The Three Most Confusing Topics in Signal Integrity and how not to - - PDF document

Slide -1 Bogatin: 3 Confusing Topics IEEE EMC Distinguished Lecturer Series

The Three Most Confusing Topics in Signal Integrity

and how not to be confused

with

• Dr. Eric Bogatin, Signal Integrity Evangelist,

Bogatin Enterprises, www.beTheSignal.com eric@beTheSignal.com

Slide -2 Bogatin: 3 Confusing Topics IEEE EMC Distinguished Lecturer Series

Overview

• Why do we care?
• Specific examples:

Characteristic Impedance Differential Impedance Inductance

• Got your own? Send me a note

Slide -3 Bogatin: 3 Confusing Topics IEEE EMC Distinguished Lecturer Series

What are the Designer’s Most Important Tools?

From Here To Here Or Here

?

Creativity and intuition are the key ingredients to the design process

Slide -4 Bogatin: 3 Confusing Topics IEEE EMC Distinguished Lecturer Series

What Does it Mean to Refer to a Cable as a “50 Ohm Cable”?

Ω 3 foot long 50 Ohm coax

Slide -5 Bogatin: 3 Confusing Topics IEEE EMC Distinguished Lecturer Series

What is the Most Important Electrical Quality the Signal Cares About?

GROUND

V Vsignal

Signal path Return path

V

Vin

Ans: the instantaneous impedance

Slide -6 Bogatin: 3 Confusing Topics IEEE EMC Distinguished Lecturer Series

Electrical Model of a Lossless Transmission Line

( ) ( )

t , x I t L t , x V x ∂ ∂ − = ∂ ∂

( ) ( )

t , x V t C t , x I x ∂ ∂ − = ∂ ∂

( ) ( )

t , x V x LC 1 t , x V t

2 2 2 2

∂ ∂ = ∂ ∂

( ) ( )

t , x I x LC 1 t , x I t

2 2 2 2

∂ ∂ = ∂ ∂ Telegraphers’ equation Wave equation

C L Z0 = LC TD =

derive

Slide -7 Bogatin: 3 Confusing Topics IEEE EMC Distinguished Lecturer Series

“…be the signal”

+ + + + +

• - - - -

Charging up a transmission line Instantaneous Impedance,

I V Z =

Slide -8 Bogatin: 3 Confusing Topics IEEE EMC Distinguished Lecturer Series

Geometry, Current and Impedance

Line width decreases, capacitance ______________, impedance ______________ Dielectric thickness increases, capacitance ___________, impedance, ___________ Line width increases, capacitance ______________, impedance______________ decreases increases increases decreases decreases increases

Slide -9 Bogatin: 3 Confusing Topics IEEE EMC Distinguished Lecturer Series

What Does it Mean to Have a 50 Ohm Line?

Ω Verrrry longgggg 50 Ohm coax

Slide -10 Bogatin: 3 Confusing Topics IEEE EMC Distinguished Lecturer Series

The Input Impedance of a Transmission Line is Time Dependent

Impedance (Ohms)

Time Many round trip time of flights

• pen

Round trip time of flight Characteristic impedance

Slide -11 Bogatin: 3 Confusing Topics IEEE EMC Distinguished Lecturer Series

“…the impedance” of a Transmission Line is Ambiguous

• The input impedance of the transmission line - may be

time dependent

• The instantaneous impedance of the transmission line
• The Characteristic impedance of the transmission line

Slide -12 Bogatin: 3 Confusing Topics IEEE EMC Distinguished Lecturer Series

2nd topic: Differential Impedance

• What is differential impedance and how does

coupling affect it?

Difference signal

Slide -13 Bogatin: 3 Confusing Topics IEEE EMC Distinguished Lecturer Series

Differential mode Common mode A Secret to Minimize Confusion About Differential Impedance

Think Think: : Differential signals Differential signals Common signals Common signals Odd mode Odd mode Even mode Even mode

Slide -14 Bogatin: 3 Confusing Topics IEEE EMC Distinguished Lecturer Series

Essential Principle Differential impedance is the instantaneous impedance the differential signal sees

Slide -15 Bogatin: 3 Confusing Topics IEEE EMC Distinguished Lecturer Series

The Differential Signal

What is the impedance the difference signal sees? What is the impedance the difference signal sees? V = 0v

• 1v

V = 1v

• 0v

Differential signal 1 2 Difference voltage = 2v : -1v

• +1v
• 1

+1

Slide -16 Bogatin: 3 Confusing Topics IEEE EMC Distinguished Lecturer Series

Zdiff Differential Impedance and Series Impedances

What is the equivalent impedance between the two signal lines?

