1.25Gbps/2.5Gbps, +3V to +5.5V, Low-Noise

Transimpedance Preamplifiers for LANs

_______________________________________________________________________________________

7

Output Filter

The MAX3266 includes a one-pole lowpass filter that

limits the circuit bandwidth and improves noise perform-

ance.

DC Cancellation Circuit

The DC cancellation circuit uses low-frequency feed-

back to remove the DC component of the input signal

(Figure 4). This feature centers the input signal within

the transimpedance amplifier’s linear range, thereby

reducing pulse-width distortion on large input signals.

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% duty cycle. A duty cycle

significantly different from 50% will cause the MAX3266

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 for signals with significant DC component (see

Typical Operating Characteristics).

Applications Information

Optical Power Relations

Many of the MAX3266 specifications relate to the input

signal amplitude. When working with fiber optic

receivers, the input is usually expressed in terms of aver-

age optical power and extinction ratio. Figure 5 shows

relations that are helpful for converting optical power to

input signal when designing with the MAX3266.

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

MAX3266 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 MAX3266

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

1

2

1000

ρ

Sensitivity

Ir

r

dBm

Ne

e

=

+

()

−

()

















10

14 1

1

21

1000

log

.

ρ

AMPLITUDE

OUTPUT (LARGE SIGNALS)

TIME

OUTPUT (SMALL SIGNALS)

Figure 3. MAX3266 Limited Output

Figure 4. DC Cancellation Effect on Input

AMPLITUDE

INPUT FROM PHOTODIODE

TIME

INPUT (AFTER DC CANCELLATION)