4813transmissionlines

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Link A/D converters andMicrocontrollers using

Long Transmission Lines John WU

Precision Analog - Data Converter Applications [email protected]


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Transmission Line EffectConsiderations

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 signalin a 2.25MHz sample clock rateADC ?


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Definition of the highestfrequency signal

The highest frequency signal is determinedby 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,t rise CLK = 10 nst rise 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

Tdisplayed 2 = T probe 2 + Tscope 2 + Tsignal 2

Tsignal 2 = T displayed 2 -T probe 2 -Tscope 2

Tsignal = SQRT(2 2 - 0.7 2 - 1 2 )=1.6ns!

1.6ns) = 220MHz


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Critical Microstrip Length

What is the critical length ofa microstrip that must be considered as

transmission line?


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Rise Time vs. Propagation Delay

0

0.51

1.52

2.5

3

3.5

0 0.5 1 1.5 2

Rise Time (100%)

V o

l t a g e (

V )

Propagationtime is15% of Tr


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Propagation Delay vs. DielectricConstant

050

100150200250

0 1 2 3 4 5 6 7 8 9

Dielectric constant

T p

d ( p s / i n c

h )


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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)

Ch i i I d f


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Characteristic Impedance ofTwisted Pair Cable & Microstrip


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Reflection Factor

If Z L >> Zo; = +1

If Z L


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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 CalculationLoad reflection factor

=1 G - 100

1 G + 100

= 1

Source reflection factor

= 20-10020+100

= -0.8

C l l t d M d


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Calculated vs. MeasuredReflection

Tr/7


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Critical Length of a Transmission Line

What is the critical length ofa transmission line that must beterminated?


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Rise Time vs. Propagation Delay

00.5

11.5

2

2.53

3.5

0 0.5 1 1.5 2

Rise Time (100%)

V o

l t a g e

( V )

Propagation timeis 50% of Tr


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Trace & Cable Terminations

Source Termination

Rt + Rs = Zo

ReceiverRt

Zo = 50Driver

Rs


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Trace & Cable Terminations

AC termination

ACTermination


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Place of Terminations

Clock SourceClockBus

TerminationResistor

Device 1 Device 2

Stub

Daisy Cha in Rou t ing w i th Stubs

Device PinBGA Ball

Sh t t b t ig l i t g it


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Short stubs create signal integrityproblems

Stub Length=0.5

Stub Length=0.25

Reference:Alteraapplication note 224

Daisy Chain Routing without


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Daisy Chain Routing withoutStubs

Clock SourceClockBus

Termination Resistor

Device PinBGA Ball

Device 1 Device 2


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Star Routing

ClockSource

Termination Resistor

Device 1

Device 2

ClockBus

Device PinBGA Ball

Device 3


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Parallel Fly-By Termination

ReceiverDevice

(BGA Package)

R2

Zo = 50

Vcc

R1

Differential Pair (LVDS) Fly By


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Differential Pair (LVDS) Fly-ByTermination

ReceiverDevice

(BGA Package)

100

Zo=50

Zo=50


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Crosstalk Analysis

Cross Talk occurs on PCB and twisted wire cable

Magnetic & Electric Fields of


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Magnetic & Electric Fields ofParallel Transmission Line

VictimTrace

AggressorTrace

Ground Plane

MagneticField

AggressorTrace

VictimTrace

ElectricField

Ground Plane


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Cross Talk Analysis


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Cross Talk Analysis


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FEXT Measurement

InductiveorCapacitiveCoupling

?


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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

Cross Talk Reduced by


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Cross Talk Reduced byTermination

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



CLOCK

DATA


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Conclusions

The highest frequency signal is determined by switchingtime

Transmission line model must be used whenpropagation delay time is greater than 15% of Tr

Termination technique dramatically reduces reflectionand crosstalk

Q&A


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Acknowledgement

Thanks Phil Lizzi for providing the real lifetransmission 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

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