Link A/D converters and Microcontrollers using Long Transmission Lines John WU Precision Analog - Data Converter Applications Engineer

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, t rise CLK = 10 ns t rise 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 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( )=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

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

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

Trace & Cable Terminations Source Termination Rt + Rs = Zo Receiver Rt Zo = 50  Driver Rs

Trace & Cable Terminations AC termination AC Termination

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

Star Routing Clock Source Terminatio n Resistor Device 1 Device 2 Clock Bus Device Pin BGA Ball Device 3

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

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

Crosstalk Analysis Cross Talk occurs on PCB and twisted wire cable

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

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: 1.“Managing Signal Quality” Mentor Graphics/Xilinx, “High-Speed Board Layout Guideline” Altera application note 224, Sept. 2003