+3.0V to +5.5V, 1.25Gbps/2.5Gbps

Limiting Amplifiers

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13

Applications Information

Optical Hysteresis

In an optical receiver, the electrical power change at

the limiting amplifier is 2x the optical power change.

As an example, 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 limiting amplifier also

increases by a factor of two.

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

+3dB.

At the limiting amplifier, the electrical power change is:

The MAX3264/MAX3265/MAX3268/MAX3269/MAX3765s’

typical voltage hysteresis is 4.4dB. This provides an opti-

cal hysteresis of 2.2dB.

GBIC Loss of Signal

In a GBIC application, the GBIC’s LOS output must be

shows the recommended circuit to maintain high

impedance. ESD protection diodes on the MAX3264/

MAX3265/MAX3268/MAX3269/MAX3765/MAX3768

PECL Terminations

The standard PECL termination (50

recommended for best performance and output char-

acteristics (see Figure 1). The data outputs operate at

high speed and should always drive transmission lines

with matched, balanced terminations.

Figure 9 shows an alternate method for terminating the

data outputs. The technique provides approximately

8mA DC bias current, with a 45

Ω AC load, for the out-

put termination. This technique is useful for viewing the

output on an oscilloscope or changing the PECL refer-

ence voltage.

Wire Bonding Dice

For high current density and reliable operation, the

MAX3264/MAX3265/MAX3268/MAX3269 use gold met-

alization. Make connections to the dice with gold wire

only, and use ballbonding techniques (wedge bonding

is not recommended). Die-pad size is 4-mils square,

with a 6-mil pitch. Die thickness is 15 mils (0.375mm).

10log

2V

/ R

V/ R

10log(2 )

20log(2)

6dB

IN

2

IN

IN

2

IN

2

()

==

= +

GBIC MODULE

4.7k

Ω

HOST

LOS

GENERAL-

PURPOSE

NPN

MAX3264

MAX3265

MAX3268

MAX3269

MAX3765

MAX3768

Figure 8. Recommended GBIC LOS Circuit

470

Ω

DRIVING 50

Ω TO GROUND

470

Ω

50

Ω

50

Ω

OUT+

OUT-

MAX3268

MAX3269

MAX3768

Figure 9. Alternative PECL Termination