SBOSA77A March   2023  – April 2024 OPA928

PRODUCTION DATA  

  1.   1
  2. 1 Features
  3. 2 Applications
  4. 3 Description
  5. 4 Pin Configuration and Functions
  6. 5 Specifications
    1. 5.1 Absolute Maximum Ratings
    2. 5.2 ESD Ratings
    3. 5.3 Recommended Operating Conditions
    4. 5.4 Thermal Information
    5. 5.5 Electrical Characteristics: 4.5V ≤ VS < 8V
    6. 5.6 Electrical Characteristics: 8V ≤ VS ≤ 16V
    7. 5.7 Electrical Characteristics: 16V < VS ≤ 36V
    8. 5.8 Typical Characteristics
  7. 6 Detailed Description
    1. 6.1 Overview
    2. 6.2 Functional Block Diagram
    3. 6.3 Feature Description
      1. 6.3.1 Guard Buffer
      2. 6.3.2 Input Protection
      3. 6.3.3 Thermal Protection
      4. 6.3.4 Capacitive Load and Stability
      5. 6.3.5 EMI Rejection
      6. 6.3.6 Common-Mode Voltage Range
    4. 6.4 Device Functional Modes
  8. 7 Application and Implementation
    1. 7.1 Application Information
      1. 7.1.1 Contamination Considerations
      2. 7.1.2 Guarding Considerations
      3. 7.1.3 Single-Supply Considerations
      4. 7.1.4 Humidity Considerations
      5. 7.1.5 Dielectric Relaxation
      6. 7.1.6 Shielding
    2. 7.2 Typical Applications
      1. 7.2.1 High-Impedance Amplifier
        1. 7.2.1.1 Design Requirements
        2. 7.2.1.2 Detailed Design Procedure
        3. 7.2.1.3 Application Curve
      2. 7.2.2 Transimpedance Amplifier
        1. 7.2.2.1 Design Requirements
        2. 7.2.2.2 Detailed Design Procedure
          1. 7.2.2.2.1 Input Bias
          2. 7.2.2.2.2 Offset Voltage
          3. 7.2.2.2.3 Stability
          4. 7.2.2.2.4 Noise
      3. 7.2.3 Improved Diode Limiter
      4. 7.2.4 Instrumentation Amplifier
    3. 7.3 Power-Supply Recommendations
    4. 7.4 Layout
      1. 7.4.1 Layout Guidelines
      2. 7.4.2 Layout Examples
  9. 8 Device and Documentation Support
    1. 8.1 Device Support
      1. 8.1.1 Development Support
        1. 8.1.1.1 PSpice® for TI
        2. 8.1.1.2 TINA-TI™ Simulation Software (Free Download)
        3. 8.1.1.3 TI Reference Designs
    2. 8.2 Documentation Support
      1. 8.2.1 Related Documentation
    3. 8.3 Receiving Notification of Documentation Updates
    4. 8.4 Support Resources
    5. 8.5 Trademarks
    6. 8.6 Electrostatic Discharge Caution
    7. 8.7 Glossary
  10. 9 Revision History
  11. 10Mechanical, Packaging, and Orderable Information

7.2.1 High-Impedance Amplifier

The OPA928 behaves very close to an ideal op amp in regards to the input current. The near-zero input bias current performance enables applications with extremely high impedance signal sources. For example, pH probes can have an output impedance of up to 10GΩ. Most op amps are inadequate to use with this kind of sensor impedance. For example, a CMOS op amp with 1pA of input bias current loads the sensor and causes a large, and unacceptable, 10mV error at the input at room temperature. This error can increase exponentially across the industrial temperature range. In Figure 7-16, the OPA928 is used as a high-impedance electrometer to amplify the small signal from the pH probe sensor. The high input impedance and ultra-low bias current into the positive input pin of the OPA928 does not load the sensor and minimizes the input bias current error.

GUID-20230118-SS0I-JHTK-Q5Z7-BH4CCBCX9J75-low.svg Figure 7-7 High Impedance pH Probe Amplifier Circuit