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
8.2.1.1.1 Designing the Transimpedance Circuit

Typically, design of a transimpedance circuit is driven by the characteristics of the current source that provides the input to the gain block. A photodiode is the most common example of a capacitive current source that interfaces with a transimpedance gain block. Continuing with the photodiode example, the system designer traditionally chooses a photodiode based on two opposing criteria: speed and sensitivity. Faster photodiodes cause a need for faster gain stages, and more sensitive photodiodes require higher gains to develop appreciable signal levels at the output of the gain stage.

These parameters affect the design of the transimpedance circuit in a few ways. First, the speed of the photodiode signal determines the required bandwidth of the gain circuit. Second, the required gain, based on the sensitivity of the photodiode, limits the bandwidth of the circuit. Third, the larger capacitance associated with a more sensitive signal source also detracts from the achievable speed of the gain block. The dynamic range of the input signal also places requirements on the amplifier dynamic range. Knowledge of the source output current levels, coupled with a desired voltage swing on the output, dictates the value of the feedback resistor, RF. The transfer function from input to output is VOUT = IINRF.

The large gain-bandwidth product of the THS4631 provides the capability for simultaneously achieving both high transimpedance gain, wide bandwidth, high slew rate, and low noise. In addition, the high-power supply rails provide the potential for a very wide dynamic range at the output, allowing for the use of input sources which possess wide dynamic range. The combination of these characteristics makes the THS4631 an excellent design option for systems that require transimpedance amplification of wideband, low-level input signals. Figure 8-2 shows a standard transimpedance circuit.

As indicated, the current source typically sets the requirements for gain, speed, and dynamic range of the amplifier. For a given amplifier and source combination, achievable performance is dictated by the following parameters: amplifier gain-bandwidth product, amplifier input capacitance, source capacitance, transimpedance gain, amplifier slew rate, and amplifier output swing. From this information, the best case performance of a transimpedance circuit using a given amplifier is determined. Best case is defined here as providing the required transimpedance gain with a maximized flat frequency response.

For the circuit shown in Figure 8-2, all but one of the design parameters is known; the feedback capacitor (CF) must still be determined. Proper selection of the feedback capacitor prevents an unstable design, controls pulse response characteristics, provides maximized flat transimpedance bandwidth, and limits broadband integrated noise. The maximized flat frequency response results with CF calculated as shown in Equation 3:

Equation 3. THS4631

where

  • CF is the feedback capacitor
  • RF is the feedback resistor
  • CF is the feedback capacitor
  • RF is the feedback resistor
  • CS is the total source capacitance (including amplifier input capacitance and parasitic capacitance at the inverting node)
  • GBP is the gain-bandwidth product of the amplifier in hertz

After the feedback capacitor has been selected, the transimpedance bandwidth is calculated with Equation 4.

Equation 4. THS4631
THS4631 Transimpedance Analysis Circuit
Note: The total source capacitance is the sum of several distinct capacitances.
Figure 8-3 Transimpedance Analysis Circuit

where

  • CI(CM) is the common-mode input capacitance
  • CI(DIFF) is the differential input capacitance
  • CD is the diode capacitance
  • CP is the parasitic capacitance at the inverting node

The feedback capacitor provides a pole in the noise gain of the circuit, counteracting the zero in the noise gain caused by the source capacitance. The pole is set such that the noise gain achieves a 20dB-per-decade rate of closure with the open-loop gain response of the amplifier, resulting in a stable circuit. As indicated, Equation 3 provides the feedback capacitance for maximized flat bandwidth. Reduction in the value of the feedback capacitor can increase the signal bandwidth, but the signal bandwidth increase occurs at the expense of peaking in the ac response.

THS4631 Transimpedance Circuit Bode PlotFigure 8-4 Transimpedance Circuit Bode Plot

The performance of the THS4631 has been measured for a variety of transimpedance gains with a variety of source capacitances. The achievable bandwidths of the various circuit configurations are summarized numerically in Table 8-1. Figure 8-6, Figure 8-7, and Figure 8-8 present the frequency responses.

Be aware the feedback capacitances do not correspond exactly with the values predicted by the equation. The capacitances have been tuned to account for the parasitic capacitance of the feedback resistor (typically 0.2pF for 0805 surface mount devices) as well as the additional capacitance associated with the printed circuit board (PCB). Use this equation as a starting point for the design, with final values for CF optimized in the laboratory.