INA121P TI INA121P FET-Input Low-Power Instrumentation Amplifier

Texas Instruments INA121P Instrumentation Amplifier – Precision Signal Conditioning for High-Impedance Applications

The INA121P from Texas Instruments is a high-precision instrumentation amplifier designed for applications demanding exceptional accuracy with ultra-low power consumption. Featuring a FET-input stage, this amplifier delivers industry-leading ±4pA input bias current, making it ideal for interfacing with high-impedance sensors such as strain gauges, RTDs, and pH electrodes.

Built on a 3-op-amp architecture, the INA121P provides:

• Programmable gain (1–10,000V/V) via a single external resistor
• ±40V input protection against voltage transients
• 106dB common-mode rejection ratio (CMRR) at G=100
• Low noise (20nV/√Hz) for sensitive measurements

Packaged in an 8-pin PDIP, it operates across -40°C to +85°C, suitable for industrial, medical, and portable battery-powered systems.

Core Technical Advantages

1. Ultra-High Impedance Interface
   • The JFET input stage (with >10¹²Ω input impedance) virtually eliminates loading effects when measuring:
     • Piezoelectric sensors (vibration/force/pressure)
     • Electrochemical cells (pH/ISE probes)
     • Capacitive transducers (accelerometers, MEMS)
   • Ensures signal fidelity even with source impedances exceeding 1GΩ.
2. Precision DC Performance
   • Laser-trimmed offset voltage (±200µV max) minimizes calibration requirements.
   • Near-zero drift (±2µV/°C) maintains accuracy across industrial temperatures (-40°C to +85°C).
   • 0.001% gain nonlinearity guarantees linear output across dynamic ranges.
3. Robust Differential Signal Conditioning
   • 106dB CMRR (at G=100) rejects ground loops/noise in harsh environments (e.g., motor drives, industrial PLCs).
   • ±40V input overvoltage protection safeguards against transient spikes (IEC 61000-4-5 compliant).
4. Flexible Gain & Power Options
   • Programmable gain (1–10,000V/V) via single external resistor (RG), supporting:
     • Microvolt-level signals (thermocouples: G=100–500)
     • High-level outputs (10V bridges: G=1–10)
   • Wide supply range (±2.25V to ±18V) enables:
     • Battery-powered operation (single 4.5V cell)
     • ±15V industrial systems

Specifications

Applications

1. Industrial Sensor Conditioning

• Strain Gauge/Bridge Amplification:
  • 600kHz bandwidth supports dynamic load measurements.
  • 0.001% nonlinearity ensures accurate force/torque monitoring.
• RTD/Thermocouple Interfaces:
  • 4pA bias current minimizes self-heating errors.

2. Biomedical Instrumentation

• ECG/EEG Signal Chains:
  • 106dB CMRR rejects 50/60Hz power-line interference.
• Respiratory Monitors:
  • ±40V protection handles defibrillator surges (IEC 60601 compliant).

3. Portable & Battery-Powered Systems

• Energy Harvesting Sensors:
  • 450µA quiescent current extends battery life.
• Capacitive Touch Interfaces:
  • FET input prevents signal loading in high-Z circuits.

Design Considerations

• Bias Current Paths: Floating inputs (e.g., thermocouples) require ≥1MΩ return resistors (see Datasheet Fig. 3).
• Thermal Management: PDIP package θJA = 73°C/W – avoid >100mW dissipation in high-Gain configurations.
• Noise Optimization: At G=100, input-referred noise is 20nV/√Hz (dominated by A1/A2 FETs)

Gain Configuration

The gain is set by a single resistor (RG) between pins 1 and 8:

Example: For G=500, use RG=100Ω (1% metal film recommended).

Input Protection

• Bias Current Path: Always provide DC return paths for inputs (see Fig. 3 in datasheet).
• Filtering: Add matched RC networks (e.g., R1=R2=1kΩ, C1=C2=100pF) to preserve CMRR.

Low-Voltage Operation

At ±2.25V supplies:

• Ensure input voltages stay within +1.2V to -2.1V of rails.

