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Link A/D converters and Microcontrollers using Long Transmission Lines John WU Precision Analog - Data Converter Applications Engineer wu_john@ti.com
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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, t rise CLK = 10 ns t 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 T displayed 2 = T probe 2 + T scope 2 + T signal 2 T signal 2 = T displayed 2 -T probe 2 -T scope 2 T signal = 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 of a microstrip that must be considered as transmission line?
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Rise Time vs. Propagation Delay
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Propagation Delay vs. Dielectric Constant
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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
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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 Г
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S Parameters for Reflection
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Reflection Analysis
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Reflection Calculation
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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
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Trace & Cable Terminations Source Termination Rt + Rs = Zo Receiver Rt Zo = 50 Driver Rs
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Trace & Cable Terminations AC termination AC Termination
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Place of Terminations Clock Source Clock Bus Termination Resistor Device 1Device 2 Stub Daisy Chain Routing with Stubs Device Pin BGA 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 Source Clock Bus Terminatio n Resistor Device Pin BGA Ball Device 1Device 2
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Star Routing Clock Source Terminatio n Resistor Device 1 Device 2 Clock Bus Device Pin BGA 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
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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 Victim Trace Aggressor Trace Ground Plane Magnetic Field Aggressor Trace Victim Trace Electric Field Ground Plane
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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
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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
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