MAX3675 Просмотр технического описания (PDF) - Maxim Integrated

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MAX3675

622Mbps, Low-Power, 3.3V Clock-Recovery and Data-Retiming IC with Limiting Amplifier

Maxim Integrated 

622Mbps, Low-Power, 3.3V Clock-Recovery

and Data-Retiming IC with Limiting Amplifier

Optical receivers, incorporating transimpedance

preamplifiers and limiting postamplifiers, can signifi-

cantly clean up the effects of dispersion and attenua-

tion. In addition, these amplifiers can provide fast

transitions with minimal aberrations to the subsequent

CDR blocks. However, these stages also add distor-

tions to the midpoint crossing, contributing to timing jit-

ter. Timing jitter is one of the most critical technical

issues to consider when developing optical receivers

and CDR circuits.

A better understanding of the different sources of jitter

helps in the design and application of optical receiver

modules and integrated CDR solutions. SDH/SONET

specifications are well defined regarding the amount of

jitter tolerance allowed at the inputs of optical receivers,

as well as jitter peaking requirements, but they do little

to define the different sources of jitter. The jitter that

must be tolerated at an optical receiver input involves

three significant sources, all of which are present in

varying degrees in typical receiver systems:

1) Random jitter (RJ)

2) Pattern-dependent jitter (PDJ)

3) Pulse-width distortion (PWD)

Random Jitter (RJ)

RJ is caused by random noise present during edge

transitions (Figure 8). This random noise results in ran-

dom midpoint crossings. All electrical systems gener-

ate some random noise; however, the faster the speed

of the transitions, the lower the effect of noise on ran-

dom jitter. The following equation is a simple worst-

case estimation of random jitter:

RJ (rms) = (rms noise) / (slew rate)

Pattern-Dependent Jitter (PDJ)

PDJ results from wide variations in the number of con-

secutive bits contained in NRZ data streams working

against the bandwidth requirements of the receiver

(Figure 9). The location of the lower -3dB cutoff fre-

quency is important, and must be set to pass the low

frequencies associated with long consecutive bit

streams. AC-coupling is common in optical receiver

design.

When using a limiting preamplifier with a highpass fre-

quency response, select the input AC-coupling capaci-

tor, CIN, to provide a low-frequency cutoff (fC) one

decade lower than the preamplifier low-frequency cut-

off. As a result, the PDJ is dominated by the low-

frequency cutoff of the preamplifier.

When using a preamplifier without a highpass response

with the MAX3675, the following equation provides a

good starting point for choosing CIN:

( ) ( )( ) CIN ≥

-tL

 PDJ BW 

1.25kΩ In 1−





0.5 

where tL = duration of the longest run of consecutive

bits of the same value (seconds); PDJ = maximum

MIDPOINT

ACTUAL

MIDPOINT

CROSSING

RANDOM

JITTER

DESIRED

MIDPOINT

CROSSING

0–1

TRANSITION

WITH RANDOM

NOISE

MIDPOINT

LF DROOP

LONG

CONSECUTIVE

BIT STREAM

0-1-0 BIT STREAM

MIDPOINT

LF PDJ

TIME

TIME

Figure 8. Random Jitter on Edge Transition

Figure 9. Pattern-Dependent Jitter Due to Low-Frequency

Cutoff

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