Texas Instruments INA121UG4

Overview

INA121UG4 from Texas Instruments (formerly Burr-Brown) is a FET-input, low-power instrumentation amplifier designed for precision signal conditioning of high-impedance transducer outputs. It delivers ±4pA bias current, ±200µV 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 INA121UG4 datasheet, INA121UG4 pinout, INA121UG4 application, or INA121UG4 equivalent, this page provides verified technical context, gain-setting resistor guidance, input bias return path requirements, and validated alternatives for low-noise, high-CMR analog front-end design.

Technical Context

The INA121UG4 uses a three-op-amp current-feedback architecture with laser-trimmed internal 40kΩ feedback resistors, enabling stable gain from 1 to 10,000 V/V via a single external RG resistor. Its FET input stage ensures ±4pA bias current and 10¹²Ω input impedance, critical for RTD and capacitive sensor interfacing.

It features integrated ±40V input overvoltage protection, 20nV/√Hz input voltage noise at 1kHz (G=100), and maintains 50kHz bandwidth at G=100. The Ref pin establishes output reference potential, requiring low-impedance grounding to preserve CMR - an 8Ω series resistance degrades typical CMR to ~80dB.

Key Specifications

Pinout & Package

INA121UG4 is supplied 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. Thermal resistance θ_JA is 150°C/W.

Key Features

Applications

Equivalent & Alternatives

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

Compared with INA121UG4, INA128UA trades 3× higher quiescent current for 4× better offset voltage, while AD8421ARMZ offers 200× higher bandwidth at the cost of pinout incompatibility and no integrated overvoltage protection - making INA121UG4 optimal for low-power, high-protection, moderate-bandwidth transducer interfaces.

Availability

INA121UG4 is available at Aetrix Electronics and suitable for bridge transducer amplifiers, portable ECG monitors, and multi-channel data acquisition systems requiring stable component supply across industrial temperature ranges (–40°C to +85°C).

Supply support for INA121UG4 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 signal conditioning ICs for industrial and medical applications.

The INA121 product line was engineered specifically for low-bias-current, high-CMR amplification of microvolt-level sensor outputs in battery-constrained and electrically noisy environments.

FAQ

What is the recommended gain-setting resistor (RG) value for G = 100 on the INA121UG4?

The INA121UG4 achieves G = 100 with RG = 505Ω, calculated from G = 1 + 50kΩ/RG. A standard 1% metal-film resistor of 511Ω is recommended per TI's application table, introducing <0.1% gain error. Layout must minimize parasitic resistance - solder joints and trace length contribute directly to gain inaccuracy at high gains.

Does the INA121UG4 require external input filtering to meet IEC 61000-4-5 surge immunity?

No. The INA121UG4 incorporates internal ±40V input overvoltage protection on both inputs, eliminating the need for external TVS diodes in most industrial surge environments. However, for Level 4 (4kV) IEC 61000-4-5 compliance, external 100Ω series resistors plus 100nF capacitors to ground are recommended to limit peak current during fast transients.

Can the INA121UG4 operate from a single +5V supply?

Yes. The INA121UG4 supports single-supply operation down to +4.5V total (±2.25V). When using +5V, connect V– to ground and V+ to +5V; configure Ref to +2.5V (mid-supply) using a precision divider or reference IC. Input common-mode range then becomes 1.2V to 3.8V, and output swing is 0.15V to 4.85V at light loads.

How does the Ref pin affect common-mode rejection ratio (CMR) in the INA121UG4?

The Ref pin sets the output common-mode level. A series impedance >8Ω degrades typical CMR from 106dB to ~80dB at G=100 due to imbalance in the output amplifier's feedback network. For optimal CMR, drive Ref with a low-impedance buffer (e.g., OPA277) or connect directly to a clean ground plane with minimal trace length.

What is the maximum safe RG resistor value to maintain stability in the INA121UG4?

RG values up to 100kΩ (G ≈ 1.5) are stable without compensation. For RG > 100kΩ (G < 1.5), phase margin decreases due to internal pole interaction; TI recommends adding a 10pF capacitor across RG when G ≤ 1.2 to prevent peaking or oscillation. This is documented in the "Dynamic Performance" section of the INA121UG4 datasheet.