INA128 — Precision Low-Power Instrumentation Amplifier

Overview

The INA128 (INA128P / INA128U) is a low-power, general-purpose precision instrumentation amplifier designed by Burr-Brown (now Texas Instruments). Built upon the classic 3-op-amp instrumentation topology with internal laser-trimmed thin-film resistors, it allows designers to configure precise gains from $1$ to $10,000$ using a single external resistor ( $R_G$ ).

Operating from power supplies spanning $\pm 2.25\text{V}$ to $\pm 18.0\text{V}$ , the INA128 consumes only $700\ \mu\text{A}$ quiescent current while delivering an exceptional $120\text{ dB}$ minimum CMRR ( $G = 100$ ). A critical differentiator of the INA128 is its rugged $\pm 40\text{V}$ input overvoltage protection, allowing the device to withstand harsh industrial signal lines, sensor fault conditions, and accidental power rail cross-wiring without external clamping diodes.

Quick reference


Amplifier Type	Precision 3-Op-Amp Monolithic Instrumentation Amplifier
Package	8-pin PDIP (INA128P) / 8-pin SOIC (INA128U)
Gain Range	$G = 1$ to $10,000$ (Set by single resistor $R_G$ )
Gain Formula	$G = 1 + 50\text{ k}\Omega / R_G \iff R_G = 50\text{ k}\Omega / (G - 1)$
Supply Voltage Range	$\pm 2.25\text{ V}$ to $\pm 18.0\text{ V}$ split (or $+4.5\text{V}$ to $+36\text{V}$ single)
Common-Mode Rejection Ratio ( $CMRR$ )	$120\text{ dB}$ min ( $G = 100$ ) / $125\text{ dB}$ typ
Input Protection	$\pm 40\text{ V}$ continuous overvoltage without damage
Input Offset Voltage ( $V_{OS}$ )	$50\ \mu\text{V}$ max ( $0.5\ \mu\text{V/}^\circ\text{C}$ drift)
Bandwidth ( $-3\text{dB}$ )	$200\text{ kHz}$ ( $G = 100$ ) / $1.3\text{ MHz}$ ( $G = 1$ )
Quiescent Current	$700\ \mu\text{A}$ typical ( $750\ \mu\text{A}$ maximum)

Pinout (DIP-8 / SOIC-8 Package)

text

        ┌──────────────┐
    RG  ─│ 1          8 │─ RG
   -IN  ─│ 2   INA    7 │─ V+
   +IN  ─│ 3   128    6 │─ Vo (Output)
    V-  ─│ 4          5 │─ Ref
        └──────────────┘

Pin	Name	Description
1, 8	`RG`	Gain setting resistor terminals (Connect single resistor $R_G$ )
2	`-IN`	Inverting differential analog signal input (Protected to $\pm 40\text{V}$ )
3	`+IN`	Non-inverting differential analog signal input (Protected to $\pm 40\text{V}$ )
4	`V-`	Negative power supply rail ( $-2.25\text{V}$ to $-18.0\text{V}$ or GND)
5	`Ref`	Output reference voltage pin (Sets output offset reference)
6	`Vo`	Single-ended amplified analog output voltage
7	`V+`	Positive power supply rail ( $+2.25\text{V}$ to $+18.0\text{V}$ )

Gain Selection & Resistor Values

The gain equation is governed by two internal $25\text{ k}\Omega$ precision resistors:

$G = 1 + \frac{50\text{ k}\Omega}{R_G} \iff R_G = \frac{50\text{ k}\Omega}{G - 1}$

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	$5.555\text{ k}\Omega$	$5.62\text{ k}\Omega$	$5.56\text{ k}\Omega$
50	$1.020\text{ k}\Omega$	$1.02\text{ k}\Omega$	$1.02\text{ k}\Omega$
100	$505.05\ \Omega$	$511\ \Omega$	$505\ \Omega$
500	$100.20\ \Omega$	$100\ \Omega$	$100\ \Omega$
1000	$50.05\ \Omega$	$49.9\ \Omega$	$50.0\ \Omega$

Standard Bridge Circuit

text

             +V_EXC Bridge Excitation (+5.0V)
                   │
             ┌─────┴─────┐
             │           │
           [ R1 ]      [ R2 ]
             │           │
             ├───────────┼──────────[Pin 2: -IN]
             │           │           INA128
      [ Bridge Sensor ] [ R3 ]       │
             │           │           [Pin 1: RG] ────[ R_G ]──── [Pin 8: RG]
             ├───────────┴──────────[Pin 3: +IN]
             │                       │
            GND                     [Pin 5: Ref] ──────────── GND (or 2.5V Offset)
                                     │
                                    [Pin 6: Vo] ───────────── Analog Out to ADC

Specifications

Parameter	Symbol	Min	Typ	Max	Unit	Conditions
Gain Non-Linearity	$NL$	—	0.001	0.005	%	$G = 1 \dots 100, R_L = 10\text{ k}\Omega$
Input Offset Voltage	$V_{OSI}$	—	25	50	$\mu\text{V}$	$G = 1000, T_A = 25^\circ\text{C}$
Input Bias Current	$I_B$	—	2.0	5.0	nA	$T_A = 25^\circ\text{C}$
Common-Mode Rejection Ratio	$CMRR$	120	125	—	dB	$G = 100, V_{CM} = \pm 10\text{V}$
Slew Rate	$SR$	—	4.0	—	$\text{V/}\mu\text{s}$	$G = 1 \dots 100$
Output Voltage Swing (Positive)	$V_{OH}$	$+V - 1.4$	$+V - 0.9$	—	V	$R_L = 10\text{ k}\Omega$
Output Voltage Swing (Negative)	$V_{OL}$	—	$-V + 0.8$	$-V + 1.2$	V	$R_L = 10\text{ k}\Omega$
Input Overvoltage Limit	$V_{IN(MAX)}$	—	—	$\pm 40$	V	Continuous differential/common-mode

Common mistakes

Comparing gain formulas with AD620: The AD620 uses $49.4\text{ k}\Omega$ , whereas the INA128 uses $50.0\text{ k}\Omega$ . Using AD620 resistor values in an INA128 circuit produces a $\approx 1.2\%$ gain error.
Driving single supply with inputs near ground: On a single $+5\text{V}$ supply, the input common-mode voltage range is limited to $+1.9\text{V} \dots +3.4\text{V}$ . Connecting a ground-referenced thermocouple will clip the input buffer stages. Use INA333 for rail-to-rail single-supply operation.
Unbuffered reference voltage on Pin 5: The Ref pin must be driven with a source impedance $< 10\ \Omega$ . A high-impedance resistor divider directly connected to Ref degrades the common-mode rejection ratio. Always buffer with an op-amp.

Notes

Sibling Part (INA129): The INA129 is the $49.4\text{ k}\Omega$ gain-equation companion to the INA128, designed for exact resistor interchangeability with the AD620.

Interface	Differential Inputs (+IN, -IN), Gain Setting Terminals (RG), Output Reference (Ref), Amplified Output (Vo)
Package	8-pin PDIP / 8-pin SOIC