The MAX3760 minimizes pulse-width distortion for data

sequences exhibiting a 50% duty cycle. A duty cycle

significantly different from 50% will cause the MAX3760

to generate pulse-width distortion.

DC cancellation current is drawn from the input and

creates noise. This is not a problem for low-level signals

with little or no DC component. Preamplifier noise

increases for signals with significant DC component

(see

Typical Operating Characteristics).

The MAX3760 operates with the photodetector cathode

Circuit. Connecting the photodetector anode to GND

and the cathode to IN defeats the DC cancellation cir-

cuit and causes pulse-width distortion.

Input Reference

INREF is the reference point for IN. Connect it as close

as possible to the photodetector diode’s AC ground. The

photodetector’s AC ground is usually the ground of the

photodetector’s filter capacitor. The total length from

INREF, through the filter capacitor and the diode, and

back to the input should be no more than 2cm.

Applications Information

Optical Power Definitions

Many of the MAX3760’s specifications relate to the

input signal amplitude. When working with fiber optic

receivers, the optical input is usually expressed in

terms of average optical power and extinction ratio.

Use the relations given in Table 1 to convert optical

power to input signal when designing with the

MAX3760.

Calculating Sensitivity,

Overload, and Linear Range

Sensitivity Calculation

generally dominates the receiver sensitivity. In a system

where the bit error rate (BER) is 1E - 10, the signal-to-

noise ratio must always exceed 12.7. The sensitivity,

expressed in average power, can be estimated as:

Where

ρ is the photodiode responsivity in A/W.

Input Overload

The overload is the largest input that the MAX3760

accepts while meeting specifications. It is calculated

as:

Linear Range

The MAX3760 has high gain, which limits the output

when the input signal exceeds 20µAp-p. It operates

with 90% linearity for inputs not exceeding the follow-

ing:

10log

20 A r

1

r

- 1

1000 dBm

e

e

µ

ρ

+

(

)

(

)

















Overload = 10log

1mA

2

ρ

1000









dBm

Sensitivity = 10log

12.7i r + 1

2 r - 1

n e

e

(

)

(

)













ρ

1000 dBm

622Mbps, Low-Noise Transimpedance

Preamplifier for LAN and WAN Optical Receivers

6

_______________________________________________________________________________________

P1

OPTICAL

POWER

P0

TIME

Figure 2. Optical Power Definitions

SYMBOL

RELATION

Average

Power

Extinction

Ratio

PARAMETER

Optical Power

of a “1”

P1

Optical Power

of a “0”

P0

Signal

Amplitude

Table 1. Optical Power Relations

Note:

Assuming a 50% average input duty cycle.

P

= P0 + P1

AVE

(

r =

1 / P0

e

P

P1

2P

r

(r

1)

AVE

e

e

=

+

P

P

r

AVE

e

0

2

1

/

=

+

(

)

P

P1 - P0

2P

r

- 1

(r

1)

IN

AVE

e

e

=

=

(

)

+