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

Slide -1 Bogatin: 3 Confusing Topics 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 IEEE EMC Distinguished Lecturer Series Slide -2 Bogatin: 3 Confusing Topics Overview • Why do we care? • Specific examples: � Characteristic Impedance � Differential Impedance � Inductance • Got your own? Send me a note IEEE EMC Distinguished Lecturer Series

Slide -3 Bogatin: 3 Confusing Topics What are the Designer’s Most Important Tools? To Here Or Here From Here ? Creativity and intuition are the key ingredients to the design process IEEE EMC Distinguished Lecturer Series Slide -4 Bogatin: 3 Confusing Topics What Does it Mean to Refer to a Cable as a “50 Ohm Cable”? 3 foot long 50 Ohm coax Ω IEEE EMC Distinguished Lecturer Series

Slide -5 Bogatin: 3 Confusing Topics What is the Most Important Electrical Quality the Signal Cares About? Ans: the instantaneous impedance V signal V Signal path V V in Return path GROUND IEEE EMC Distinguished Lecturer Series Slide -6 Bogatin: 3 Confusing Topics Electrical Model of a Lossless Transmission Line ∂ ∂ ∂ ∂ ( ) ( ) ( ) ( ) I x , t = − C V x , t Telegraphers’ equation V x , t = − L I x , t ∂ x ∂ t ∂ x ∂ t 2 2 2 2 ∂ 1 ∂ ∂ 1 ∂ ( ) ( ) ( ) ( ) V x , t = V x , t I x , t = I x , t Wave equation 2 2 2 2 ∂ t LC ∂ x ∂ t LC ∂ x L Z 0 = TD = LC derive C IEEE EMC Distinguished Lecturer Series

Slide -7 Bogatin: 3 Confusing Topics “…be the signal” Charging up a transmission line + + + + + - - - - - V Z = Instantaneous Impedance, I IEEE EMC Distinguished Lecturer Series Slide -8 Bogatin: 3 Confusing Topics Geometry, Current and Impedance Line width increases, capacitance ______________, impedance______________ increases decreases Line width decreases, capacitance ______________, impedance ______________ decreases increases Dielectric thickness increases, capacitance ___________, impedance, ___________ decreases increases IEEE EMC Distinguished Lecturer Series

Slide -9 Bogatin: 3 Confusing Topics What Does it Mean to Have a 50 Ohm Line? Verrrry longgggg 50 Ohm coax Ω IEEE EMC Distinguished Lecturer Series Slide -10 Bogatin: 3 Confusing Topics The Input Impedance of a Transmission Line is Time Dependent open Impedance (Ohms) Characteristic impedance Time Round trip Many round trip time of time of flights flight IEEE EMC Distinguished Lecturer Series

Slide -11 Bogatin: 3 Confusing Topics “…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 IEEE EMC Distinguished Lecturer Series Slide -12 Bogatin: 3 Confusing Topics 2 nd topic: Differential Impedance • What is differential impedance and how does coupling affect it? Difference signal IEEE EMC Distinguished Lecturer Series

Slide -13 Bogatin: 3 Confusing Topics A Secret to Minimize Confusion About Differential Impedance Differential mode Think: : Think Common mode Differential signals Differential signals Common signals Common signals Odd mode Odd mode Even mode Even mode IEEE EMC Distinguished Lecturer Series Slide -14 Bogatin: 3 Confusing Topics Essential Principle Differential impedance is the instantaneous impedance the differential signal sees IEEE EMC Distinguished Lecturer Series

Slide -15 Bogatin: 3 Confusing Topics The Differential Signal Differential signal V = 0v � � � 1v � V = 1v � � � � 0v 1 2 +1 Difference voltage = 2v : -1v � � � +1v � -1 What is the impedance the difference signal sees? What is the impedance the difference signal sees? IEEE EMC Distinguished Lecturer Series Slide -16 Bogatin: 3 Confusing Topics Differential Impedance and Series Impedances What is the equivalent impedance between the two signal lines? with no coupling: with no coupling: Z diff Z diff = Z 0 + Z 0 Z 0 Z 0 Z diff = 2 x Z 0 What happens to the impedance of one line when we turn on coupling? IEEE EMC Distinguished Lecturer Series

Slide -17 Bogatin: 3 Confusing Topics Other Line Is Tied Low Z 0 , Second Trace Pegged Low 58 Single-Ended Impedance (Ohms) 56 54 52 50 Z 0 , Second Trace Pegged Low 48 46 44 42 40 0 5 10 15 20 25 30 35 40 45 50 Edge to Edge Spacing Between the Traces (mils) IEEE EMC Distinguished Lecturer Series Slide -18 Bogatin: 3 Confusing Topics Other Line Driven Opposite 58 s) hm 56 pedance (O 54 V diff = V 1 – V 2 Z 0 , Second Trace Pegged Low 52 50 Single-Ended Im 48 +1 v -1 v 46 44 Z 0 , Both Traces Driven Opposite 42 return 40 0 5 10 15 20 25 30 35 40 45 50 ODD MODE Edge to Edge Spacing Between the Traces (mils) IEEE EMC Distinguished Lecturer Series

Slide -19 Bogatin: 3 Confusing Topics Relating the Modes’ Impedance to the Impedance the Signals See What is the equivalent impedance between the two signal lines? with coupling: with coupling: Z differential Z odd Z odd = Z 2 x Z differenti al odd decrease decrease As coupling increases, differential impedance will: _______________ IEEE EMC Distinguished Lecturer Series Slide -20 Bogatin: 3 Confusing Topics What’s Inductance? � � � � IEEE EMC Distinguished Lecturer Series

Slide -21 Bogatin: 3 Confusing Topics Deficiencies with Text Book Definitions • Too mathematical to provide insight • Deals with coils, not traces on a board IEEE EMC Distinguished Lecturer Series Slide -22 Bogatin: 3 Confusing Topics Inductance Principles -1 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 Right hand rule photo source: Halliday and Resnick, Physics, 1962 What influences the total number of rings of field lines? IEEE EMC Distinguished Lecturer Series

Slide -23 Bogatin: 3 Confusing Topics 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 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 Many flavors of inductance: self � � mutual self mutual � partial loop � loop partial total, net or effective total, net or effective IEEE EMC Distinguished Lecturer Series Slide -24 Bogatin: 3 Confusing Topics 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 IEEE EMC Distinguished Lecturer Series

Slide -25 Bogatin: 3 Confusing Topics Most Important Principle (#3) of Inductance V � � I � � � � B � � ?B � � � V � 1831 Christmas lecture to the Royal Society: Faraday demonstrates “switching noise” Michael Faraday's discovery: Induction : dB/dt � V � I Changing magnetic field Changing lines around a conductor induce a voltage, which drives a current. IEEE EMC Distinguished Lecturer Series Slide -26 Bogatin: 3 Confusing Topics Summary • Why do we care? • Specific examples: � Characteristic Impedance � Differential Impedance � Inductance • Got your own? Send me a note IEEE EMC Distinguished Lecturer Series

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