AD620 — Low-Cost Low-Power Precision Instrumentation Amplifier

Overview

The AD620 (AD620AN / AD620AR) is the quintessential low-cost, high-accuracy monolithic instrumentation amplifier manufactured by Analog Devices. Replacing discrete three-op-amp instrumentation circuits with laser-trimmed monolithically matched internal resistors, it allows the user to program precise voltage gains from $1$ to $10,000$ using just a single external resistor ( $R_G$ ).

Operating from dual power supplies ranging from $\pm 2.3\text{V}$ to $\pm 18.0\text{V}$ (or single supplies from $+4.6\text{V}$ to $+36.0\text{V}$ ), the AD620 draws only $1.3\text{ mA}$ maximum supply current while achieving an exceptional common-mode rejection ratio of $> 100\text{ dB}$ ( $G = 10$ ) and low input voltage noise of $9\text{ nV}/\sqrt{\text{Hz}}$ at $1\text{ kHz}$ . It is universally employed in precision load-cell weight scales, strain gauge sensor interfaces, RTD temperature bridges, and medical bio-potential monitoring (ECG/EEG/EMG front-ends).

Quick reference


Amplifier Type	Monolithic 3-Op-Amp Precision Instrumentation Amplifier
Package	8-pin PDIP (AD620ANZ) / 8-pin SOIC (AD620ARZ)
Gain Range	$G = 1$ to $10,000$ (Set by single resistor $R_G$ )
Gain Formula	$R_G = 49.4\text{ k}\Omega / (G - 1)$
Supply Voltage Range	$\pm 2.3\text{ V}$ to $\pm 18.0\text{ V}$ split (or $+4.6\text{V}$ to $+36\text{V}$ single)
Common-Mode Rejection Ratio ( $CMRR$ )	$100\text{ dB}$ min ( $G = 10$ ) / $130\text{ dB}$ typ ( $G = 100$ )
Input Offset Voltage ( $V_{OS}$ )	$50\ \mu\text{V}$ max ( $0.6\ \mu\text{V/}^\circ\text{C}$ drift)
Input Bias Current ( $I_B$ )	$1.0\text{ nA}$ maximum ( $0.5\text{ nA}$ typical)
Bandwidth ( $-3\text{dB}$ )	$120\text{ kHz}$ ( $G = 100$ ) / $1.0\text{ MHz}$ ( $G = 1$ )
Quiescent Current	$0.9\text{ mA}$ typical / $1.3\text{ mA}$ maximum

Pinout (DIP-8 / SOIC-8 Package)

text

        ┌──────────────┐
   -RG  ─│ 1          8 │─ +RG
   -IN  ─│ 2   AD     7 │─ +VS
   +IN  ─│ 3   620    6 │─ OUTPUT
   -VS  ─│ 4          5 │─ REF
        └──────────────┘

Pin	Name	Description
1	`-RG`	Gain setting resistor negative terminal
2	`-IN`	Inverting differential analog signal input
3	`+IN`	Non-inverting differential analog signal input
4	`-VS`	Negative power supply rail ( $-2.3\text{V}$ to $-18.0\text{V}$ or GND)
5	`REF`	Output reference voltage pin (Connect to Ground or $V_{ADC}/2$ for level-shifting)
6	`OUTPUT`	Single-ended amplified analog output voltage
7	`+VS`	Positive power supply rail ( $+2.3\text{V}$ to $+18.0\text{V}$ )
8	`+RG`	Gain setting resistor positive terminal

Gain Selection & Resistor Values

The gain is programmed by selecting resistor $R_G$ placed between Pin 1 and Pin 8 according to the formula:

$G = 1 + \frac{49.4\text{ k}\Omega}{R_G} \iff R_G = \frac{49.4\text{ k}\Omega}{G - 1}$

