Texas Instruments INA121UAG4

Overview

INA121UAG4 from Texas Instruments (originally Burr-Brown) is a FET-input, low-power instrumentation amplifier designed for precision signal conditioning of high-impedance, low-level transducer outputs. It delivers ±4pA input bias current, ±200µV max input offset voltage, 106dB CMR at G=100, and operates from ±2.25V to ±18V supplies - enabling use in battery-powered ECG systems and bridge sensor interfaces.

For engineers reviewing the INA121UAG4 datasheet, INA121UAG4 pinout, INA121UAG4 application, or INA121UAG4 equivalent, key selection criteria include its laser-trimmed gain accuracy (±0.05% typ at G=100), 50kHz bandwidth at G=100, ±40V input overvoltage protection, and SOIC-8 package compatibility with industrial temperature range (–40°C to +85°C).

Technical Context

The INA121UAG4 uses a three-op-amp current-feedback architecture with internal 40kΩ/25kΩ resistor networks and laser-trimmed 50kΩ feedback elements. Its FET input stage enables direct interfacing with >1MΩ source impedances while maintaining 20nV/√Hz input voltage noise at 1kHz (G=100).

Gain is set externally via a single resistor RG between pins 1 and 8 per G = 1 + 50kΩ/RG, supporting 1V/V to 10,000V/V. Input common-mode range extends to within 1.2V of V+ and 2.1V beyond V−, and internal overvoltage protection clamps inputs safely to ±40V without damage.

Key Specifications

Pinout & Package

INA121UAG4 is housed in an 8-pin SOIC (SO-8) surface-mount package with 1.27mm lead pitch and 1.75mm max height, compliant with JEDEC MS-012 variation AA and RoHS requirements.

Key Features

Applications

Equivalent & Alternatives

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

Compared with INA128UA and AD8421ARMZ, the INA121UAG4 uniquely balances ultra-low bias current, integrated ±40V protection, and sub-500µA quiescent consumption - making it optimal for battery-powered, field-rugged instrumentation where reliability trumps raw speed or ultimate precision.

Availability

INA121UAG4 is available at Aetrix Electronics and suitable for ECG monitoring systems, industrial load cell interfaces, portable gas analyzers, and capacitive touch sensing requiring stable component supply across multi-year production cycles.

Supply support for INA121UAG4 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 full production, qualification, and technical support for legacy precision analog products including the INA121 series.

The INA121 product line was engineered specifically for low-power, high-accuracy signal conditioning in medical, industrial, and environmental sensing - emphasizing FET input integrity, laser-trimmed DC performance, and robust input protection.

FAQ

What is the maximum gain achievable with INA121UAG4?

The INA121UAG4 supports gains from 1V/V to 10,000V/V using a single external resistor RG per the equation G = 1 + 50kΩ/RG. At G=10,000, RG = 5.001Ω (4.99Ω standard value); layout parasitics and resistor TCR become critical, and gain accuracy degrades to ±0.5%. For stable operation above G=1000, use low-TC metal-film resistors and minimize trace inductance near pins 1 and 8. The INA121UAG4 datasheet confirms this range under "Gain Range" in the specifications table.

Does INA121UAG4 require external input filtering for ECG applications?

No, the INA121UAG4 does not require external RC filtering for basic ECG amplification due to its FET input architecture, which eliminates bias-current-induced offsets even with large series resistors. However, Figure 5 of the INA121UAG4 datasheet recommends matched R/C networks (R1=R2, C1=C2, C3≈10×C1) on pins 2 and 3 to preserve high-frequency CMR against 60Hz interference. Unmatched capacitance degrades CMR by converting common-mode noise to differential error - a documented behavior verified in the "Common-Mode Rejection vs Frequency" curve.

Can INA121UAG4 operate from a single +5V supply?

Yes, the INA121UAG4 operates from split supplies as low as ±2.25V (i.e., +4.5V total), enabling true single-supply use with V+ = +5V and V– = 0V. In this configuration, the Ref pin must be biased to mid-supply (e.g., +2.5V) using a low-impedance source to center the output swing. The "Input Common-Mode Range vs Output Voltage" curves in the INA121UAG4 datasheet confirm linear operation with VS = ±2.5V, and output swing reaches ±1.2V into 10kΩ - sufficient for driving ADC inputs directly.

What is the thermal resistance (θJA) of the INA121UAG4 SOIC-8 package?

According to the INA121UAG4 datasheet, the SOIC-8 package has a junction-to-ambient thermal resistance (θJA) of 150°C/W under standard JEDEC test conditions (1-layer board, 1in² copper pad). This value assumes no additional copper pour or thermal vias. With 2oz copper and 4 thermal vias under the exposed pad (if present), θJA can improve to ~65°C/W. The datasheet specifies this in the "Thermal Resistance" section of the Absolute Maximum Ratings table, confirming 150°C/W for SO-8 surface-mount variants including INA121UAG4.

How does the INA121UAG4 handle input overvoltage conditions?

The INA121UAG4 incorporates internal overvoltage protection diodes on both inputs, rated for continuous safe operation at ±40V regardless of supply voltage - verified in the Absolute Maximum Ratings table. During overvoltage, input current is limited to <1mA (per Figure "Input Over-Voltage V/I Characteristics"), preventing latch-up or parametric shift. Unlike unprotected amplifiers, the INA121UAG4 resumes normal operation immediately after fault removal with no calibration needed. This behavior is explicitly documented in the "Input Protection to ±40V" feature and confirmed in the typical performance curves.