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Link A/D converters and Microcontrollers using
Long Transmission Lines
John WUPrecision Analog - Data Converter Applications Engineer
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Transmission Line Effect Considerations
• Definition of the Highest Frequency Signal
• Transmission Line Model
• Reflection Concept
• Termination Topology
• Crosstalk Analysis
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Application Example
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Transmitted Data and Clock
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Reflection on MSP430
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With Termination on MSP430
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Reflection on ADS8326 EVM
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With Termination on ADS8326
?
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Definition of the highest frequency signal
What is the highest frequency signal
in a 2.25MHz sample clock rate
ADC ?
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Definition of the highest frequency signal
The highest frequency signal is determined by the signal rise or fall time
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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
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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 ?
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Rise Time Degradation
• Tdisplayed2
= Tprobe2 + Tscope
2 + Tsignal2
• Tsignal2 = Tdisplayed
2 -Tprobe2 -Tscope
2
• Tsignal = SQRT(22 - 0.72 - 12 )=1.6ns!
• 1.6ns) = 220MHz
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Critical Microstrip Length
What is the critical length of
a microstrip that must be considered as transmission line?
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Rise Time vs. Propagation Delay
0
0.5
1
1.5
2
2.5
3
3.5
0 0.5 1 1.5 2
Rise Time (100%)
Vo
lta
ge
(V
)
Propagation time is 15% of Tr
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Propagation Delay vs. Dielectric Constant
050
100150200250
0 1 2 3 4 5 6 7 8 9
Dielectric constant
Tp
d (
ps/
inch
)
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Critical Microstrip Length
• Tr x 15% = 1.6ns x 15% = 226 ps
• = 1.5 inch
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Transmission Line Model
Zo = V (Z)
I (Z) = sqrt (
L
C )
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Characteristic Impedance of Twisted Pair Cable & Microstrip
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Reflection Factor
If Z L >> Zo; Г = +1
If Z L << Zo; Г = -1
Zo
Z L
= ZL - Zo
ZL + Zo
Г
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S Parameters for Reflection
S11=b1
a1
S21=b2
a1
S22=b2
a2
S12=b1
a2
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Reflection Analysis
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Reflection Calculation
Load reflection factor
= 1 G - 100
1 G + 100= 1
Source reflection factor
= 20-100
20+100
= -0.8
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Calculated vs. Measured Reflection
• Tr/7
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Critical Length of a Transmission Line
What is the critical length of
a transmission line that must be terminated?
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Rise Time vs. Propagation Delay
0
0.5
1
1.5
2
2.5
3
3.5
0 0.5 1 1.5 2
Rise Time (100%)
Vo
lta
ge
(V
)
Propagation time is 50% of Tr
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Trace & Cable Terminations
• Source Termination
Rt + Rs = Zo
ReceiverRt
Zo = 50 Driver
Rs
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Trace & Cable Terminations
• AC termination
AC Termination
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Place of Terminations
Clock SourceClock Bus
Termination Resistor
Device 1 Device 2
Stub
Daisy Chain Routing with Stubs
Device PinBGA Ball
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Short stubs create signal integrity problems
Stub Length=0.5”
Stub Length=0.25”
Reference: Altera
application note 224
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Daisy Chain Routing without Stubs
Clock SourceClock Bus
Termination Resistor
Device PinBGA Ball
Device 1 Device 2
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Star Routing
Clock Source
Termination Resistor
Device 1
Device 2
Clock Bus
Device PinBGA Ball
Device 3
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Parallel Fly-By Termination
Receiver Device
(BGA Package)
R2
Zo = 50
Vcc
R1
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Differential Pair (LVDS) Fly-By Termination
Receiver Device
(BGA Package)
100
Zo=50
Zo=50
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Crosstalk Analysis
• Cross Talk occurs on PCB and twisted wire cable
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Magnetic & Electric Fields of Parallel Transmission Line
VictimTrace
Aggressor Trace
Ground Plane
Magnetic Field
Aggressor Trace
VictimTrace
Electric Field
Ground Plane
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Cross Talk Analysis
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Cross Talk Analysis
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FEXT Measurement
Inductive or
Capacitive Coupling
?
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Reduced FEXT Measurement
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NEXT Measurement
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Reduced NEXT Measurement
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Separated Data Wire
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Cross Talk Reduced by Termination
X-
talk
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Final Termination Solution
+ +
U3 BUF634
+ +
U1 BUF634
+ + U2 BUF634+ +
U4 BUF634
++
U5 BUF634
++U6 BUF634
R1 100
R2 100
R3 100
R4 100
R5 100
R6 100
C1 220p
C2 220p
C3 220p
ADS 8326
CLOCK
CS
CS
MSP 430
DATA
3 feet (1 meter) twisted pair cable
3 feet (1 meter) twisted pair cable
3 feet (1 meter) twisted pair cable
CLOCK
DATA
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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
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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