Applications Information

Optical Power Relations

Many of the MAX3864 specifications relate to the input

signal amplitude. When working with fiber optic receivers,

the input is usually expressed in terms of average optical

are helpful for converting optical power to input signal

when designing with the MAX3864.

Optical power relations are shown in Table 1; the defini-

tions are true if the average duty cycle and mark densi-

ty of the input data are 50%.

Optical Sensitivity Calculations

generally determines the receiver sensitivity. To obtain

a system bit-error rate (BER) of 1E-10, the minimum sig-

nal-to-noise ratio (SNR) is 12.7. The input sensitivity,

expressed in average power, can be estimated as:

where

ρ is the photodiode responsivity, including fiber-

ρ = 1, then sensitivity is -24dBm.

Input Optical Overload

The overload is the largest input that the MAX3864

accepts while meeting deterministic jitter specifications.

The optical overload can be estimated in terms of aver-

age power with the following equation (assumes

∞):

Optical Linear Range

The MAX3864 has high gain, which limits the outputs

when the input signal exceeds 40µAp-p. The MAX3864

operates in a linear range for inputs not exceeding:

Layout Considerations

Use good high-frequency design and layout techniques.

The use of a multilayer circuit board with separate ground

and power planes is recommended. Connect the GND

pins to the ground plane with the shortest possible traces.

Noise performance and bandwidth will be adversely

affected by capacitance at the IN pin. Minimize capaci-

tance on this pin, and select a low-capacitance photodi-

ode. Assembling the MAX3864 in die form using chip and

wire technology provides the best possible performance.

Figure 5 shows the recommended layout for a TO

header.

The SO package version of the MAX3864 is offered as an

easy way to characterize the circuit and to become famil-

iar with the circuit’s operation, but it does not offer opti-

mum performance. When using the SO version of the

MAX3864, the package capacitance adds approximately

0.3pF at the input. The PC board between the MAX3864

input and the photodiode also adds parasitic capaci-

tance. Keep the input line short, and remove power and

ground planes beneath it.

GND

Connect GND as close to the AC ground of the photode-

tector diode as possible. The photodetector AC ground is

usually the ground of the filter capacitor from the photode-

tector cathode. The total loop (from GND, through the

bypass capacitor and the diode, and back to IN) should

be no more than approximately 1/5th of a wavelength.

Overload

10log

2mAp - p

1000

2

dBm

=

×













ρ

Sensitivity =10log

SNR

2

dBm

×+

()

−













Ir

r

Ne

e

1

1

1000

ρ

2.5Gbps, +3V to +5.5V, Wide Dynamic Range

Transimpedance Preamplifier

6

_______________________________________________________________________________________

PO

PI

TIME

POWER

Figure 4. Optical Power Relations

PARAMETER

SYMBOL

RELATION

Average Power

Extinction Ratio

Optical Power of a 1

Optical Power of a 0

Signal Amplitude

Table 1. Optical Power Relations

Note: Assuming a 50% average input duty cycle and mark

density.

Linear Range = 10log

4-

2

dBm

0

1

1000

1

µ+

−

()













Ap p r

r

e

e

ρ