Slide -1VL-155 Practical Differential Pair Design
Bogatin Enterprises 2010 www.beTheSignal.com
Dr. Eric Bogatin,
Signal Integrity Evangelist,
Bogatin Enterprises
www.beTheSignal.com
April 2010Presented at the Huntsville EMC Symposium, April 2010
Practical Differential Pair Design
Slide -2VL-155 Practical Differential Pair Design
Bogatin Enterprises 2010 www.beTheSignal.com
Copyright © 2010 by
Bogatin Enterprises, LLC
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Please respect the great deal of effort that
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Bogatin Enterprises, LLC26235 W. 110th Terr.Olathe, KS 66061v: 913-393-1305f: 913-393-0929e: [email protected]
Slide -3VL-155 Practical Differential Pair Design
Bogatin Enterprises 2010 www.beTheSignal.com
For More Information
www.BeTheSignal.com
� Signal Integrity Certification Programs
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� Signal integrity public classes
� No Myths Allowed webinar series
� Streaming recorded lectures
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� Feature articles and columns
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Slide -4VL-155 Practical Differential Pair Design
Bogatin Enterprises 2010 www.beTheSignal.com
Outline
• Download a copy of the presentation from beTheSignal.com: under SI content, select “slides presentation”, PPT-VL-155
• Design Methodology
• Problems to avoid
• Decision factors for coupling
• Exploring Design Space
Slide -5VL-155 Practical Differential Pair Design
Bogatin Enterprises 2010 www.beTheSignal.com
Pop Quiz
• Which is better:
� Tightly coupled diff pairs?
� Loosely coupled diff pairs?
Slide -6VL-155 Practical Differential Pair Design
Bogatin Enterprises 2010 www.beTheSignal.com
What is the most common answer to
all SI questions?
“>it depends”
We answer it depends questions by
“putting in the numbers” with analysis:
� Rules of thumb
� Approximations
� Numerical simulation tools
Slide -7VL-155 Practical Differential Pair Design
Bogatin Enterprises 2010 www.beTheSignal.com
Differential mode
Common mode
A Secret to Minimize Confusion
About Differential Impedance
Think:
Differential signals
Common signals
Odd mode
Even mode
Slide -8VL-155 Practical Differential Pair Design
Bogatin Enterprises 2010 www.beTheSignal.com
Differential and Common Signals
V1
V2
• Definitions:
� Vdiff = V1 – V2
� Vcomm = ½ (V1 + V2)
2 4 6 8 10 12 14 16 180 20
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
0.0
2.0
time, nsec
V1, V
V2, V
Typical LVDS levels
2 4 6 8 10 12 14 16 180 20
0.0
0.5
1.0
-0.5
1.5
time, nsec
Vcom
mV
diff
common
differential
Slide -9VL-155 Practical Differential Pair Design
Bogatin Enterprises 2010 www.beTheSignal.com
Every Pair of Signals Has a
Differential and Common Component
2 4 6 8 10 12 14 16 180 20
0.2
0.4
0.6
0.8
1.0
1.2
1.4
1.6
1.8
0.0
2.0
time, nsec
V1
, V
V2
, V
2 4 6 8 10 12 14 16 180 20
0.0
0.5
1.0
-0.5
1.5
time, nsec
Vco
mm
Vdiff
Added Skew = RT to one line
common
differential
• Differential and common signals
propagate independently and DO
NOT Interact on the board
• They each see a different
electrical environment:� Diff and comm impedance
� Diff and comm prop velocity
� Diff and comm attenuation
� Diff and comm cross talk
• But>.� Any line to line asymmetry will convert
diff into comm signal and vis versa
Definitions:
Vdiff = V1 – V2
Vcomm = ½ (V1 + V2)
Slide -10VL-155 Practical Differential Pair Design
Bogatin Enterprises 2010 www.beTheSignal.com
A New Design Methodology to Eliminate SI Problems
Before the Design is Released
}Understand the essential
principles
-15 -10 -5 0 5 10 15-20 20
0.05
0.10
0.15
0.00
0.20
Center to Center Pitch, mils
NE
XT
, fr
actio
n
pitch
•An efficient methodology:
� Identify the SI problems
� Find the root cause
� Establish design guidelines to minimize them- balancing tradeoffs
� “correct by design”: use analysis tools to develop pre-layout design rules specific to your design
� Use post layout analysis tools to verify the final design
Slide -11VL-155 Practical Differential Pair Design
Bogatin Enterprises 2010 www.beTheSignal.com
Practical Design Considerations
• Always do what is free
• Explore design space with simple estimates, then more accurate analysis
• Explore cost- performance trade offs with “virtual prototypes”
• The most difficult tradeoffs: higher component cost for lower system cost
• Consider product lifetime performance
• Manage risk: buy “insurance” by adding design margin
Cost factors:
Performance(meet specs)
Slide -12VL-155 Practical Differential Pair Design
Bogatin Enterprises 2010 www.beTheSignal.com
Establish a Design Guideline by
Applying the “Youngman Principle”
Read www.bethesignal.com/blog, Nov 9, 2008
“If your arm hurts when you raise it, don't raise your arm.”
