Texas Instruments INA121P

Overview

INA121P from Texas Instruments (formerly Burr-Brown) is a FET-input, low-power instrumentation amplifier optimized for high-impedance sensor interfaces. It delivers ±4pA bias current, ±200µV input offset voltage, 106dB CMR at G=100, and operates from ±2.25V to ±18V supplies - enabling precision signal conditioning in battery-powered ECG, bridge transducer, and RTD measurement systems.

For engineers reviewing the INA121P datasheet, INA121P pinout, INA121P application, or INA121P equivalent, key selection considerations include its laser-trimmed gain accuracy (±0.05% typ at G=100), 50kHz bandwidth at G=100, ±40V input protection, and compatibility with single-resistor gain setting (RG) across 1–10,000 V/V range.

Technical Context

The INA121P 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-referred noise at 1kHz (G=100).

Gain is set externally via RG between pins 1 and 8 per G = 1 + 50kΩ/RG; internal metal-film resistors contribute to ±25ppm/°C gain drift above G=1. Input overvoltage protection clamps to ±40V without damage, and the Ref pin requires ≤8Ω impedance to preserve CMR performance.

Key Specifications

Pinout & Package

INA121P is housed in an 8-pin plastic DIP (PDIP-8) package with 0.300" wide body, RoHS-compliant lead finish, and JEDEC MS-001 compliant outline. Thermal resistance θ_JA is 100°C/W.

Key Features

Applications

Equivalent & Alternatives

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

Compared with INA121P, INA128P offers superior CMR and noise at the cost of 3.3× higher supply current, while AD620ANZ provides lower bias current but requires external input protection circuitry - making INA121P the optimal balance of protection, power, and precision for industrial sensor interfaces.

Availability

INA121P is available at Aetrix Electronics and suitable for ECG monitoring systems, strain gauge-based load cells, RTD temperature sensing, and portable gas detectors requiring stable component supply with guaranteed long-term availability.

Supply support for INA121P 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 and technical support for legacy precision analog products including the INA121 series.

The INA121 product line was designed specifically for low-power, high-accuracy instrumentation applications requiring robust input protection and flexible gain configuration in space-constrained industrial and medical systems.

FAQ

What is the maximum gain achievable with INA121P using external resistor RG?

The INA121P supports gains from 1 V/V to 10,000 V/V using a single external resistor RG connected between pins 1 and 8, per the equation G = 1 + 50kΩ/RG. For G = 10,000, RG = 5.001Ω (4.99Ω standard value); layout parasitics and resistor tolerance become critical at such low values. The INA121P datasheet confirms this range is fully specified and tested.

Does INA121P require external input protection when used with ±15V supplies?

No - the INA121P integrates internal overvoltage protection that withstands ±40V on input pins regardless of supply voltage, including ±15V operation. This eliminates the need for external clamping diodes in most industrial sensor applications, as confirmed by Absolute Maximum Ratings and Input Over-Voltage V/I Characteristic curves in the INA121P datasheet.

Can INA121P operate from a single +5V supply?

Yes - the INA121P functions with total supply voltage as low as +4.5V (±2.25V), so a single +5V rail with ground is viable. However, the input common-mode range shrinks to approximately –0.3V to +3.8V, and output swing reduces to ~0.3V to 4.7V. These limitations are documented in the "Input Common-Mode Range vs Output Voltage" typical curves for VS = ±2.5V in the INA121P datasheet.

What is the recommended PCB layout practice for maintaining CMR in INA121P circuits?

To preserve the INA121P's 106dB CMR, route VIN+ and VIN– traces as a tightly coupled differential pair with identical length and spacing, place a solid ground plane beneath them, and keep the Ref pin connection impedance below 8Ω using short, wide traces. Any series resistance >8Ω on Ref degrades CMR to ~80dB, as quantified in the INA121P Application Information section.

How does the INA121P's current-feedback architecture affect settling time at high gain?

The INA121P's current-feedback topology maintains excellent settling time even at high gains: 35µs to 0.01% for G=100 and 260µs for G=1000. This contrasts with conventional voltage-feedback instrumentation amps that degrade significantly at high gain. These values are measured and specified in the INA121P Electrical Characteristics table under "Settling Time, 0.01%".