Desired Gain ( $G$ )	Exact Calculated $R_G$	Nearest 1% Standard Resistor	Nearest 0.1% Standard Resistor
1	$\infty$ (Open circuit)	None (Pins 1 & 8 left open)	None
10	$5.489\text{ k}\Omega$	$5.49\text{ k}\Omega$	$5.49\text{ k}\Omega$
50	$1.008\text{ k}\Omega$	$1.00\text{ k}\Omega$	$1.01\text{ k}\Omega$
100	$498.99\ \Omega$	$499\ \Omega$	$499\ \Omega$
500	$98.99\ \Omega$	$100\ \Omega$	$98.8\ \Omega$
1000	$49.45\ \Omega$	$49.9\ \Omega$	$49.3\ \Omega$

Standard Wheatstone Bridge Interface Circuit

text

             +V_EXC Bridge Excitation (+5.0V)
                   │
             ┌─────┴─────┐
             │           │
           [ R1 ]      [ R2 ]
             │           │
             ├───────────┼──────────[Pin 2: -IN]
             │           │           AD620
      [ Strain Gauge ] [ R3 ]        │
             │           │           [Pin 1: -RG] ──[ R_G ]── [Pin 8: +RG]
             ├───────────┴──────────[Pin 3: +IN]
             │                       │
            GND                     [Pin 5: REF] ──────────── GND (or 2.5V Offset)
                                     │
                                    [Pin 6: OUTPUT] ───────── Analog Out to ADC

Specifications

Parameter	Symbol	Min	Typ	Max	Unit	Conditions
Gain Non-Linearity	$NL$	—	10	40	ppm	$G = 1 \dots 100, R_L = 10\text{ k}\Omega$
Input Offset Voltage (Laser Trimmed)	$V_{OSI}$	—	30	50	$\mu\text{V}$	$G = 1000, T_A = 25^\circ\text{C}$
Input Offset Current	$I_{OS}$	—	0.3	0.5	nA	$T_A = 25^\circ\text{C}$
Common-Mode Rejection Ratio	$CMRR$	93	100	—	dB	$G = 10, V_{CM} = \pm 10\text{V}$
Common-Mode Rejection ( $G = 100$ )	$CMRR$	110	130	—	dB	$G = 100, V_{CM} = \pm 10\text{V}$
Slew Rate	$SR$	—	1.2	—	$\text{V/}\mu\text{s}$	$G = 1 \dots 100$
Output Voltage Swing (Positive)	$V_{OH}$	$+VS - 1.4$	$+VS - 1.1$	—	V	$R_L = 2\text{ k}\Omega$
Output Voltage Swing (Negative)	$V_{OL}$	—	$-VS + 1.2$	$-VS + 1.5$	V	$R_L = 2\text{ k}\Omega$

Common mistakes

Leaving no DC return path for input bias currents: Because the AD620 inputs are bipolar transistors, both +IN and -IN must have a DC resistive path to ground ( $COM$ ). Connecting AC-coupled capacitors or a floating thermocouple directly to the inputs causes charge accumulation, driving the amplifier into saturation. Connect a $100\text{ k}\Omega \dots 1\text{ M}\Omega$ resistor from each input to ground.
Driving the output outside headroom limits on single supply: The AD620 is not a rail-to-rail op-amp. On a single $+5\text{V}$ supply (with $-V_S = \text{GND}$ ), output voltage swing is limited to $+1.5\text{V} \dots +3.8\text{V}$ . For true $0\text{V} \dots 5\text{V}$ single-supply operation, use the INA333.
Leaving the REF pin floating: Pin 5 (REF) sets the zero-reference voltage for the output ( $V_{OUT} = G \times (V_+ - V_-) + V_{REF}$ ). Leaving REF floating causes unpredictable output offsets. Tie REF to ground (dual supply) or to a low-impedance mid-supply reference ( $2.5\text{V}$ ).

Notes

Reference Pin Drive: Always drive the REF pin with a low-impedance source (such as an op-amp buffer or ground plane). Adding series resistance to REF directly degrades the common-mode rejection ratio.

Interface	Differential Inputs (+IN, -IN), Single Resistor Gain Pins (+RG, -RG), Reference Pin (REF), Output
Package	8-pin PDIP / 8-pin SOIC