps are general-purpose building blocks.
What standard defines the instrumentation amplifier symbol?
ANSI/IEEE 315-1975 defines amplifier symbols including differential amplifiers; the designator is A (amplifier) or U (integrated circuit). IEC 60617-02 uses the triangular amplifier block convention. Neither standard defines a unique glyph specifically for instrumentation amplifiers — the device is identified by labelled differential inputs (IN+ and IN−) and the INA or in-amp annotation.
What are common instrumentation amplifier ICs?
Widely used instrumentation amplifier ICs include the INA128 and INA129 (Texas Instruments, precision, ±18 V), AD620 and AD8221 (Analog Devices, low-power), INA333 (Texas Instruments, micropower 50 µA, single-supply 1.8 V), and AD8237 (Analog Devices, zero-drift). The choice depends on gain range, supply voltage, input offset, noise, bandwidth, and power consumption requirements.
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