High Speed Analog Design and Application Seminar - Section 5 - High Speed PCB Layout Techniques

Oct 29, 2026

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High Speed Analog Design and Application Seminar Section 5 explains how high speed PCB layout fundamentals prevent circuit failures that occur when schematic simulations (e.g., Spice) disagree with real hardware. It covers key PCB elements—copper traces, copper planes, and vias—focusing on how geometry and dielectric properties drive inductance, capacitance, and characteristic impedance. Practical guidance emphasizes minimizing unintended inductance/capacitance, choosing appropriate plane strategies, and understanding via effects for signals above high frequency ranges.

LanguageEnglish

CategoryTechnology

FormatPDF

Pages41

Reading Time1 h 43 min

AccessFree to download

Section 5 - High Speed PCB Layout Techniques

High speed PCB layout scenario and motivation

PCB Components

Copper Traces

Copper Planes

Vias

High Speed Analog Design and Application Seminar Section 5 High Speed PCB Layout Techniques Scenario: You have spent several days, no maybe weeks, perfecting a design on paper and also using Spice to ensure the design exceeds all expectations. You hand the schematic to your layout person who puts all everything on a printed circuit board (PCB) . The PCB comes back in a week or two and is finally populated and ready to test. But it doesn’t work!!!! Why not? On paper it works!! Spice said it works!! But it doesn’t work!! This scenario happens more often than not and the reason many circuits do not work as expected is due primarily to the PCB layout. This section looks at some key fundamentals of high speed PCB layout techniques so that hopefully the above scenario will never happen to you. 5-1 Texas Instruments High Speed Analog Design and Application Seminar PCB Components Component: Copper Traces Purpose: Interconnect two or more points Problem: Inductance and Capacitance x = length of trace (cm) w = width of trace (cm) L(nH) ≈ 2x ln ⎜⎛ 5.98 h ⎞⎟ ⎝ 0.8 w + t ⎠ C(pF) ≈ 0.264x (ε r + 1.41 ) ln ⎜ ~~ ~~058.98w~~ ~~~~ ~~t~~ ~~ ⎟ h = height of trace (cm) t = thickness of trace (cm) er = PCB Permeability Z0 ( Ω) = 31.6 | L(nH) | | --- | | C(pF) | Tp ( ps/cm ) = 31.6L(nH)C(pF) 0.8mm (0.031”) trac˜e on 0.8mm (0.031”) thick PCB (FR-4) has: 4nH and 0.8pF per cm er = PCB materi˜al ˜ 10nH and 2.0pF per inch permeability (FR-4 4.5) The PCB consists of layers of metal and insulator and can consist of several layers. Examining some common elements of a PCB will help the reader understand what many people believe is “Black Magic”. Copper traces are utilized to connect one element node to another node. The shape of these traces determine one very important aspect of a PCB – the characteristic inductance, capacitance, and ultimately the characteristic impedance. Resistance is generally ignored as most designs do not carry more than several mA of current and the results can often be negligible. Characteristic impedance (Z0 ) was covered previously, so this will not be discussed here. But what is important is the inductance and capacitance as determined by the trace dimensions and the PCB dielectric (εr) . FR-4, probably the most common PCB material used by manufacturers today and has a permeability range normally from 4.0 to 5.0, but 4.5 is often used as a typical permeability. Check with the PCB manufacturer to determine what material they utilize and the associated permeability. NOTE: Reference the book entitled “High-Speed Digital Design – A Handbook for Black Magic” written by Howard Johnson and Martin Graham, 1993, Prentice-Hall, ISBN 0-13-395724-1. 5-2 Texas Instruments High Speed Analog Design and Application Seminar | PCB Components | | | | --- | --- | --- | | Component: Copper Planes Purpose: Used For Ground Planes and Power Planes Problem: Stray Capacitance on Signal Traces Benefit: Large Bypass Capacitance & Minimal Inductance | | | | h |
| C(pF) ≈ 0.0886h~~ ~~ε~~ ~~r~~ ~~A
h = separation between planes (cm) A = area of common planes = l*w (cm2) er = PCB Permeability | | 0.8mm (0.03˜ 1”) thick PCB (FR-4) has:
0.5pF per cm2 peremrePbiCBlity m(FatR-e4ria˜l4.5)˜ 32.7pF per inch2 | | | Copper planes are typically found when power planes and ground planes are utilized. Planes make an excellent high frequency capacitor and can often be utilized for high frequency bypassing in complement with traditional capacitors. The use of a solid ground plane is generally preferred over a grid plane. A solid plane minimizes inductance to the absolute minimum which is a desirable trait for high speed signals – which includes both Analog and Digital signals. But, as will be discussed later, this plane can cause capacitance problems to sensitive nodes of the circuit. Be aware of all attributes of the circuit and do not blindly use planes everywhere. A side benefit of a solid plane is i

FAQs

Why can a circuit work in Spice but fail after the PCB is fabricated?

The document attributes most mismatches to PCB layout effects. Trace and structure geometry create real inductance and capacitance that simulations may not capture accurately, leading to unexpected behavior.

How do copper trace dimensions affect high speed performance?

Copper trace length, width, and height relative to the PCB dielectric determine inductance and capacitance. These values in turn influence characteristic impedance and overall signal integrity.

What is the role of copper planes in PCB design, and what risk comes with them?

Copper planes are used for ground and power and can provide large bypass capacitance with minimal inductance. However, they may introduce capacitance problems at sensitive circuit nodes, so plane use should be deliberate rather than automatic.

Oct 29, 2026

Upload by Finn