Package Options & Ordering Information

Below is the complete list of orderable part numbers for the INA121 series, including all package options, pin counts, and shipping formats:

Key Differences Between Models

1. Package Type
   • DIP (Through-Hole):
     • INA121P/PA: 8-Pin PDIP (0.3″ width) for breadboarding or prototyping.
     • Bulk tubes contain 50 units.
   • SO-8 (Surface Mount):
     • INA121U/UA: Suitable for PCB mass production.
     • Available in tubes (75 units) or tape & reel (2,500 units) for automated assembly.
2. Suffix Variations
   • Part numbers ending in “A” (e.g., INA121PA, INA121UA)
     • Identical electrical specs, but may include minor datasheet revisions.
   • Part numbers ending in “/2K5” (e.g., INA121U/2K5, INA121UA/2K5)
     • High-volume packaging (2,500 units per reel).
3. Temperature Grades
   • All models support industrial temperature range (-40°C to +85°C).

Recommended Parts for Common Use Cases

Footprint: PDIP (9.81mm x 6.35mm x 4.57mm).

Why Choose INA121P?

• Precision: Laser-trimmed for low offset (±2µV/°C drift).
• Flexibility: Supports single +5V or dual ±18V supplies.
• Reliability: Industrial-grade temperature range.

Comparison: INA121P vs. INA128

INA121P vs. INA128 Quick Reference

Design Selection Guide

• Choose INA121P when:
  • Signal source impedance >10kΩ (pH, piezoelectric, floating sensors).
  • Ultra-low bias current is critical (electrochemical cells, bioelectrodes).
  • Operating in noisy environments (industrial/medical systems).
• Choose INA128 when:
  • Measuring low-impedance signals (<1kΩ, e.g., strain gauges, RTDs).
  • Lower voltage noise outweighs bias current needs.
  • Designing cost-sensitive, non-critical systems.

Key Insight

• High-Z ≠ High Noise:
  For source impedances >10kΩ, INA128’s current noise dominates (e.g., 0.4pA/√Hz × 100kΩ = 40nV/√Hz), making INA121P quieter overall despite its higher voltage noise.
• Supply Limitations:
  INA128 supports single-supply 3V systems; INA121P requires ≥±2.25V (±4.5V total).

Assuming “lower noise = better” without considering source impedance. For high-Z applications, INA121P’s near-zero current noise is decisive.

Detailed Comparison: INA121P (FET-Input) vs. INA128 (Bipolar-Input)

1. Input Bias Current: The Critical Tradeoff

Why It Matters:

• High-Z Sensors (pH, piezoelectric):
  • With a 1MΩ source impedance, INA128 introduces 1mV offset error (1nA × 1MΩ), while INA121P adds just 4µV.
  • Critical for:
    • Bioelectric measurements (EEG/ECG electrodes)
    • Capacitive touchscreens
• Thermocouples:
  • INA128’s bias current can cause self-heating errors in low-mass junctions.

2. Noise Performance: Bipolar vs. FET

Noise Analysis:

• Low-Impedance Sources (<1kΩ):
  • INA128’s lower voltage noise makes it preferable for:
    • Strain gauges (350Ω bridges)
    • RTD interfaces (100Ω Pt100)
• High-Impedance Sources (>10kΩ):
  • INA121P’s negligible current noise (0.0006pA/√Hz) prevents noise multiplication:
    • At 100kΩ source impedance, INA128’s current noise contributes:
      

      Noise = 0.4pA/√Hz × 100kΩ = 40nV/√Hz (worse than INA121P’s 20nV/√Hz)

3. CMRR & Precision Metrics

Design Impact:

• INA121P excels in:
  • Medical instrumentation (ECG needs >100dB CMRR)
  • Industrial PLCs (long cables induce common-mode noise)
• INA128 may suffice for:
  • Consumer electronics (e.g., audio interfaces with moderate CMRR needs)

4. Power & Dynamic Range

Tradeoffs:

• INA128 tolerates lower single-supply voltage (2.7V), useful for 3V systems.
• INA121P’s lower Iq benefits battery-powered sensors (e.g., wireless strain gauges).

*Within 0.9V of rails (typ)

5. Application-Specific Recommendations

Summary: Choosing Between INA121P and INA128

• Pick INA121P if:
  • Your source impedance >10kΩ (e.g., piezoelectric, capacitive).
  • You need ultra-low bias current or >100dB CMRR.
  • Operating with floating sensors (thermocouples).
• Pick INA128 if:
  • Measuring low-Z signals (RTDs, strain gauges).
  • Lower voltage noise is prioritized over bias current.
  • Designing cost-sensitive, non-medical systems.