Link A/D converters and Microcontrollers using

Long Transmission Lines

John WUPrecision Analog - Data Converter Applications Engineer

wu_john@ti.com

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 + Tscope

2 + Tsignal2

• Tsignal2 = Tdisplayed

2 -Tprobe2 -Tscope

2

• 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

0

0.5

1

1.5

2

2.5

3

3.5

0 0.5 1 1.5 2

Rise Time (100%)

Vo

lta

ge

(V

)

Propagation time is 15% of Tr

Propagation Delay vs. Dielectric Constant

050

100150200250

0 1 2 3 4 5 6 7 8 9

Dielectric constant

Tp

d (

ps/

inch

)

Critical Microstrip Length

• Tr x 15% = 1.6ns x 15% = 226 ps

• = 1.5 inch

Transmission Line Model

Zo = V (Z)

I (Z) = sqrt (

L

C )

Characteristic Impedance of Twisted Pair Cable & Microstrip

Reflection Factor

If Z L >> Zo; Г = +1

If Z L << Zo; Г = -1

Zo

Z L

= ZL - Zo

ZL + Zo

Г

S Parameters for Reflection

S11=b1

a1

S21=b2

a1

S22=b2

a2

S12=b1

a2

Reflection Analysis

Reflection Calculation

Load reflection factor

= 1 G - 100

1 G + 100= 1

Source reflection factor

= 20-100

20+100

= -0.8

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

0

0.5

1

1.5

2

2.5

3

3.5

0 0.5 1 1.5 2

Rise Time (100%)

Vo

lta

ge

(V

)

Propagation time is 50% of Tr

Trace & Cable Terminations

• Source Termination

Rt + Rs = Zo

ReceiverRt

Zo = 50 Driver

Rs

Trace & Cable Terminations

• AC termination

AC Termination

Place of Terminations

Clock SourceClock Bus

Termination Resistor

Device 1 Device 2

Stub

Daisy Chain Routing with Stubs

Device PinBGA 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 SourceClock Bus

Termination Resistor

Device PinBGA Ball

Device 1 Device 2

Star Routing

Clock Source

Termination Resistor

Device 1

Device 2

Clock Bus

Device PinBGA 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

Zo=50

Crosstalk Analysis

• Cross Talk occurs on PCB and twisted wire cable

Magnetic & Electric Fields of Parallel Transmission Line

VictimTrace

Aggressor Trace

Ground Plane

Magnetic Field

Aggressor Trace

VictimTrace

Electric Field

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

+ +

U3 BUF634

+ +

U1 BUF634

+ + U2 BUF634+ +

U4 BUF634

++

U5 BUF634

++U6 BUF634

R1 100

R2 100

R3 100

R4 100

R5 100

R6 100

C1 220p

C2 220p

C3 220p

ADS 8326

CLOCK

CS

CS

MSP 430

DATA

3 feet (1 meter) twisted pair cable

3 feet (1 meter) twisted pair cable

3 feet (1 meter) twisted pair cable

CLOCK

DATA

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, 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