Identify the root cause of a problem and fix the root cause
“If problem A happens when your design has feature B, then eliminate feature B from your design”
Slide -13VL-155 Practical Differential Pair Design
Bogatin Enterprises 2010 www.beTheSignal.com
Four Chief Problems to Manage in
High Speed Serial Links
• Losses
� Conductor loss
� Dielectric loss
• Ripple
� Impedance mismatches from: TX, channel, vias, connectors, RX
• Noise: cross talk
� Diff to diff and comm to diff coupling
• Mode conversion
� Line to line asymmetry
50 100 150 200 250 300 3500 400
0.0
0.2
0.4
0.6
-0.2
0.8
time, psec
Eye_uniform
.Density
0.8
What you think you have
Mantra: “losses, ripple, noise, mode conversion”
What you actually have
Slide -14VL-155 Practical Differential Pair Design
Bogatin Enterprises 2010 www.beTheSignal.com
Design Solution Options to Achieve
Performance Goals
• Lowest attenuation
� Low dissipation factor laminate
� Lowest Dk laminate
� Wide lines
� Smooth copper
� Lower impedance
• Lowest ripple noise
� Controlled impedance to a target impedance
� Lower target impedance to match lower via impedance
• Lowest cross talk
� Avoid microstrip
� Large spacing between channels
� Tight coupling when return path is screwed up
• Lowest mode conversion
� Matched length, or length compensation
� Matched cross section lines
� Mitigate glass weave skew
• Lowest cost features
� FR4
� Highest interconnect density
1
2
Slide -15VL-155 Practical Differential Pair Design
Bogatin Enterprises 2010 www.beTheSignal.com
A “Hidden Variable” to Real World
Performance
• Identical boards from different suppliers
• Very different insertion loss: 2x difference- why?
example courtesy of Cisco
Slide -16VL-155 Practical Differential Pair Design
Bogatin Enterprises 2010 www.beTheSignal.com
Solutions are Available for
Smoother Copper
Courtesy of John Andresakis
(dual flat foil)
Push your fab vendors for:
1. rms roughness characterization data
2. Smoother copper foil
Cost will be driven by the market
If you do not ask for it, there will be no market need
The higher the volume the lower the cost
Slide -17VL-155 Practical Differential Pair Design
Bogatin Enterprises 2010 www.beTheSignal.com
Tight or Loose Coupling?
