__________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.

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

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:

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

Optical gain sensitivity (in dBm) is:

In a receiver with G = 6k

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:

150kHz cutoff)

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.

10log

S

2G

x

x

GAIN

ρ

r

r

e

e

+

−













1

1

1000

10

10

2

20

2

6

2

2

2

log

log(

)

log( )

2V

/ R

V/ R

IN

IN

()

==

= + dB

Low-Power, 622Mbps Limiting Amplifiers

with Chatter-Free Power Detect for LANs

_______________________________________________________________________________________

9

Table 1. Optical-Power Relations*

TIME

P0

P1

Figure 4. Optical-Power Relations

SYMBOL

RELATION

Average

Power

PARAMETER

Extinction

Ratio

Optical Power

of a “1”

P1

Optical Power

of a “0”

P0

Signal

Amplitude

P

= P0 + P1

AVE

r =

1 / P0

e

P

PP

r

r

AVE

e

e

12

1

=

+

PP

r

AVE

e

02

1

=+

()

/

PP

P

P

r

r

IN

AVE

e

e

=−

=

−

()

+

10

2

1

1

OPTICAL

POWER

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