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AD620 Low Cost Low Power Instrumentation Amplifier

Update time : 2025-12-11 17:21:47

The AD620 is a high-performance instrumentation amplifier launched by Analog Devices, featuring core advantages of low cost, low power consumption, and ease of use. It is widely used in precision measurement fields such as medical instruments, industrial control, and sensor interfaces. Its single external resistor programmable gain (1 to 10,000), wide power supply range, and excellent DC/AC performance make it an ideal alternative to traditional discrete three-op-amp designs. This article will provide a comprehensive analysis from aspects of features, pin definitions, absolute maximum ratings, specifications, operating principles, applications, and models.

AD620 Instrumentation Amplifier Overview

The AD620, a high-performance instrumentation amplifier from Analog Devices, combines low cost, low power consumption, and ease of use. It supports programmable gain from 1 to 10,000, set via a single external resistor, simplifying circuit design.
With a wide power supply range (±2.3V to ±18V), it works with both battery-powered (e.g., ±3V) and industrial systems (e.g., ±15V). Its max supply current of 1.3mA suits portable devices, far lower than traditional discrete op-amp setups.
Notable performance includes low input offset voltage (50μV max for B grade), high common-mode rejection (100dB min at gain 10), and low noise (9nV/√Hz at 1kHz). It offers 120kHz bandwidth at gain 100 and 15μs settling time, ideal for medical instruments (ECG), industrial control, and sensor interfaces, replacing traditional designs effectively.
AD620 Instrumentation Amplifier Pinouts

The AD620 adopts an 8-pin package, with the top-view pin distribution as follows :

Pin Number	Name	Function Description
1	RG	Gain setting terminal (cooperating with Pin 8); gain is set by connecting an external resistor RG between Pin 1 and Pin 8.
2	−IN	Inverting input terminal.
3	+IN	Non-inverting input terminal.
4	−VS	Negative power supply terminal (range: ±2.3 V to ±18 V).
5	REF	Reference terminal, used to set the zero point of the output voltage (can be connected to system ground or offset voltage).
6	OUTPUT	Output terminal; the output signal is the amplified differential signal, referenced to the REF terminal potential.
7	+VS	Positive power supply terminal (range: ±2.3 V to ±18 V).
8	RG	Gain setting terminal (cooperating with Pin 1).

Note: For a gain of 1, Pins 1 and 8 must be left floating (RG=∞).

Instrumentation Amplifier AD620 Absolute Maximum Ratings

Exceeding the following ratings may cause permanent damage to the device, so strict compliance is required during use:

Parameter	Rating
Supply voltage	±18 V
Internal power dissipation (free air)	650 mW
Common-mode input voltage	±VS (power supply voltage range)
Differential input voltage	25 V
Output short-circuit duration	Unlimited (can be shorted long-term)
Storage temperature (N, R packages)	-65°C to +125°C
Storage temperature (Q package)	-65°C to +150°C
Operating temperature (Models A, B)	-40°C to +85°C
Operating temperature (Model S)	-55°C to +125°C
Soldering temperature (10 seconds)	300°C

AD620 Instrumentation Amplifier Electrical Specifications

The following parameters are tested at TA=25°C, VS=±15V, and RL=2kΩ (unless otherwise specified):

1. Gain Characteristics

Parameter	Condition	AD620A (max)	AD620B (max)	Unit
Gain range	—	1 to 10,000	1 to 10,000	—
Gain error	G=1	0.10%	0.02%	%
	G=100	0.30%	0.15%	%
Nonlinearity	G=1−1000	40 ppm	40 ppm	ppm

2. DC Error Characteristics

Parameter	Condition	AD620A (max)	AD620B (max)	Unit
Input offset voltage	VS=±5Vto±15V	125 μV	50 μV	μV
Input offset voltage drift	—	1.0 μV/°C	0.6 μV/°C	μV/°C
Input bias current	Ambient temperature	2.0 nA	1.0 nA	nA
Common Mode Rejection Ratio (CMRR)	G=100	110 dB	120 dB	dB

3. AC and Noise Characteristics

Parameter	Condition	Typical Value	Unit
Bandwidth	G=1	1000 kHz	kHz
	G=100	120 kHz	kHz
Settling time (to 0.01%)	G=1−100	15 μs	μs
Voltage noise (1 kHz)	—	9 nV/√Hz	nV/√Hz
0.1 Hz to 10 Hz noise	G=100	0.28 μV p-p	μV p-p

Instrumentation Amplifier AD620 Theory of Operation

The AD620 is based on an improved three-op-amp architecture, consisting of an input stage, a gain stage, and an output stage:

Input stage: Uses Superβ transistors Q1 and Q2 to reduce input bias current; the input differential voltage is applied to the external resistor RG through a feedback loop (A1, A2, R1, R2), generating a current proportional to the input voltage.
Gain setting: Internal resistors R1 and R2 are laser-trimmed to 24.7 kΩ, with a total resistance of 49.4 kΩ. The gain formula is , which can be programmed with only one external resistor.
Output stage: Op-amp A3 forms a unity-gain subtractor to eliminate common-mode signals and output a single-ended signal (referenced to the REF terminal potential), realizing differential-to-single-ended conversion.

