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
8.2.1.1.2 Measuring Transimpedance Bandwidth

While there is no substitute for measuring the performance of a particular circuit under the exact conditions that are used in the application, the complete system environment often makes measuring harder. Measuring the frequency response of a transimpedance circuit is difficult with traditional laboratory equipment because the circuit requires a current as an input rather than a voltage. Also, the capacitance of the current source has a direct effect on the frequency response. A simple interface circuit can be used to emulate a capacitive current source with a network analyzer. With this circuit, transimpedance bandwidth measurements are simplified, making amplifier evaluation easier and faster.

THS4631 Emulating a Capacitive Current Source With a Network Analyzer
Note: The interface network creates a capacitive, constant current source from a network analyzer and properly terminates the network analyzer at high frequencies.
Figure 8-5 Emulating a Capacitive Current Source With a Network Analyzer

The transconductance transfer function of the interface circuit is:

Equation 5. THS4631

The transfer function contains a zero at dc and a pole at THS4631 .

The transconductance is constant for signal source frequencies greater than the pole frequency, THS4631 , providing a controllable ac current source. This circuit also properly terminates the network analyzer with 50Ω at high frequencies. The second requirement for this current source is to provide the desired output impedance, emulating the output impedance of a photodiode or other current source. The output impedance of this circuit is given by:

Equation 6. THS4631

Assuming C1 >> C2, the equation reduces to: THS4631 , giving the appearance of a capacitive source at a higher frequency.

When selecting capacitor values, the designer must consider two requirements. First, C2 represents the anticipated capacitance of the true source. Second, C1 is chosen so that the corner frequency of the transconductance network is much less than the transimpedance bandwidth of the circuit. Choosing this corner frequency properly leads to more accurate measurements of the transimpedance bandwidth. If the interface-circuit corner frequency is too close to the bandwidth of the circuit, determining the power level in the flat band is difficult. A decade or more of flat bandwidth provides a good basis for determining the proper transimpedance bandwidth.