Download Bouncing Out Ground Bounce to Improve Signal Integrity and EMC

Slide -1
Ground Bounce and EMC
Bouncing Out Ground Bounce to
Improve Signal Integrity and EMC
Dr. Eric Bogatin,
Signal Integrity Evangelist,
Bogatin Enterprises, a LeCroy Company
www.beTheSignal.com
Sept 2011
 Bogatin Enterprises, LLC, a LeCroy Company 2011
www.beTheSignal.com
Slide -2
Ground Bounce and EMC
Overview
• A methodology for solving problems
• The ground bounce problem
• The essential principles
• Why ground bounce is also an EMC problem
• Solutions
 Bogatin Enterprises, LLC, a LeCroy Company 2011
www.beTheSignal.com
Slide -3
Ground Bounce and EMC
For More Information
www.beTheSignal.com
Recent Publications
Future class schedules
My Blog: What I learned this month
Recorded webinars
www.PrintedCircuitUniversity.com
for online training
Published by Prentice Hall, 2009
 Bogatin Enterprises, LLC, a LeCroy Company 2011
www.beTheSignal.com
Slide -4
Ground Bounce and EMC
Fastest Way to Solve a Problem is
to Identify its Root Cause
If you have the wrong
root cause, you will only
fix the problem by luck
 Bogatin Enterprises, LLC, a LeCroy Company 2011
www.beTheSignal.com
Slide -5
Ground Bounce and EMC
Identify the Problem:
Ground Bounce
On Chip
Icharge
Idischarge
Switching lines
Quiet data line
V SS
V CC
L Bonding
common
lead
inductance
GND
•
•
•
•
•
Power
L Bonding
15836
© 1991 Integrated Circuit Engineering Corporation
Identify the problem
Understand the essential principles
Find the root cause
Establish the design guideline
Create a specific design rule
 Bogatin Enterprises, LLC, a LeCroy Company 2011
www.beTheSignal.com
Slide -6
Ground Bounce and EMC
The Origin of Ground Bounce
Signal 2
Voltage
across
signal 2 has
ground
bounce
voltage in
series with it
- crosstalk
I1
L11
Signal 1
1
L21
I2
2
L22
VgndBounce
Common return
I1 = I2
dNtotal
=
dt
VgndBounce = (L22 − L21 )
Ntotal = L22I 2 − L21I1
dI1
dI
= Ltotal 1
dt
dt
In what interconnect elements will return currents overlap?
In what signaling scheme will net return currents be mostly constant?
 Bogatin Enterprises, LLC, a LeCroy Company 2011
www.beTheSignal.com
Slide -7
Ground Bounce and EMC
Confirm Root Cause with Simple
Simulation (using QUCS)
n lines
2v/div
1v/div
Data line
Quiet line
RT = 1 nsec
L_comm = 4 nH
n = 4 switching
TD = 1.1 nsec
R_source = 20 Ohms
common lead inductance
 Bogatin Enterprises, LLC, a LeCroy Company 2011
www.beTheSignal.com
Slide -8
Ground Bounce and EMC
Turn the Root Cause into a Design
Guideline with the “Youngman Principle”
Read www.bethesignal.com/blog, Nov 9, 2008
“If your arm hurts when you raise it, don't raise your arm.”
“If problem A happens when your design has feature B,
then eliminate feature B from your design”
Identify the root cause of a problem and fix the root cause
 Bogatin Enterprises, LLC, a LeCroy Company 2011
www.beTheSignal.com
Slide -9
Ground Bounce and EMC
In Which Interconnect Structures
Will Ground Bounce Arise?
•
Two ingredients for ground bounce:
Screwed up return path: higher inductance than a wide plane
Overlapping return currents for different signal lines
•
Applying the Youngman Principle: how to reduce ground bounce?
