SBOS622C July   2018  – January 2023 OPA855

PRODUCTION DATA  

  1. 1 Features
  2. 2 Applications
  3. 3 Description
  4. 4 Revision History
  5. 5 Device Comparison Table
  6. 6 Pin Configuration and Functions
  7. 7 Specifications
    1. 7.1 Absolute Maximum Ratings
    2. 7.2 ESD Ratings
    3. 7.3 Thermal Information
    4. 7.4 Recommended Operating Conditions
    5. 7.5 Electrical Characteristics
    6. 7.6 Typical Characteristics
  8. 8 Parameter Measurement Information
  9. 9 Detailed Description
    1. 9.1 Overview
    2. 9.2 Functional Block Diagram
    3. 9.3 Feature Description
      1. 9.3.1 Input and ESD Protection
      2. 9.3.2 Feedback Pin
      3. 9.3.3 Wide Gain-Bandwidth Product
      4. 9.3.4 Slew Rate and Output Stage
    4. 9.4 Device Functional Modes
      1. 9.4.1 Split-Supply and Single-Supply Operation
      2. 9.4.2 Power-Down Mode
  10. 10Application, Implementation, and Layout
    1. 10.1 Application Information
    2. 10.2 Typical Application
      1. 10.2.1 Design Requirements
      2. 10.2.2 Detailed Design Procedure
      3. 10.2.3 Application Curves
    3. 10.3 Typical Application
      1. 10.3.1 Design Requirements
      2. 10.3.2 Detailed Design Procedure
      3. 10.3.3 Application Curves
    4. 10.4 Power Supply Recommendations
    5. 10.5 Layout
      1. 10.5.1 Layout Guidelines
      2. 10.5.2 Layout Example
  11. 11Device and Documentation Support
    1. 11.1 Device Support
      1. 11.1.1 Development Support
    2. 11.2 Documentation Support
      1. 11.2.1 Related Documentation
    3. 11.3 Receiving Notification of Documentation Updates
    4. 11.4 Support Resources
    5. 11.5 Trademarks
    6. 11.6 Electrostatic Discharge Caution
    7. 11.7 Glossary
  12. 12Mechanical, Packaging, and Orderable Information

10.5.2 Layout Example

GUID-CCF34F26-98D1-47D2-9F3B-2DE709150EC0-low.gifFigure 10-12 Layout Recommendation

When configuring the OPA855 as a transimpedance amplifier additional care must be taken to minimize the inductance between the avalanche photodiode (APD) and the amplifier. Always place the photodiode on the same side of the PCB as the amplifier. Placing the amplifier and the APD on opposite sides of the PCB increases the parasitic effects due to via inductance. APD packaging can be quite large which often requires the APD to be placed further away from the amplifier than ideal. The added distance between the two device results in increased inductance between the APD and op amp feedback network as shown in Figure 10-13. The added inductance is detrimental to a decompensated amplifiers stability since it isolates the APD capacitance from the noise gain transfer function. The noise gain is given by Equation 3. The added PCB trace inductance between the feedback network increases the denominator in Equation 3 thereby reducing the noise gain and the phase margin. In cases where a leaded APD in a TO can is used inductance should be further minimized by cutting the leads of the TO can as short as possible.

The layout shown in Figure 10-13 can be improved by following some of the guidelines shown in Figure 10-14. The two key rules to follow are:

  • Add an isolation resistor RISO as close as possible to the inverting input of the amplifier. Select the value of RISO to be between 10 Ω and 20 Ω. The resistor dampens the potential resonance caused by the trace inductance and the amplifiers internal capacitance.
  • Close the loop between the feedback elements (RF and CF) and RISO as close to the APD pins as possible. This ensures a more balanced layout and reduces the inductive isolation between the APD and the feedback network.

Equation 3. GUID-D489C94B-8D80-456E-A7F6-3D209809A21A-low.gif

where

  • ZF is the total impedance of the feedback network.
  • ZIN is the total impedance of the input network.

GUID-E735490A-7F77-498D-9E93-DA4DCDBAE291-low.gif Figure 10-13 Non-Ideal TIA Layout
GUID-7791EE20-FD1F-4620-BBF8-9F559FD1A47B-low.gif Figure 10-14 Improved TIA Layout