Transcript Considerations for High Speed PCB design

Link A/D converters and
Microcontrollers using
Long Transmission Lines
John WU
Precision Analog - Data Converter Applications Engineer
[email protected]
Transmission Line Effect
Considerations
• Definition of the Highest Frequency Signal
• Transmission Line Model
• Reflection Concept
• Termination Topology
• Crosstalk Analysis
Application Example
Transmitted Data and Clock
Reflection on MSP430
With Termination on MSP430
Reflection on ADS8326 EVM
With Termination on ADS8326
?
Definition of the highest frequency signal
What is the highest frequency signal
in a 2.25MHz sample clock rate
ADC ?
Definition of the highest
frequency signal
The highest frequency signal is determined
by the signal rise or fall time
Rise Time vs. Bandwidth
The rise time and bandwidth are related by:
For example:
clock rate = 2.25 MHz,
trise CLK = 10 ns
trise Data = 2 ns
The highest frequency signal or bandwidth is:
= 175MHz
Rise Time Measurement
• The displayed Tr = 2ns (10-90%) on an oscilloscope
• 3-dB bandwidth of a probe is 500MHz (Tr = 0.7ns)
• 3-dB bandwidth of an oscilloscope input is 350MHz (Tr=1ns)
• What is the measured input signal Tr ?
Rise Time Degradation
• Tdisplayed2 = Tprobe2 + Tscope2 + Tsignal2
• Tsignal2 = Tdisplayed2 -Tprobe2 -Tscope2
•
Tsignal = SQRT(22 - 0.72 - 12 )=1.6ns!
•
1.6ns) = 220MHz
Critical Microstrip Length
What is the critical length of
a microstrip that must be considered as
transmission line?
Rise Time vs. Propagation Delay
Propagation
time is
15% of Tr
Voltage (V)
3.5
3
2.5
2
1.5
1
0.5
0
0
0.5
1
Rise Time (100%)
1.5
2
Tpd (ps/inch)
Propagation Delay vs. Dielectric
Constant
250
200
150
100
50
0
0
1
2
3
4
5
6
Dielectric constant
7
8
9
Critical Microstrip Length
• Tr x 15% = 1.6ns x 15% = 226 ps
•
= 1.5 inch
Transmission Line Model
Zo =
V (Z)
I (Z)
= sqrt (
L
C
)
Characteristic Impedance of
Twisted Pair Cable & Microstrip
Reflection Factor
Zo
ZL
Г
ZL - Zo
=
ZL + Zo
If Z L >> Zo; Г = +1
If Z L << Zo; Г = -1
S Parameters for Reflection
b2
S21=
a1
b1
S11=
a1
b2
S22=
a2
b1
S12=
a2
Reflection Analysis
Reflection Calculation
Source reflection factor
20-100
=
20+100
= -0.8
Load reflection factor
1 G - 100
=
1 G + 100
=1
Calculated vs. Measured
Reflection
• Tr/7
Critical Length of a Transmission Line
What is the critical length of
a transmission line that must be
terminated?
Voltage (V)
Rise Time vs. Propagation Delay
3.5
3
2.5
2
1.5
1
0.5
0
Propagation time
is 50% of Tr
0
0.5
1
Rise Time (100%)
1.5
2
Trace & Cable Terminations
• Source Termination
Driver
Rt
Zo = 50 W
Rs
Rt + Rs = Zo
Receiver
Trace & Cable Terminations
• AC termination
AC
Termination
Place of Terminations
Daisy Chain Routing with Stubs
Clock Source
Clock
Bus
Termination
Resistor
Stub
Device 1
Device Pin
BGA Ball
Device 2
Short stubs create signal integrity
problems
Stub Length=0.5”
Stub Length=0.25”
Reference:
Altera
application note 224
Daisy Chain Routing without
Stubs
Device Pin
BGA Ball
Clock Source
Clock
Bus
Device 1
Terminatio
n Resistor
Device 2
Star Routing
Clock
Bus
Device 1
Terminatio
n Resistor
Device 2
Clock
Source
Device 3
Device Pin
BGA Ball
Parallel Fly-By Termination
Vcc
R1
Zo = 50 W
Receiver
Device
(BGA Package)
R2
Differential Pair (LVDS) Fly-By
Termination
Receiver
Device
(BGA Package)
Zo=50 W
100 W
Zo=50 W
Crosstalk Analysis
• Cross Talk occurs on PCB and twisted wire cable
Magnetic & Electric Fields of
Parallel Transmission Line
Aggressor
Trace
Victim
Trace
Aggressor
Trace
Victim
Trace
Magnetic
Field
Ground Plane
Electric
Field
Ground Plane
Cross Talk Analysis
Cross Talk Analysis
FEXT Measurement
Inductive
or
Capacitive
Coupling
?
Reduced FEXT Measurement
NEXT Measurement
Reduced NEXT Measurement
Separated Data Wire
Cross Talk Reduced by
Termination
Xtalk
Final Termination Solution
ADS
8326
U3 BUF634
U5 BUF634
MSP
430
R1 100
+
3 feet (1 meter) twisted pair cable
+
+
+
CLOCK
+
U2 CS
BUF634
R4 100
U1
CLOCK
BUF634
C1 220p
R2 100
+
+
3 feet (1 meter) twisted pair cable
+
R5 100
U4 CS
BUF634
C2 220p
+
DATA
3 feet (1 meter) twisted pair cable
+
R6 100
C3 220p
R3 100
+
+
U6
DATA
BUF634
Conclusions
• The highest frequency signal is determined by switching
time
• Transmission line model must be used when
propagation delay time is greater than 15% of Tr
• Termination technique dramatically reduces reflection
and crosstalk
Q&A
Acknowledgement
Thanks Phil Lizzi for providing the “real life”
transmission line application example
Reference:
1.
“Managing Signal Quality” Mentor Graphics/Xilinx, 2005
http://www.xilinx.com/publications/xcellonline/xcell_53/xc_pdf/xc_mentor53.pdf
2.
“High-Speed Board Layout Guideline” Altera application note 224, Sept. 2003