SLOS451C December   2004  – March 2025 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
8.2.1.1.4 Selection of Feedback Resistors

Feedback-resistor selection can have a significant effect on the performance of the THS4631 in a given application, especially in configurations with low closed-loop gain. If the amplifier is configured for unity gain, connect the output directly to the inverting input. Any resistance between these two points interacts with the input capacitance of the amplifier and causes an additional pole in the frequency response. For non-unity gain configurations, low resistances are desirable for a flat frequency response. However, do not load the amplifier too heavily with the feedback network if large output signals are expected. In most cases, a tradeoff is made between the frequency response characteristics and the loading of the amplifier. For a gain of 2, a 499Ω feedback resistor is the recommended operating point from both perspectives. Resistor values that are too large subject the THS4631 to oscillation problems. For example, an inverting amplifier configuration with a 5kΩ gain resistor and a 5kΩ feedback resistor develops an oscillation due to the interaction of the large resistors with the input capacitance. In low-gain configurations, avoid feedback resistors that are too large or anticipate using an external compensation scheme to stabilize the circuit. Using a simple capacitor in parallel with the feedback resistor makes the amplifier more stable (see also the Typical Characteristics graphs).

Table 8-1 Transimpedance Performance Summary for Various Configurations
SOURCE
CAPACITANCE (PF)		 TRANSIMPEDANCE
GAIN (Ω)		 FEEDBACK
CAPACITANCE (PF)		 –3dB
FREQUENCY (MHZ)
18 10k 2 15.8
18 100k 0.5 3
18 1M 0 1.2
47 10k 2.2 8.4
47 100k 0.7 2.1
47 1M 0.2 0.52
100 10k 3 5.5
100 100k 1 1.4
100 1M 0.2 0.37