Texas Instruments INA121UA/2K5G4

Overview

INA121UA/2K5G4 from Texas Instruments (formerly Burr-Brown) is a FET-input, low-power instrumentation amplifier optimized for high-impedance sensor interfaces. It delivers ±4pA input bias current, ±200µV max input offset voltage, 106dB CMR at G=100, ±2.25V to ±18V dual-supply operation, and gain programmability from 1 to 10,000 V/V via a single external resistor - enabling precision bridge, thermocouple, and ECG signal conditioning in battery-powered medical and industrial data acquisition systems.

For engineers reviewing the INA121UA/2K5G4 datasheet, INA121UA/2K5G4 pinout, INA121UA/2K5G4 application, or INA121UA/2K5G4 equivalent, key selection considerations include its laser-trimmed offset drift (±2µV/°C), 50kHz bandwidth at G=100, SOIC-8 surface-mount package with MSL Level-3 rating, input protection to ±40V, and compatibility with low-voltage (±2.25V) and single-5V supply configurations.

Technical Context

The INA121UA/2K5G4 employs a three-op-amp current-feedback architecture that enables wide bandwidth (600kHz at G=1) while maintaining low quiescent current (±450µA). Its internal 50kΩ laser-trimmed feedback network ensures accurate gain setting and low temperature drift, with gain equation G = 1 + 50kΩ/R_G.

Input stage FET topology provides 10¹²Ω differential impedance and ±40V absolute maximum input voltage tolerance. The Ref pin establishes output reference potential, requiring low-impedance grounding to preserve CMR; >8Ω series resistance degrades CMR to ~80dB at G=1.

Key Specifications

Pinout & Package

INA121UA/2K5G4 is housed in an 8-pin SOIC (SO-8) surface-mount package per JEDEC MS-012 variation AA, with 1.27mm pitch, 3.9mm width, and 1.75mm max height. Thermal resistance θ_JA = 150°C/W.

Key Features

Applications

Equivalent & Alternatives

The following parts are listed as comparable options for similar instrumentation amplifier applications.

Compared with INA121UA/2K5G4, INA128UA/2K5 trades 0.85mA extra quiescent current for 14dB CMR improvement and 150µV lower offset, while AD8421ARMZ sacrifices input ruggedness and low-frequency noise performance for 20× higher small-signal bandwidth - making INA121UA/2K5G4 optimal for cost-sensitive, low-power, high-reliability sensor front-ends.

Availability

INA121UA/2K5G4 is available at Aetrix Electronics and suitable for bridge transducer amplifiers, portable ECG monitors, thermocouple interfaces, and multi-channel data acquisition systems requiring stable component supply, long-term lifecycle support, and RoHS-compliant SOIC packaging.

Supply support for INA121UA/2K5G4 includes scheduled delivery planning, volume procurement assistance, BOM continuity management, traceable sourcing, and lifecycle availability coordination for OEM customers, industrial embedded developers, connected-device designers, and electronics production programs.

Manufacturer

Texas Instruments acquired Burr-Brown in 2000 and maintains its precision analog portfolio, emphasizing high-accuracy, low-power, and robust signal-conditioning ICs for industrial, medical, and test equipment markets.

The INA121 product line was designed specifically for low-power, high-input-impedance instrumentation amplifier applications - targeting battery-operated medical devices, portable DAQ, and industrial sensor interfaces where bias current, offset drift, and supply flexibility are critical.

FAQ

What is the maximum gain achievable with INA121UA/2K5G4, and how is it set?

The INA121UA/2K5G4 supports gains from 1 V/V to 10,000 V/V, set by a single external resistor RG connected between pins 1 and 8 using the formula G = 1 + 50kΩ/RG. For G=10,000, RG = 5.001Ω (4.99Ω standard value); layout parasitics become critical below 100Ω, so Kelvin connections and low-inductance routing are recommended. The INA121UA/2K5G4's internal laser-trimmed 50kΩ network ensures accuracy without external calibration.

Does INA121UA/2K5G4 support single-supply operation?

Yes, the INA121UA/2K5G4 operates with total supply voltages as low as +4.5V (±2.25V), enabling true single-5V operation when V+ = +5V and V– = 0V (ground). In this configuration, the Ref pin must be biased to mid-supply (e.g., 2.5V) to center the output swing; input common-mode range shrinks to approximately 1.2V above ground and 2.1V below V+, per the "Input Common-Mode Range vs Output Voltage" curve. The INA121UA/2K5G4 maintains ±450µA quiescent current and 106dB CMR under these conditions.

How does the Ref pin affect common-mode rejection in INA121UA/2K5G4?

The Ref pin defines the output common-mode level and directly impacts CMR: a series resistance >8Ω degrades typical CMR from 106dB to ~80dB at G=1 due to imbalance in the output amplifier's feedback path. To preserve performance, Ref must connect to a low-impedance source - such as a buffered voltage divider or op-amp follower - especially in high-CMR applications like ECG. The INA121UA/2K5G4's internal architecture makes Ref sensitivity a critical layout consideration, not a specification limitation.

What is the input protection capability of INA121UA/2K5G4, and how is it implemented?

The INA121UA/2K5G4 features integrated overvoltage protection on both inputs, rated for continuous ±40V differential and common-mode voltage beyond supply rails. This is achieved via internal 25kΩ series resistors and clamp diodes to V+ and V–, visible in the functional diagram. Unlike external protection schemes, this architecture avoids added noise or leakage, preserving the ±4pA bias current and 10¹²Ω input impedance. The INA121UA/2K5G4 sustains this rating across its full –40°C to +85°C operating range without derating.

Can INA121UA/2K5G4 drive ADC inputs directly, and what are the settling requirements?

Yes, the INA121UA/2K5G4 can directly drive SAR and delta-sigma ADCs with input capacitance ≤1000pF. Settling time to 0.01% is 20µs at G=1, 35µs at G=100, and 260µs at G=1000 - meaning a 16-bit ADC sampling at 100kSPS requires G≤100 to ensure full settling. For G=1000, maximum sample rate drops to ~3.8kSPS. The INA121UA/2K5G4's ability to drive capacitive loads without external isolation resistors simplifies anti-aliasing filter design in precision DAQ systems.