Link a d converters and microcontrollers using long transmission lines
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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

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Link A/D converters and Microcontrollers using Long Transmission Lines

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Link A/D converters and Microcontrollers using Long Transmission Lines

John WUPrecision Analog - Data Converter Applications [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 Delay vs. Dielectric Constant


Critical Microstrip Length

  • Tr x 15% = 1.6ns x 15% = 226 ps

  • = 1.5 inch


Transmission Line Model


Characteristic Impedance of Twisted Pair Cable & Microstrip


Г

Reflection Factor

Zo

Z L

If Z L >> Zo; Г = +1

If Z L<< Zo; Г = -1


S Parameters for Reflection


Reflection Analysis


Reflection Calculation


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?


Rise Time vs. Propagation Delay


Source Termination

Trace & Cable Terminations

Driver

Receiver

Zo = 50 W

Rt

Rs

Rt + Rs = Zo


Trace & Cable Terminations

  • AC termination

AC Termination


Daisy Chain Routing with Stubs

Place of Terminations

Clock Source

Clock Bus

Termination Resistor

Stub

Device 1

Device 2

Device Pin

BGA Ball


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

Device 2

Termination Resistor


Star Routing

Clock Bus

Device 1

Termination 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

Electric Field

Ground Plane

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

X-talk


Final Termination Solution


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:

  • “Managing Signal Quality”Mentor Graphics/Xilinx, 2005http://www.xilinx.com/publications/xcellonline/xcell_53/xc_pdf/xc_mentor53.pdf

  • “High-Speed Board Layout Guideline” Altera application note 224, Sept. 2003


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