IMPEDANCE MATCHING FOR HIGH-SPEED SIGNALS IN PCB DESIGN (1)

• The process for impedance matching in high speed PCB designs depends on the signaling standard, supply voltage levels, and signal levels. We often say that PCB traces should simply have 50 Ohm impedance, but this becomes more complicated when dealing with coupling between differential pairs. High speed digital designers should take time to understand this important process and design interconnects with proper termination to match their signaling standard.

  IMPEDANCE MATCHING FOR SINGLE-ENDED SIGNALS

  Because impedance mismatches between components, traces, and vias depend on trace geometry, logic family, and coupling, you need to carefully design traces to have the proper single-ended and differential impedance. Different logic families have signaling standards that define how a driver, trace, and receiver need to be terminated to provide consistent impedance matching in PCB interconnects. While we’ll look specifically at PCB transmission lines between integrated circuits, the same ideas apply to interconnects inside an integrated circuit, where transmission lines need to be carefully designed to ensure impedance matching.

  For single-ended signals, you simply need to worry about the input impedance (for receivers) and output impedance (for drivers). Note that dispersion in the PCB substrate can complicate impedance matching, and your goal should be to adjust the geometry to ensure impedance matching throughout the signal bandwidth. This can become a complex optimization problem, which I’ve discussed elsewhere on this blog and in a few articles on Altium’s PCB Design Blog.

  IC manufacturers have gotten much better about reporting the input and output impedance spectra for their components. At minimum, they should tell you the pin-package lead inductance, input capacitance, and equivalent input resistance. All these values depend on logic family. You can then use any of the standard impedance matching schemes for high speed interconnects to prevent signal reflection and ensure power transfer. The standard methods used for impedance matching in single-ended signaling standards are shown below. Note that each of these has a fly-by variation, where the termination resistor is connected directly to the component pad rather than the trace.