1. Power Distribution Network
2. Gigahertz Channel Design Considerations
3. PCB and Stack-Up Design Considerations
4. Device Pin-Map, Checklists, and Connection Guidelines
5. General Board Design Considerations/Guidelines
6. Memory Interfacing Guidelines
7. Power Dissipation and Thermal Management
8. Tools, Models, and Libraries
9. Reference Designs and Development Kits
10. Document Revision History for AN 958: Board Design Guidelines
4.1. High Speed Board Design Advisor
4.2. Complete Pin Connection Table by Device
4.3. Pin Connection Guidelines By Device
4.4. Design for Debug with JTAG Pins
4.5. Hot Socketing, POR and Power Sequencing Support
4.6. Implementing OCT
4.7. Unused I/O Pins Guidelines
4.8. Device Breakout Guidelines
4.9. Additional Resources
5.1.1. Material Selection and Loss
5.1.2. Cross Talk Minimization
5.1.3. Power Filtering/Distribution
5.1.4. Unused I/O Pins
5.1.5. Signal Trace Routing
5.1.6. Ground Bounce
5.1.7. Understanding Transmission Lines
5.1.8. Impedance Calculation
5.1.9. Coplanar Wave Guides
5.1.10. Simultaneous Switching Noise Guidelines
5.4.2. Inductors and Ferrite Beads
A good ferrite bead has low DC resistance, high AC impedance, and high current handling capability. However, as the current handling capability increases, the AC impedance tends to drop, so there is a tradeoff involved. Figure 84 shows the impedance versus frequency plot of a typical ferrite bead.
Figure 84. Typical Impedence Profile of a Ferrite Bead
In Figure 84, the impedance at 2 GHz is more than 100 Ω. The ratio between that impedance and the power supply impedance, which is often lower than 1 Ω, is more than 100. As a result, most of the noise is blocked by the ferrite bead and is shunted to ground instead.
Some ferrite beads that meet performance are:
- The Steward MI0805M221R-00 ferrite beads for transceiver power and ground planes. The DC resistance for this part is lower than 50 mΩ, and it can handle 2.5 A of current. The impedance is over 200 Ω at 1 GHz.
- Two Steward ferrite beads connected in parallel can provide 5 A of current capability with 25 mΩ of DC resistance and over 100 Ω of AC impedance. This performance level is adequate for most applications.
- The Murata BLM31PG500SN1 ferrite bead has 25 mΩ of DC resistance, 3 A of current, and 75 Ω of AC impedance at 1 GHz.