Don’t screw up the return path
Don’t overlap return currents
•
Where we expect to see ground bounce:
Resistor SIPS
Narrow package traces, Narrow connector pins
Gaps in planes
Vias, connectors
 Bogatin Enterprises, LLC, a LeCroy Company 2011
www.beTheSignal.com
Slide -10
Ground Bounce and EMC
Return Bounce in
Resistor Networks
TT electronics
Bourns
muRata
 Bogatin Enterprises, LLC, a LeCroy Company 2011
www.beTheSignal.com
Slide -11
Ground Bounce and EMC
Calculating Total Inductance of
the Return Path is Hard!
w
“if all you have is a
hammer, everything
looks like a nail”
Signal path
s
Total Inductance per length, nH/inch
Return path
Total inductance of return path
20
18
16
14
12
s = 40 mils
s = 30 mils
s = 20 mils
s = 10 mils
10
8
6
4
2
0
5
10
15
20
25
30
35
40
45
For w = s
Ltotal ~ 10 nH/inch
50
Trace Width, mils
 Bogatin Enterprises, LLC, a LeCroy Company 2011
www.beTheSignal.com
Slide -12
Ground Bounce and EMC
Estimating Ground Bounce Noise
Vgnd = L totaln
Vgnd
dI
dt
Vsig
=
L totaln
L
xn
= 2% x total
Z0RT
RT
Ltotal[nH] = total inductance of return path
n = number of simultaneous signals switching
RT[nsec] = 10-90 rise time of signal current
V
dI
= sig
dt Z 0RT
Ltotal ~ 10 nH/inch x Len
Ltotal ~ 5 nH
Ltotal ~ 2 nH
Example: n = 3, RT = 0.5 nsec
gb noise ~ 2% x 2 x 3 / 0.5 = 24%
Example: n = 2, RT = 1 nsec
gb noise ~ 2% x 5 x 2 / 1 = 20%
Do you wonder why ground bounce is so common in packages and connectors?
 Bogatin Enterprises, LLC, a LeCroy Company 2011
www.beTheSignal.com
Slide -13
Ground Bounce and EMC
Overlapping Return Currents
= Ground Bounce
•
Ltotal ~ 10 nH/inch x Len of overlapping return
currents
•
Example: 1 signal switching, with returns
overlapping for 0.25 inches
Vgnd
•
L n
= total
Vsig Z 0RT
Ltotal = 2.5 nH x 2 legs = 5 nH
•
Z0 = 50 Ohms, RT = 1 nsec, n = 1
Ltotal ~ 10 nH/inch x 0.25
inches = 2.5 nH
0.25 inches
Vgnd
Vsig
L n
5 x1
= total =
= 10%
Z 0RT 50 x 1
If there are 3 signals switching, noise ~ 30%,
more if RT < 1 nsec
 Bogatin Enterprises, LLC, a LeCroy Company 2011
www.beTheSignal.com
Slide -14
Ground Bounce and EMC
Radiated Emissions from
Antennas
1000000
(at 3 meters)
R
E=
5.5
Pradiated
R
E = far field strength in V/m
R = distance from source in m
Pradiated = total radiated power in W
Field Strength ( V/m)
100000
10000
1000
100 µV/m
100
10
1.E-09
1.E-08
1.E-07
1.E-06
1.E-05
1.E-04
1.E-03
1.E-02
1.E-01
Radiated power (Watts)
A few nanowatts of radiated power will fail FCC class B!
 Bogatin Enterprises, LLC, a LeCroy Company 2011
www.beTheSignal.com
Slide -15
Ground Bounce and EMC
Preventing EMI Problems is Hard!
“There are two kinds of designers:
those who are trying to design
antennas on purpose and those
that aren’t doing it on purpose”
 Bogatin Enterprises, LLC, a LeCroy Company 2011
www.beTheSignal.com
Slide -16
Ground Bounce and EMC
Two Fundamental Types of
Antennas- and Currents
Electric dipole
Cable shield
+
+
Noisy plane
I
~
-
chassis
If C = 1 pF,
Z = 1600 Ω
@ 100 MHz
I
Magnetic dipole
Transmission line
Where is the return current path in each antenna?
