Designing with INA128: Datasheet, Pinout, and Precision Sensor Amplification Guide

Executive Summary: What is the INA128?

The INA128 is a low-power, general-purpose precision instrumentation amplifier designed for accurate signal conditioning in bridge, thermocouple, and medical applications. It utilizes a classic 3-op amp architecture to provide excellent accuracy and wide bandwidth even when operating at high gain.

• Market Position: An industry-standard, high-precision component known for its versatility and reliability in professional-grade electronics.

• Top Features: Extremely low offset voltage (50µV max), high common-mode rejection (120dB min), and a wide supply range of ±2.25V to ±18V.

• Primary Audience: Ideal for medical device designers, industrial process control engineers, and high-end sensor developers.

• Supply Status: Active; widely available through global distributors.1. Technical Specifications & Performance Analysis

The INA128 is engineered to extract small differential signals from noisy environments, a task critical in precision measurement.

1.1 Core Architecture (3-Op Amp Design)

The INA128 uses a classic three-operational-amplifier configuration. This architecture allows for a high input impedance and simplified gain setting. By using a single external resistor ($R_G$), users can set the gain anywhere from 1 to 10,000 V/V. This eliminates the need for complex matched resistor networks, significantly reducing BOM complexity and board space.

1.2 Key Electrical Characteristics

For procurement and design, the following parameters define the INA128's performance: 

- Input Offset Voltage: A maximum of 50 µV, ensuring minimal error in low-level signal amplification. 

- Quiescent Current: Only 700 µA, making it suitable for systems where power efficiency is a priority. 

- Input Protection: The inputs are internally protected up to ±40V beyond the power supplies, preventing damage during over-voltage events. 

- Bandwidth: Offers a 1.3 MHz bandwidth at a gain of 1, maintaining signal integrity across a wide frequency range.

1.3 Interfaces and Connectivity

As an analog component, the INA128 interfaces directly with sensors. Its high common-mode rejection ratio (CMRR) of 120 dB ensures that noise common to both input lines (like 60Hz hum) is effectively cancelled out before reaching the ADC or output stage.

2. Pinout, Package, and Configuration

Understanding the physical layout is essential for PCB design and troubleshooting.

2.1 Pin Configuration Guide

The INA128 typically comes in an 8-pin configuration: 

- Pins 1 & 8 ($R_G$): Connection points for the external gain-setting resistor. 

- Pins 2 & 3 ($-V_{IN}, +V_{IN}$): Differential inputs for the sensor signal. 

- Pins 4 & 7 ($V-, V+$): Negative and positive power supply rails. 

- Pin 5 (Ref): Reference pin; determines the output voltage level for zero differential input. 

- Pin 6 (Output): The amplified signal output.

2.2 Naming Convention & Ordering Codes

Understanding the Part Numbers:- INA128P: Plastic DIP (Dual In-line Package), preferred for prototyping and through-hole boards. 

- INA128U: SOIC-8 (Small Outline IC), the standard for high-density surface-mount production. 

- INA128UA: Often denotes a different grade or packaging style (refer to the specific TI datasheet for precision grade variations).

2.3 Available Packages

3. Design & Integration Guide (For Engineers & Makers)

Pro Tip: Always verify the common-mode input range when operating on low-voltage single supplies to avoid signal clipping.

3.1 Hardware Implementation

• Bypass Capacitors: Use 0.1µF ceramic capacitors placed as close as possible to the supply pins (Pins 4 and 7) to minimize noise.

• PCB Layout: Keep the traces to the gain-setting resistor ($R_G$) extremely short. Parasitic resistance in these traces will directly cause gain errors.

• Thermal Management: With a low quiescent current of 700µA, the INA128 rarely requires a heatsink under standard operating conditions.

3.2 Common Design Challenges

• Issue: Common Mode Range Violation: Using a single 5V supply often leads to output saturation.

• Fix: Ensure the input signal stays at least 1.4V away from the supply rails. If necessary, use a dual-supply configuration (e.g., ±5V).

• Issue: Gain Accuracy: Gain is determined by $G = 1 + (50k\Omega / R_G)$.

• Fix: Use 0.1% tolerance resistors for $R_G$ to maintain the precision the INA128 is capable of providing.

4. Typical Applications & Use Cases

📺 Video Recommendation: INA128 Guide

4.1 Real-World Example: Medical ECG Monitor

In medical instrumentation like an ECG, the INA128 is used to amplify the tiny millivolt signals from the heart while rejecting the massive common-mode noise from the patient's body and the environment.

5. Alternatives and Cross-Reference Guide

The INA128 is a classic, but newer or specialized alternatives may be better for specific BOM requirements.

• Direct Replacements:

• AD620 (Analog Devices): The most common industry alternative with very similar specs and pinout.

• LT1168: A low-power, high-accuracy alternative from Linear Technology (now ADI).

• Better Performance (The Upgrade Path):

• INA828: TI's next-generation replacement offering lower power, lower noise, and improved DC precision in the same pinout.

• Cost-Effective Options:

• INA818: Optimized for lower-cost applications while maintaining high precision.

6. Frequently Asked Questions (FAQ)

• Q: What is the difference between INA128 and AD620?

• A: They are very similar and often pin-compatible. The INA128 generally offers a wider bandwidth at higher gains, while the AD620 is a long-standing industry favorite for general-purpose use.

• Q: Can INA128 be used with an Arduino or STM32?

• A: Yes, but since these microcontrollers usually run on single supplies (3.3V or 5V), you must bias the INA128's Ref pin to mid-supply (e.g., 2.5V) to ensure the output stays within the ADC's range.

• Q: Is the INA128 rail-to-rail?

• A: No. The output typically swings to within roughly 1.2V–1.4V of the supply rails. For rail-to-rail performance, consider the INA333.