INA333 — MicroPower (50µA) Precision Rail-to-Rail Output Instrumentation Amplifier

Overview

The INA333 (INA333AIDGKT / INA333AIDR) is an ultra-low-power, zero-drift precision instrumentation amplifier engineered by Texas Instruments. Designed for battery-operated medical devices, wearable biosignal monitors, and portable 3.3V/5V microcontroller systems, it draws an astonishingly low $50\ \mu\text{A}$ quiescent current while operating down to a single supply voltage of $1.8\text{ V}$ .

Utilizing a 3-op-amp architecture with patented internal auto-zeroing topology, the INA333 delivers true rail-to-rail output voltage swing (within $50\text{ mV}$ of the power rails), an ultra-low input offset voltage of $25\ \mu\text{V}$ maximum with practically zero drift ( $0.1\ \mu\text{V/}^\circ\text{C}$ ), and high common-mode rejection of $100\text{ dB}$ ( $G \ge 10$ ). A single external resistor ( $R_G$ ) programs precise voltage gains from $1$ to $1000$ .

Quick reference


Amplifier Type	MicroPower Zero-Drift Rail-to-Rail Instrumentation Amplifier
Package	8-pin VSSOP (MSOP-8) / 8-pin SOIC / 8-pin WSON (3mm × 3mm)
Gain Range	$G = 1$ to $1000$ (Set by single resistor $R_G$ )
Gain Formula	$G = 1 + 100\text{ k}\Omega / R_G \iff R_G = 100\text{ k}\Omega / (G - 1)$
Supply Voltage Range	$+1.8\text{ V}$ to $+5.5\text{ V}$ single supply (or $\pm 0.9\text{V}$ to $\pm 2.75\text{V}$ split)
Quiescent Current	$50\ \mu\text{A}$ typical ( $75\ \mu\text{A}$ maximum)
Input Offset Voltage ( $V_{OS}$ )	$25\ \mu\text{V}$ max ( $0.1\ \mu\text{V/}^\circ\text{C}$ drift)
Input Bias Current ( $I_B$ )	$70\text{ pA}$ typical / $200\text{ pA}$ maximum
Output Architecture	Rail-to-Rail Output ( $V_{OL} \le 50\text{ mV}, V_{OH} \ge V_+ - 50\text{ mV}$ )
Common-Mode Rejection ( $CMRR$ )	$100\text{ dB}$ min ( $G \ge 10$ ) / $115\text{ dB}$ typ

Pinout (VSSOP-8 / SOIC-8 Package)

text

        ┌──────────────┐
    RG  ─│ 1          8 │─ RG
  VIN-  ─│ 2   INA    7 │─ V+
  VIN+  ─│ 3   333    6 │─ VOUT
    V-  ─│ 4          5 │─ REF
        └──────────────┘

Pin	Name	Description
1, 8	`RG`	Gain setting resistor terminals (Connect single resistor $R_G$ )
2	`VIN-`	Inverting differential analog signal input
3	`VIN+`	Non-inverting differential analog signal input
4	`V-`	Negative supply terminal (Connect to GND for single-supply operation)
5	`REF`	Output reference voltage pin (Connect to $V_{DD}/2$ for single-supply bipolar signals)
6	`VOUT`	Rail-to-rail single-ended amplified output voltage
7	`V+`	Positive power supply rail (+1.8V to +5.5V DC)

Gain Selection & Resistor Values

The gain is set by two internal $50\text{ k}\Omega$ feedback resistors:

$G = 1 + \frac{100\text{ k}\Omega}{R_G} \iff R_G = \frac{100\text{ k}\Omega}{G - 1}$

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	$11.111\text{ k}\Omega$	$11.3\text{ k}\Omega$	$11.1\text{ k}\Omega$
50	$2.041\text{ k}\Omega$	$2.05\text{ k}\Omega$	$2.05\text{ k}\Omega$
100	$1.010\text{ k}\Omega$	$1.02\text{ k}\Omega$	$1.01\text{ k}\Omega$
500	$200.40\ \Omega$	$200\ \Omega$	$200\ \Omega$
1000	$100.10\ \Omega$	$100\ \Omega$	$100\ \Omega$

Single-Supply ECG / Biosignal Circuit (3.3V)

text

        +3.3V Single Supply
           │
     ┌─────┴──────────────────────┐
     │                            │
   [ V+ Pin 7 ]              [ 1.65V Mid-Supply Ref Buffer ]
     INA333                       │
   [Pin 5: REF] ──────────────────┘
   [Pin 2: VIN-] ──[ 10k ]──[ Lead RA (Right Arm) ]
   [Pin 3: VIN+] ──[ 10k ]──[ Lead LA (Left Arm) ]
   [Pin 1: RG ] ────[ R_G = 1.02kΩ (Gain=100) ]──── [Pin 8: RG]
   [Pin 6: VOUT] ───────────────────────────────── Analog Output to MCU ADC (0V - 3.3V)
   [Pin 4: V-  ]
           │
          GND

Specifications

Parameter	Symbol	Min	Typ	Max	Unit	Conditions
Input Offset Voltage	$V_{OS}$	—	10	25	$\mu\text{V}$	$G = 100, T_A = 25^\circ\text{C}$
Input Offset Voltage Drift	$dV_{OS}/dT$	—	0.05	0.1	$\mu\text{V/}^\circ\text{C}$	$-40^\circ\text{C} \le T_A \le 125^\circ\text{C}$
Input Bias Current	$I_B$	—	70	200	pA	$T_A = 25^\circ\text{C}$
Common-Mode Rejection Ratio	$CMRR$	100	115	—	dB	$G \ge 10, V_{CM} = 0.1\text{V} \dots V_+ - 0.1\text{V}$
Bandwidth ( $-3\text{dB}$ )	$BW$	—	35	—	kHz	$G = 10$
Slew Rate	$SR$	—	0.16	—	$\text{V/}\mu\text{s}$	$G = 1 \dots 100$
Output Low Swing	$V_{OL}$	—	20	50	mV	$R_L = 10\text{ k}\Omega$ to $V_+/2$
Output High Swing	$V_{OH}$	$V_+ - 50\text{mV}$	$V_+ - 20\text{mV}$	—	V	$R_L = 10\text{ k}\Omega$ to $V_+/2$
Quiescent Current	$I_Q$	—	50	75	$\mu\text{A}$	$T_A = 25^\circ\text{C}$

Common mistakes

Exceeding input common-mode range ( $V_{CM}$ ) on single supply: The input common-mode voltage range extends from $(V-) + 0.1\text{V}$ to $(V+) - 0.1\text{V}$ . Operating inputs below ground or within $100\text{mV}$ of the supply rail will introduce non-linearities.
Using high-impedance divider for REF pin: Pin 5 (REF) sets the baseline output level. Driving REF from an unbuffered resistor divider impairs internal resistor matching, severely reducing common-mode rejection. Always use an active op-amp buffer (like OPA333 or TLV9001) to drive REF.
Forgetting that the gain formula uses 100kΩ: Unlike the AD620 ( $49.4\text{ k}\Omega$ ) and INA128 ( $50\text{ k}\Omega$ ), the INA333 uses $100\text{ k}\Omega$ ( $2 \times 50\text{ k}\Omega$ ). Using an AD620 gain resistor will result in roughly half the intended gain.

Notes

Zero-Drift Architecture: The internal auto-zeroing topology continuously removes $1/f$ low-frequency flicker noise, making the INA333 exceptionally accurate for sub-Hz biomedical and strain measurements.