+3.0V to +5.5V, 125Mbps to 266Mbps

Limiting Amplifiers with Loss-of-Signal Detector

_______________________________________________________________________________________

7

sensitivity. The offset-correction circuitry is less sensi-

tive to variations of input duty cycle (for example, the

40% to 60% duty cycle encountered in 4B/5B coding)

Loss-of-Signal Comparator

The LOS comparator indicates when the input signal

power is below the programmed LOS threshold. To

ensure supply and temperature independence, VTH is

generated by a 1.2V bandgap reference. The op amp’s

external gain-setting resistors (R1 and R2) can be

chatter-free operation, the LOS comparator is designed

with approximately 5dB of hysteresis.

Squelch

The squelch function disables the data outputs by forc-

ing OUT- low and OUT+ high during a LOS condition.

This function ensures that when there is a loss of sig-

nal, the limiting amplifier (and all downstream devices)

does not respond to input noise or corrupt data.

Connect SQUELCH to GND or leave it unconnected to

data squelching.

Applications Information

Program the LOS Threshold

Figure 2 provides information for selecting the LOS

responsivities of the photodiode and preamplifier are

known, then the value of R2 can be selected from

Figure 2 to provide LOS assert at the desired input

power.

Select Capacitors

A typical MAX3964A/MAX3965/MAX3968 implementa-

and two input coupling capacitors). For all applications

up to 266Mbps, Maxim recommends the following:

Wire Bonding

For high-current density and reliable operation, the

MAX3964A series uses gold metalization. Diepad size

is 4mils square with a 6mil pitch. Die thickness is

15mils.

0

20

80

60

40

120

100

-40

-36

-34

-38

-32

-30

-28

-26

OPTICAL INPUT POWER AT LOS ASSERT (dBm)

200kV/W

30kV/W

10kV/W

15kV/W

20kV/W

100kV/W

Figure 2. LOS Assert Programming Resistor vs. LOS Assert

Power (for Various PIN-TIA Gains )

PART

DATA RATE (Mbps)

LOS OUTPUTS

MAX3964A*

125 to 155

PECL

MAX3965

125 to 155

TTL

MAX3968

125 to 266

PECL

Selector Guide

*The MAX3964A is functionally equivalent to MAX3964, but offers

slightly improved ESD tolerance. The MAX3969 is a recommend-

ed upgrade for the MAX3964, MAX3964A, MAX3965, and

MAX3968.