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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

Link A/D converters and Microcontrollers using Long Transmission Lines

John WUPrecision Analog - Data Converter Applications [email protected]

transmission line effect considerations
Transmission Line Effect Considerations
  • Definition of the Highest Frequency Signal
  • Transmission Line Model
  • Reflection Concept
  • Termination Topology
  • Crosstalk Analysis
definition of the highest frequency signal
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 signal1
Definition of the highest frequency signal

The highest frequency signal is determined by the signal rise or fall time

rise time vs bandwidth
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
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
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
Critical Microstrip Length

What is the critical length of

a microstrip that must be considered as transmission line?

critical microstrip length1
Critical Microstrip Length
  • Tr x 15% = 1.6ns x 15% = 226 ps
  • = 1.5 inch
reflection factor

Г

Reflection Factor

Zo

Z L

If Z L >> Zo; Г = +1

If Z L<< Zo; Г = -1

critical length of a transmission line
Critical Length of a Transmission Line

What is the critical length of

a transmission line that must be terminated?

trace cable terminations
Source TerminationTrace & Cable Terminations

Driver

Receiver

Zo = 50 W

Rt

Rs

Rt + Rs = Zo

trace cable terminations1
Trace & Cable Terminations
  • AC termination

AC Termination

place of terminations
Daisy Chain Routing with StubsPlace of Terminations

Clock Source

Clock Bus

Termination Resistor

Stub

Device 1

Device 2

Device Pin

BGA Ball

short stubs create signal integrity problems
Short stubs create signal integrity problems

Stub Length=0.5”

Stub Length=0.25”

Reference:

Altera

application note 224

daisy chain routing without stubs
Daisy Chain Routing without Stubs

Device Pin

BGA Ball

Clock Source

Clock Bus

Device 1

Device 2

Termination Resistor

star routing
Star Routing

Clock Bus

Device 1

Termination Resistor

Device 2

Clock Source

Device 3

Device Pin

BGA Ball

parallel fly by termination
Parallel Fly-By Termination

Vcc

R1

Zo = 50 W

Receiver Device

(BGA Package)

R2

differential pair lvds fly by termination
Differential Pair (LVDS) Fly-By Termination

Receiver Device

(BGA Package)

Zo=50 W

100 W

Zo=50 W

crosstalk analysis
Crosstalk Analysis
  • Cross Talk occurs on PCB and twisted wire cable
magnetic electric fields of parallel transmission line
Magnetic & Electric Fields of Parallel Transmission Line

Aggressor

Trace

Victim

Trace

Aggressor

Trace

Victim

Trace

Magnetic Field

Electric Field

Ground Plane

Ground Plane

fext measurement
FEXT Measurement

Inductive or Capacitive Coupling

?

conclusions
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
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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