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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 WU Precision Analog - Data Converter Applications Engineer wu_john@ti.com. 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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  1. Link A/D converters and Microcontrollers using Long Transmission Lines John WUPrecision Analog - Data Converter Applications Engineerwu_john@ti.com

  2. Transmission Line Effect Considerations • Definition of the Highest Frequency Signal • Transmission Line Model • Reflection Concept • Termination Topology • Crosstalk Analysis

  3. Application Example

  4. Transmitted Data and Clock

  5. Reflection on MSP430

  6. With Termination on MSP430

  7. Reflection on ADS8326 EVM

  8. With Termination on ADS8326 ?

  9. Definition of the highest frequency signal What is the highest frequency signal in a 2.25MHz sample clock rate ADC ?

  10. Definition of the highest frequency signal The highest frequency signal is determined by the signal rise or fall time

  11. 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

  12. 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 ?

  13. Rise Time Degradation • Tdisplayed2= Tprobe2 + Tscope2 + Tsignal2 • Tsignal2 = Tdisplayed2 -Tprobe2 -Tscope2 • Tsignal = SQRT(22 - 0.72 - 12 )=1.6ns! • 1.6ns) = 220MHz

  14. Critical Microstrip Length What is the critical length of a microstrip that must be considered as transmission line?

  15. Rise Time vs. Propagation Delay

  16. Propagation Delay vs. Dielectric Constant

  17. Critical Microstrip Length • Tr x 15% = 1.6ns x 15% = 226 ps • = 1.5 inch

  18. Transmission Line Model

  19. Characteristic Impedance of Twisted Pair Cable & Microstrip

  20. Г Reflection Factor Zo Z L If Z L >> Zo; Г = +1 If Z L<< Zo; Г = -1

  21. S Parameters for Reflection

  22. Reflection Analysis

  23. Reflection Calculation

  24. Calculated vs. Measured Reflection • Tr/7

  25. Critical Length of a Transmission Line What is the critical length of a transmission line that must be terminated?

  26. Rise Time vs. Propagation Delay

  27. Source Termination Trace & Cable Terminations Driver Receiver Zo = 50 W Rt Rs Rt + Rs = Zo

  28. Trace & Cable Terminations • AC termination AC Termination

  29. Daisy Chain Routing with Stubs Place of Terminations Clock Source Clock Bus Termination Resistor Stub Device 1 Device 2 Device Pin BGA Ball

  30. Short stubs create signal integrity problems Stub Length=0.5” Stub Length=0.25” Reference: Altera application note 224

  31. Daisy Chain Routing without Stubs Device Pin BGA Ball Clock Source Clock Bus Device 1 Device 2 Termination Resistor

  32. Star Routing Clock Bus Device 1 Termination Resistor Device 2 Clock Source Device 3 Device Pin BGA Ball

  33. Parallel Fly-By Termination Vcc R1 Zo = 50 W Receiver Device (BGA Package) R2

  34. Differential Pair (LVDS) Fly-By Termination Receiver Device (BGA Package) Zo=50 W 100 W Zo=50 W

  35. Crosstalk Analysis • Cross Talk occurs on PCB and twisted wire cable

  36. Magnetic & Electric Fields of Parallel Transmission Line Aggressor Trace Victim Trace Aggressor Trace Victim Trace Magnetic Field Electric Field Ground Plane Ground Plane

  37. Cross Talk Analysis

  38. Cross Talk Analysis

  39. FEXT Measurement Inductive or Capacitive Coupling ?

  40. Reduced FEXT Measurement

  41. NEXT Measurement

  42. Reduced NEXT Measurement

  43. Separated Data Wire

  44. Cross Talk Reduced by Termination X-talk

  45. Final Termination Solution

  46. 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

  47. 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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