SLAAET6 August   2025

 

  1.   1
  2.   Abstract
  3.   Trademarks
  4. 1S-Parameter Definition
    1. 1.1 Insertion Loss (S21)
    2. 1.2 Return Loss (S11)
  5. 2High-Speed Signal Design Example Of FPD-Link™ Serializer Body
    1. 2.1 Design Example Overview
    2. 2.2 Key Points in High-Speed FPD-Link Layout Design
  6. 3Factors Impacting Return Loss and Optimization Guidelines
    1. 3.1 Transmission Line Impedance Impact
    2. 3.2 AC Coupling Capacitor Landing Pad Impact And Optimization
      1. 3.2.1 Mitigation Strategy: Anti-Pad Implementation
      2. 3.2.2 Simulation Results With Ansys® HFSS
    3. 3.3 Through-Hole Connector Footprint Impact and Optimization
      1. 3.3.1 Through-Hole Connector Via Anti-Pad Impact
        1. 3.3.1.1 Simulation Results With Ansys® HFSS
      2. 3.3.2 Surrounding Ground Vias Impact
        1. 3.3.2.1 Simulation Results (Surrounding Ground Vias Impact)
      3. 3.3.3 Non-Functional Pad Impact
        1. 3.3.3.1 Simulation Results (Non-Functional Pad Impact)
    4. 3.4 Generic Signal Via Impact and Optimization
      1. 3.4.1 Simulation Results
    5. 3.5 ESD Diode Parasitic Capacitance Impact and Optimization
  7. 4Summary

3.2.1 Mitigation Strategy: Anti-Pad Implementation

To counteract pad-introduced impedance deviation, create a ground cutout (anti-pad) under the landing pads on the adjacent reference layer (for example, layer 2). This forces the pad to reference a more distant layer (for example, layer 3), effectively increases the dielectric thickness and raising impedance to offset the capacitive effect of the bigger pad.

Anti-pad size requires careful consideration. Oversized or undersized anti-pad can disrupt the electromagnetic fields and affect signal coupling between the trace and reference plane, which in turn affects the signal impedance.

The best anti-pad size depends on various factors such as the dielectric thickness, trace width, and pad dimensions. Use simulation tools (for example, Ansys® HFSS) to analyze pad effects and determine the best possible anti-pad size.