• Performance drivers:
� Target impedance
� Widest line width
� Channel to channel cross talk
� Glass weave pitch
• Cost drivers:
� Fewest layers
� Lowest cost laminate
� Highest interconnect density
� Narrowest line that is free
� Tightest pitch that is high yield
Cost factors:
Performance(meet specs)
Slide -18VL-155 Practical Differential Pair Design
Bogatin Enterprises 2010 www.beTheSignal.com
What is NOT influenced by Coupling
• The degree of coupling has NO impact on reflections or mismatch
• The differential signal only sees the differential impedance-
Symmetric,
uncoupled lines
make a perfectly
good differential
pair
uncoupled tightly coupled
HyperLynx 8.0
Slide -19VL-155 Practical Differential Pair Design
Bogatin Enterprises 2010 www.beTheSignal.com
Microstrip:
Differential Impedance and Coupling
Increasing coupling decreases
differential impedance
Slide -20VL-155 Practical Differential Pair Design
Bogatin Enterprises 2010 www.beTheSignal.com
Compensate Line Width for Separation to
Keep Differential Impedance Constant
This line defines design space for 100 Ohm differential impedance
h = 3.5 mils
h = 2.7 mils, Dk = 4
Slide -21VL-155 Practical Differential Pair Design
Bogatin Enterprises 2010 www.beTheSignal.com
Attenuation and Line Width
• Consequence of wider line width, w:
� Lower Resistance
� If impedance is constant, lower attenuation
� Need thicker b to keep impedance constant
• What if dielectric thickness, b is fixed? What is impact of wider w?
wb
−=
0
LenLen
Z
Rx34.421SFor ALL transmission lines:
dB/inchRL in Ohms/inZ0 in Ohms
Slide -22VL-155 Practical Differential Pair Design
Bogatin Enterprises 2010 www.beTheSignal.com
Attenuation and Line Width
Stripline, ½ oz copper
Fixed total thickness b = 12.2 mils
- Increasing line width, decreases R,
decreasing S21
- Increasing line width decreases Z0,
increasing S21.
Which has lower S21, narrow or wide w?
Lower Z0 than 50 Ohms is lower loss
wb
5 10 15 20 25 30 35 40 450 50
10
20
30
40
50
60
70
0
80
Line Width, w, mils
Ch
ara
cte
ristic Im
ped
an
ce
, O
hm
s
5 10 15 20 25 30 35 40 450 50
-0.15
-0.10
-0.05
-0.20
0.00
Line Width, w, mils
Insert
ion L
oss, d
B/inch
Optimum impedance for lowest
conductor loss ~ 35 Ohms
−=
0
LenLen
Z
Rx34.421S
@ 1 GHz
Conductor loss, rms = 0
Slide -23VL-155 Practical Differential Pair Design
Bogatin Enterprises 2010 www.beTheSignal.com
How Else to Enable Wide Lines,
And Tight Pitch?
• Compromise: Loose coupling
� s = 2 x w, w = 5 mils
� Dk = 4
• Lower target impedance
� 85 Ohms is target in PCIeII
• Thicker H1 = H2 = 13 mils
� s = w = 5 mils
� Dk = 4
• Lower Dk = 3.1
� s = w = 5 mils
� H1 = H2 = 6.5 mils
Slide -24VL-155 Practical Differential Pair Design
Bogatin Enterprises 2010 www.beTheSignal.com
Worse Case: Far End Cross Talk in
Microstrip: Single-ended to Diff
+ -aggressorvictim
spacingCoupling
0.2 0.4 0.6 0.8 1.0 1.2 1.4 1.6 1.80.0 2.0
-0.20
-0.15
-0.10
-0.05
0.00
-0.25
0.05
time, nsec
Diffe
rential N
ois
e, fr
action
100 Ohm diff
5 mil line, spacing
RT = 100 psec
Len = 10 inchesRT
Len~FEXT
Coupling cases:
Uncoupled: s = 3 x w
Loose: s = 2 x w
Tight s = w
~ 5 dB reduction in cross talk from tightest coupling
Differential cross talk from common sources can be -20 dB!
FEXT often limits max trace length in PCIe to < ~16 inches
(non-interleaved)
10 15 20 25 30 35 40 455 50
-40
-30
-20
-10
-50
0
Spacing, mils
FarE
nd N
ois
e,
fraction
Far
End N
ois
e in d
B
Slide -25VL-155 Practical Differential Pair Design
Bogatin Enterprises 2010 www.beTheSignal.com
Worst Case Near End Cross Talk in
Stripline
+ -aggressorvictim
spacingCoupling
1 2 3 40 5
-90
-80
-70
-60
-50
-40
-30
-20
-10
-100
0
Spacing/line width
Near
End C
ross T
alk
, in
dB
For less than -50 dB xtk,
keep spacing > 3 x w
Three coupling cases:
Tight s = w
Loose: s = 2 x w
Uncoupled: s = 3 x w
Which is more important
influencing NEXT: coupling or
spacing?
