MAX3762(2004) 查看數據表（PDF） - Maxim Integrated

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MAX3762
(Rev.:2004)

Low-Power, 622Mbps Limiting Amplifiers with Chatter-Free Power Detect for LANs

Maxim Integrated 

Low-Power, 622Mbps Limiting Amplifiers

with Chatter-Free Power Detect for LANs

__________Applications Information

Converting Average Optical Power to

Signal Amplitude

Many of the MAX3761/MAX3762’s specifications relate

to input-signal amplitude. When working with fiber optic

receivers, the input is usually expressed in terms of

average optical power and extinction ratio. The rela-

tions given in Table 1 are helpful for converting optical

power to input signal when designing with the

MAX3761/MAX3762.

Table 1. Optical-Power Relations*

PARAMETER SYMBOL

RELATION

Average

Power

PAVE

PAVE =(P0 + P1) / 2

Extinction

Ratio

re

Optical Power

of a “1”

P1

Optical Power

of a “0”

P0

Signal

Amplitude

PIN

re = P1 / P0

P1 =

2PAVE

re

re + 1

( ) P0 = 2PAVE / re +1

( ) PIN = P1− P0 = 2PAVE

re − 1

re + 1

* Assuming a 50% average input data duty cycle (true for SONET/ATM data).

In an optical receiver, the input voltage to the limiting

amplifier can be found by multiplying the relationship in

Table 1 with the photodiode responsivity (p) and tran-

simpedance amplifier gain (G).

Optical Hysteresis

Power and hysteresis are often expressed in decibels.

By definition, decibels are always 10log (power). At the

inputs to the MAX3761/MAX3762 limiting amplifier, the

power is VIN2/R. If a receiver’s optical input power (x)

increases by a factor of two, and the preamplifier is lin-

ear, then the voltage input to the MAX3761/MAX3762

also increases by a factor of two.

The optical power change is 10log(2x / x) = 10log(2) =

+3dB

At the MAX3761/MAX3762, the voltage change is:

( ) 10log 2VIN 2 / R = 10log(22) = 20log(2) = + 6dB

VIN2/ R

OPTICAL

POWER P1

PAVE

P0

TIME

Figure 4. Optical-Power Relations

In an optical receiver the dB change at the MAX3761/

MAX3762 will always equal 2x the optical dB change.

The MAX3761/MAX3762’s typical voltage hysteresis is

3.6dB. This provides an optical hysteresis of 1.8dB.

Input Sensitivity

The receiver’s gain sensitivity defines the smallest signal

input that results in fully limited PECL-compatible data

outputs. Smaller signals result in nonlimited outputs. The

MAX3761/MAX3762’s input sensitivity (SGAIN) is 4mVp-p:

SGAIN = 4mV

Optical gain sensitivity (in dBm) is:

10log







SGAIN

2Gρ

x

re

re

+1

−1

x



1000



In a receiver with G = 6kΩ, re = 10, and ρ = 0.8A/W,

gain sensitivity is 510nW, or -32.9dBm.

622Mbps ATM Component Selection

As an example, a preamplifier with a 150kHz low-

frequency cutoff and a 950mVp-p maximum output has

the best performance with the following selections:

CIN = 5.6nF, so that fC = 15kHz (one decade below the

150kHz cutoff)

CAZ = 150pF, so that fOC < 1.5kHz (one decade

below fC)

CFILTER = 100pF, so that the integration frequency

equals 3MHz.

These selections should provide data-dependent jitter

less than 110ps p-p when the input consists of PRBS

data with no more than 72 consecutive bits.

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