DC Cancellation Circuit

The direct current (DC) cancellation circuit uses low-

frequency feedback to remove the DC component of

the input signal. This feature centers the input signal

within the transimpedance amplifier’s linear range,

thereby reducing pulse-width distortion caused by

large input signals (Figure 3).

The DC cancellation circuit is internally compensated

and therefore does not require external capacitors. This

circuit minimizes pulse-width distortion for data

sequences that exhibit a 50% mark density. A mark

density significantly different from 50% will cause the

MAX3271 to generate pulse-width distortion.

DC cancellation current is drawn from the input and

creates noise. For low-level signals with little or no DC

component, this is not a problem. Amplifier noise will

increase slightly for signals with significant DC compo-

nent.

Applications Information

Optical Power Relations

Many of the MAX3271 specifications relate to the input

signal amplitude. When working with fiber optic

receivers, the input is sometimes expressed in terms of

average optical power and extinction ratio. Figure 4

shows relations that are helpful for converting optical

power to input signal when designing with the MAX3271.

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

tions are true if the average duty cycle of the input data

is 50%.

Optical Sensitivity Calculation

MAX3271 generally determines the receiver sensitivity.

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

SNR ratio must always exceed 14.1. The input sensitivi-

ty, expressed in average power, can be estimated as:

where

ρ is the photodiode responsivity in A/W.

Input Optical Overload

The overload is the largest input that the MAX3271

accepts while meeting specifications. The optical over-

load can be estimated in terms of average power with

the following equation:

Overload

mA

dBm

log

=









10

2

2

1000

ρ

Sensitivity

Ir

r

dBm

Ne

e

=

+

()

()













−

10

14 1

1

21

1000

log

.

ρ

+3.3V, 2.5Gbps Low-Power

Transimpedance Amplifier

6

_______________________________________________________________________________________

AMPLITUDE

OUTPUT (LARGE SIGNALS)

TIME

OUTPUT (SMALL SIGNALS)

Figure 2. MAX3271 Limited Output

AMPLITUDE

INPUT FROM PHOTODIODE

TIME

INPUT AFTER DC CANCELLATION

Figure 3. DC Cancellation Effect on Input

P0

P1

TIME

Figure 4. Optical Power Relations