~ 1 dB reduction in cross
talk from tightest
coupling
Reduce cross talk by
increasing spacing to
aggressor!
Slide -26VL-155 Practical Differential Pair Design
Bogatin Enterprises 2010 www.beTheSignal.com
Coupling and Differential Cross Talk
• When the return path is a wide, uniform plane, tighter coupling has little impact on differential cross talk ( ie, in controlled impedance board traces)
• When the return path is not a uniform plane, tighter coupling can dramatically decrease differential cross talk
• Always use tight coupling between lines in a differential pair when the return path is not a wide uniform plane:
� Gaps
� Vias
� Connectors
� Leaded, 2 layer packages
� Sockets/interposers
� Flex/ribbon cable
Slide -27VL-155 Practical Differential Pair Design
Bogatin Enterprises 2010 www.beTheSignal.com
Local Dk Variation Causes
“Weave Induced Skew”
Worst case if pitch = (1/2 + n) x glass pitch
Typical glass weave pitch ~ 15-25 mils
1080, 2116 are 17 mils pitch
Best case is if routing pitch matches glass weave
pitch: ~16-20 mils
Brist et al. PCD&F Nov 2004
8 mil wide line, 17 mil pitch
Slide -28VL-155 Practical Differential Pair Design
Bogatin Enterprises 2010 www.beTheSignal.com
Time Delay Measurements of
Different Traces
straight
zig-zag
Courtesy of Altera
8 inch long Stripline in
2116 glass
8 psec in 8
inches ~ 1
psec/inch
Slide -29VL-155 Practical Differential Pair Design
Bogatin Enterprises 2010 www.beTheSignal.com
Measured Skew in 4 inch Test Lines
Typical glass weave skew ~ 2.5
psec/inch
Worst case glass weave skew maybe
~15 psec/inch Courtesy of Jeff Loyer, Intel Corp.
+ -
Higher local Dk
Slower speed
Longer delay
Lower local Dk
higher speed
shorter delay
Glass Dk ~ 6
Resin Dk ~ 3
Photo courtesy of Jeff Loyer, Intel
40,000 TDR measurements
Typical case: 20 inches x 2.5 psec/inch = 50 psec skew. Possible problem for > 2 Gbps
Slide -30VL-155 Practical Differential Pair Design
Bogatin Enterprises 2010 www.beTheSignal.com
Which is Better:
Tight or Loose Coupling?
tight loose
Higher Interconnect Density
Lower Conductor Loss
Sweet spot s ~ 2w
If interconnect density is
most important, always
use tight coupling
If loss is important,
consider using
loose coupling
Slide -31VL-155 Practical Differential Pair Design
Bogatin Enterprises 2010 www.beTheSignal.com
Which is Better,
Tight or Loose Coupling?
It depends:
• Why loose coupling:
� Lower loss
� Risk reduction for glass weave skew mitigation
• Why tight coupling
� Higher interconnect density
� Lower cost
• What is not critical
� Differential impedance control
� Channel to channel cross talk
• What else:
� Lower Df
� Lower Dk
� Smoother copper
Slide -32VL-155 Practical Differential Pair Design
Bogatin Enterprises 2010 www.beTheSignal.com
Practical Guidelines
• If bit rate is < ~ 1 Gbps
� Loss not an important driver
� Always consider tight coupling
• If bit rate is > ~ 5 Gbps
� Loss very important
� Consider looser coupling
� Route on a pitch equal to the glass weave pitch
• Regardless of bit rate, always do your own analysis
Slide -33VL-155 Practical Differential Pair Design
Bogatin Enterprises 2010 www.beTheSignal.com
The End!
www.BeTheSignal.com
� Signal Integrity Certification Programs
� Continuing Education Curriculums
� Signal integrity public classes
� No Myths Allowed webinar series
� Streaming recorded lectures
� Hands on labs
� Feature articles and columns
� SI-Insights quarterly publication
� Monthly Pop Quiz
� My Blog: What I learned this monthPublished by Prentice Hall, 2009