SLOS451D December   2004  – March 2026 THS4631

PRODUCTION DATA  

  1.   1
  2. 1 Features
  3. 2 Applications
  4. 3 Description
  5. 4 Related Products
  6. 5 Pin Configuration Functions
  7. 6 Specifications
    1. 6.1 Absolute Maximum Ratings
    2. 6.2 ESD Ratings
    3. 6.3 Recommended Operating Conditions
    4. 6.4 Thermal Information
    5. 6.5 Electrical Characteristics
    6. 6.6 Typical Characteristics
  8. 7 Parameter Measurement Information
  9. 8 Application and Implementation
    1. 8.1 Application Information
      1. 8.1.1 Transimpedance Fundamentals
      2. 8.1.2 Noise Analysis
    2. 8.2 Typical Applications
      1. 8.2.1 Wideband Photodiode Transimpedance Amplifier
        1. 8.2.1.1 Detailed Design Procedure
          1. 8.2.1.1.1 Designing the Transimpedance Circuit
          2. 8.2.1.1.2 Measuring Transimpedance Bandwidth
          3. 8.2.1.1.3 Summary of Key Decisions in Transimpedance Design
          4. 8.2.1.1.4 Selection of Feedback Resistors
        2. 8.2.1.2 Application Curves
      2. 8.2.2 Alternative Transimpedance Configurations
    3. 8.3 Power Supply Recommendations
      1. 8.3.1 Slew-Rate Performance With Varying Input-Step Amplitude and Rise-and-Fall Time
    4. 8.4 Layout
      1. 8.4.1 Layout Guidelines
        1. 8.4.1.1 Printed-Circuit Board (PCB) Layout Techniques for High Performance
        2. 8.4.1.2 PowerPAD Design Considerations
        3. 8.4.1.3 PowerPAD PCB Layout Considerations
        4. 8.4.1.4 Power Dissipation and Thermal Considerations
      2. 8.4.2 Layout Example
  10. 9 Device and Documentation Support
    1. 9.1 Device Support
      1. 9.1.1 Design Tools Evaluation Fixture, Spice Models, and Applications Support
        1. 9.1.1.1 Bill of Materials
        2. 9.1.1.2 EVM
        3. 9.1.1.3 EVM Warnings and Restrictions
    2. 9.2 Documentation Support
      1. 9.2.1 Related Documentation
    3. 9.3 Receiving Notification of Documentation Updates
    4. 9.4 Support Resources
    5. 9.5 Trademarks
    6. 9.6 Electrostatic Discharge Caution
    7. 9.7 Glossary
  11. 10Revision History
  12. 11Mechanical, Packaging, and Orderable Information

8.1.2 Noise Analysis

High slew rate, unity gain stable, voltage-feedback operational amplifiers typically achieve high slew rate at the expense of a higher-input noise voltage. However, the 7nV/√Hz input voltage noise for the THS4631 is much lower than comparable amplifiers while achieving high slew rates. The input-referred voltage noise and the input-referred current noise term combine to give low output noise under a wide variety of operating conditions. Figure 8-1 shows the amplifier noise analysis model with all the noise terms included. In this model, all noise terms are taken to be noise voltage or current density terms in either nV/√Hz or fA/√Hz.

THS4631 Noise Analysis Model Figure 8-1 Noise Analysis Model

The total output noise voltage is computed as the square root of all square output noise voltage contributors. Equation 1 shows the general form for the output noise voltage using the terms shown in Figure 8-1.

Equation 1. THS4631

Equation 2 shows that dividing this expression by the noise gain [NG = (1+ Rf/Rg)] gives the equivalent input-referred spot noise voltage at the noninverting input.

Equation 2. THS4631

High resistor values can dominate the total equivalent input-referred noise. Use a 3kΩ source-resistance (RS) value to add a voltage noise term of approximately 7nV/√Hz. This noise term is equivalent to the amplifier voltage noise term. Higher resistor values dominate the noise of the system. Although the THS4631 JFET input stage is advantageous for high-source impedance because of the low-bias currents, the system noise and bandwidth is limited by a high-source (RS) impedance.