 Bogatin Enterprises, LLC, a LeCroy Company 2011
www.beTheSignal.com
Slide -17
Ground Bounce and EMC
Common Currents and
Failing FCC
+
+
Len
I
~
I
Direction of
max radiation
~
-
(conductive plate)
Isolated dipole
E = 4π x 10 −7
f x I x Len
= 0.4 µV / m x f x I
R
f in MHz, I in microAmps
@ 100 MHz, E ~ 100 uV/m is FCC, class B limit
Imax =
100uV / m
= 3uA
0.4uV / m x 100MHz
Only takes ~ 3 uA common
current to fail FCC!
 Bogatin Enterprises, LLC, a LeCroy Company 2011
www.beTheSignal.com
Slide -18
Ground Bounce and EMC
Example: Common Currents and
Radiation from Ground Bounce
E = 4π x 10 −7
E=
I=
Len =
R=
f=
f x I x Len
R
field in V/m
current in Amps
length of monopole in m
distance to point in m
sine wave frequency in Hz
E = 1.2 x 10
−6
Cable shield
Noisy plane
chassis
10 8 x 10 −4 x 1
= 4000 µV / m
3
Voltage noise = 100 mV
Impedance ~ 1 kΩ
Ω
CM Current ~ 100 µA
Len ~ 1 m
R=3m
@ 100 MHz
Should you be worried?
 Bogatin Enterprises, LLC, a LeCroy Company 2011
www.beTheSignal.com
Slide -19
Ground Bounce and EMC
Apply Youngman’s Principle to Fix EMC
Problems from Ground Bounce
Cable shield
Noisy plane
Icm
chassis
Icm
•
Solutions:
Reduce total inductance
of return path: short, wide
conductors, signal and
return currents in
proximity
L total dIplane
=
Z cm dt
Iplane = current in the planes
Ltotal = total inductance of the return plane
Zcm = impedance the common current sees
Reduce dI/dt in the
planes- diff signaling, ondie, on-pkg caps
Increase comm signal
impedance: ferrites!
 Bogatin Enterprises, LLC, a LeCroy Company 2011
www.beTheSignal.com
Slide -20
Ground Bounce and EMC
Finite Width of Return Path
Cable shield
Noisy plane
D. S. Britt, D. H. Hockanson, F. Sha, J. Drewniak, T. Hubbing
and T. Van Doran, “Effects of Gapped Ground Planes and
Guard Traces on Radiated EMI”, IEEE Symposium on
Electromagnetic Compatibility, 1997, p. 159
chassis
Gap just under
the trace
Common
current
Keep return path width > 3 x w
 Bogatin Enterprises, LLC, a LeCroy Company 2011
www.beTheSignal.com
Slide -21
Ground Bounce and EMC
Radiated Emission from Cables
Really Comes from the Connectors
Enclosure
IC =
Signal I/O
1
dI 1
20mA
L total =
0.1nH
= 10µA
ZC
dt 200
1n sec
• How much external magnetic fields
around a coax?
0.1 nH
asymmetric connector, causing
some total inductance
• What is the total inductance of the
return path of a coax?
• How much ground bounce will
appear in an ideal coax connector?
•
Asymmetrical connector causes total inductance in return path
•
Return current through total inductance causes voltage noise
•
Voltage noise drives common currents in cable
•
Cable radiates.
 Bogatin Enterprises, LLC, a LeCroy Company 2011
www.beTheSignal.com
Slide -22
Ground Bounce and EMC
Common Currents and Connector
Quality
IEEE EMC Symposium 2009
 Bogatin Enterprises, LLC, a LeCroy Company 2011
www.beTheSignal.com
Slide -23
Ground Bounce and EMC
Ground Bounce Can Ruin Your Day
• Causes excessive cross talk
Can be long range
Grows with more signals switching simultaneously
Grows inversely with rise time
• Causes EMI
In connectors
In planes
• Fix ground bounce by
Don’t screw up the return path
Don’t share return paths
 Bogatin Enterprises, LLC, a LeCroy Company 2011
www.beTheSignal.com
Slide -24
Ground Bounce and EMC
For More Information
www.beTheSignal.com
Recent Publications
Future class schedules
My Blog: What I learned this month
Recorded webinars
www.PrintedCircuitUniversity.com
for online training
Published by Prentice Hall, 2009
 Bogatin Enterprises, LLC, a LeCroy Company 2011
www.beTheSignal.com