The advantages of this architecture are:

Gain increases as RG decreases, while open-loop gain improves, reducing gain error;
Gain-bandwidth product is optimized with increasing gain, balancing high gain and bandwidth requirements;
Laser trimming ensures internal resistor matching, improving CMRR and linearity.

AD620 Settling Time Test Circuit

The AD620 settling time test circuit is designed to evaluate the device’s dynamic response to step signals (time to settle within 0.01% of the final value). In the circuit, a 10V p-p differential input signal connects to the AD620’s ±IN pins. A DIP switch selects among three gain - setting resistors (18.1 kΩ, 1.81 kΩ, 180 Ω), corresponding to gains of approximately 1, 10, and 100. The power supply operates over a ±VS range (e.g., ±15 V), and the output (Pin 6) directly monitors voltage transients.
The circuit focuses on dynamic performance verification: when the input steps, the AD620 must quickly overcome offsets, noise, and other disturbances to stabilize the output within the target accuracy. While 1% - tolerance resistors affect the absolute gain, they do not interfere with trend - based settling - time testing. The typical 15 μs settling time, validated in this setup, confirms the AD620’s suitability for high - speed data acquisition (e.g., multiplexed systems), guiding timing design in industrial control and instrumentation applications.

Simplified Schematic of AD620

The simplified schematic of the AD620 reveals its improved three - op - amp architecture. Input stage op - amps A1/A2 are biased by 30μA current sources, with Superβ transistors Q1/Q2 minimizing input bias current. Laser - trimmed resistors R1/R2 (24.7 kΩ each) and an external gain resistor (between pins 1/8) form the gain network. Output stage A3 acts as a differential - to - single - ended subtractor, with the REF pin setting the output zero. Laser - trimmed matching and symmetric design enhance CMRR for precision signal amplification.

AD620 A Pressure Monitor Circuit that Operates on a 5 V Single Supply

AD620 A Medical ECG Monitor Circuit

AD620 Diode Protection for Voltages Beyond Supply

Instrumentation Amplifier AD620 Applications

The low power consumption and high precision of the AD620 make it suitable for multiple fields:

Medical instruments: Such as ECG monitoring (Figure 39), low noise and high CMRR can capture weak ECG signals, and ±3 V power supply is suitable for portable devices.
Weighing and pressure measurement: Cooperating with strain gauge bridge sensors (350 Ω or 3 kΩ), a 20 mV full-scale signal is amplified to 2 V with a gain of 100, and the total power consumption under 5 V single power supply is only 3.8 mA (including the sensor).
Data acquisition systems: As a front-end amplifier, it cooperates with ADC to achieve high-precision data acquisition, and the 15 μs settling time supports multi-channel multiplexing switching.
Industrial control: Used in transmitter interfaces and temperature measurement (e.g., thermocouple signal amplification), with a wide power supply range adapting to industrial environments.
Precision V-I conversion: Forms a voltage-to-current converter with an external op-amp (Figure 40), with a power consumption of 1.8 mA under ±3 V power supply, suitable for low-power closed-loop control systems.

AD620 Seires Models

The AD620 is available in multiple models, differing mainly in package and temperature range:

Model	Temperature Range	Package Type	Package Option
AD620AN	-40°C to +85°C	8-pin PDIP	N-8
AD620AR	-40°C to +85°C	8-pin SOIC	R-8
AD620BRZ	-40°C to +85°C	8-pin SOIC (RoHS)	R-8
AD620SQ/883B	-55°C to +125°C	8-pin CERDIP	Q-8

Note: The suffix "Z" indicates RoHS compliance, and "REEL" indicates tape and reel packaging, suitable for automated assembly.

Conclusion

The AD620 achieves the core performance of high-precision instrumentation amplifiers at a low cost. Its flexible gain setting, wide power supply range, and low power consumption make it an ideal choice from portable devices to industrial systems. By reasonably designing external circuits (such as gain resistors, filtering, and grounding), its performance advantages can be fully utilized to meet precision measurement requirements.

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