Zdiff = Z0 + Z0 Zdiff = 2 x Z0

with no coupling: with no coupling: Z0 Z0 What happens to the impedance of one line when we turn on coupling?

Slide -17 Bogatin: 3 Confusing Topics IEEE EMC Distinguished Lecturer Series

Other Line Is Tied Low

Z0, Second Trace Pegged Low

40 42 44 46 48 50 52 54 56 58 5 10 15 20 25 30 35 40 45 50 Edge to Edge Spacing Between the Traces (mils) Single-Ended Impedance (Ohms)

Z0, Second Trace Pegged Low

Slide -18 Bogatin: 3 Confusing Topics IEEE EMC Distinguished Lecturer Series

Other Line Driven Opposite

40 42 44 46 48 50 52 54 56 58 5 10 15 20 25 30 35 40 45 50 Edge to Edge Spacing Between the Traces (mils) Single-Ended Im pedance (O hm s)

Z0, Second Trace Pegged Low Z0, Both Traces Driven Opposite

return

+1 v

• 1 v

Vdiff = V1 – V2 ODD MODE

Slide -19 Bogatin: 3 Confusing Topics IEEE EMC Distinguished Lecturer Series

Relating the Modes’ Impedance to the Impedance the Signals See

Zodd Zodd Zdifferential

• dd

al differenti

Z x 2 Z =

What is the equivalent impedance between the two signal lines? with coupling: with coupling: As coupling increases, differential impedance will: _______________ decrease decrease

Slide -20 Bogatin: 3 Confusing Topics IEEE EMC Distinguished Lecturer Series

What’s Inductance?

Slide -21 Bogatin: 3 Confusing Topics IEEE EMC Distinguished Lecturer Series

Deficiencies with Text Book Definitions

• Too mathematical to

provide insight

• Deals with coils, not traces
• n a board

Slide -22 Bogatin: 3 Confusing Topics IEEE EMC Distinguished Lecturer Series

Inductance Principles -1

photo source: Halliday and Resnick, Physics, 1962

wire carrying a current

Rings of magnetic field lines are Rings of magnetic field lines are around all current carrying around all current carrying conductors conductors

What influences the total number of rings of field lines? Right hand rule

Slide -23 Bogatin: 3 Confusing Topics IEEE EMC Distinguished Lecturer Series

Inductance Principles -2

• 2. Inductance is the number of rings of magnetic field
• 2. Inductance is the number of rings of magnetic field

lines around a conductor, per amp of current lines around a conductor, per amp of current through it through it

Units: Webers/amp = Henry nH more common

Many flavors of inductance: self self   mutual mutual loop loop   partial partial total, net or effective total, net or effective

Inductance is a measure of the efficiency of a conductor to create rings of magnetic field lines at the cost of current

• high inductance, lots of field lines

Slide -24 Bogatin: 3 Confusing Topics IEEE EMC Distinguished Lecturer Series

Inductance Plays a Pivotal Role in Signal Integrity

• Signal propagation:
• loop self inductance
• Discontinuities:
• loop self inductance
• Cross talk:
• loop mutual inductance
• PDN and rail collapse:
• loop self inductance
• Ground bounce:
• total inductance of the return path
• Hacking interconnects: performance physical design
• Partial self and partial mutual inductance

Slide -25 Bogatin: 3 Confusing Topics IEEE EMC Distinguished Lecturer Series

Most Important Principle (#3) of Inductance

1831 Christmas lecture to the Royal Society: Faraday demonstrates “switching noise” Michael Faraday's discovery: Induction: dB/dt V I Changing Changing magnetic field lines around a conductor induce a voltage, which drives a current. V

• I
• B

?B

• V

Slide -26 Bogatin: 3 Confusing Topics IEEE EMC Distinguished Lecturer Series

Summary

• Why do we care?
• Specific examples:

Characteristic Impedance Differential Impedance Inductance

• Got your own? Send me a note

Slide -27 Bogatin: 3 Confusing Topics IEEE EMC Distinguished Lecturer Series