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MAX3793

1Gbps to 4.25Gbps Multirate Transimpedance Amplifier with Photocurrent Monitor

Maxim Integrated 

1Gbps to 4.25Gbps Multirate Transimpedance

Amplifier with Photocurrent Monitor

Applications Information

Optical Power Relations

Many of the MAX3793 specifications relate to the input

signal amplitude. When working with optical receivers,

the input is sometimes expressed in terms of average

optical power and extinction ratio. Figure 5 and Table 1

show relations that are helpful for converting optical

power to input signal when designing with the

MAX3793.

Optical Sensitivity Calculation

The input-referred RMS noise current (IN) of the

MAX3793 generally determines the receiver sensitivity.

PI

PAVG

PO

TIME

Figure 5. Optical Power Relations

To obtain a system bit-error rate (BER) of 1E-12, the

signal-to-noise ratio must always exceed 14:1. The

input sensitivity, expressed in average power, can be

estimated as:

Sensitivity

=

⎛ 14.1 ×

10log

⎜

⎝

2

×

ρ

IN (re + 1)

× (re − 1))

×

⎞

1000⎟

⎠

dBm

where, ρ is the photodiode responsivity in A/W, and IN

is the RMS noise current in amps.

Input Optical Overload

Overload is the largest input that the MAX3793 accepts

while meeting DJ specifications. Optical overload can

be estimated in terms of average power with the follow-

ing equation:

Overload

=

10log

⎛

⎝⎜

IOL

2×

ρ

×

1000⎞⎠⎟

dBm

Optical Linear Range

The MAX3793 has high gain, which limits the output

when the input signal exceeds 50µAP-P. The MAX3793

operates in a linear range for inputs not exceeding:

Linear

Range

=

10log

⎛

⎜

⎝

50μA

2× ρ

×

×

(re

(re

+ 1)

− 1)

×

⎞

1000⎟

⎠

dBm

Table 1. Optical Power Relations*

PARAMETER

Average power

Extinction ratio

SYMBOL

PAVG

re

Optical power of a 1

P1

Optical power of a 0

P0

Optical modulation amplitude

PIN

*Assuming a 50% average mark density.

RELATION

PAVG = (P0 + P1) / 2

re = P1 / P0

P1 = 2PAVG re

re + 1

P0 = 2PAVG / (re + 1)

PIN = P1 − P0 = 2PAVG re −

re +

_______________________________________________________________________________________ 9

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