For optimum supply-noise rejection, the MAX3864

should be terminated with a differential load. If a single-

ended output is required, the unused output should be

terminated with 50

a DC-coupled, 50

Ω grounded load.

Output Filter

The MAX3864 includes a one-pole lowpass filter that

limits the circuit bandwidth and improves noise perfor-

mance.

DC Cancellation Circuit

The DC cancellation circuit uses low-frequency feed-

back to remove the DC component of the input signal

(Figure 3). This feature centers the input signal within the

transimpedance amplifier’s linear range, thereby reduc-

ing pulse-width distortion (PWD) on large input signals.

The DC cancellation circuit is internally compensated

and therefore does not require external capacitors. This

circuit minimizes PWD for data sequences that exhibit a

50% duty cycle and mark density. A duty cycle or mark

density significantly different from 50% causes the

MAX3864 to generate PWD.

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).

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

Transimpedance Preamplifier

_______________________________________________________________________________________

5

VOLTAGE

AMPLIFIER

TRANSIMPEDANCE

AMPLIFIER

OUTPUT

BUFFER

OUTPUT

FILTER

OUT+

50

Ω

50

Ω

OUT-

LOWPASS FILTER

750

Ω

FILTER

IN

GND

DC CANCELLATION

CIRCUIT

DISABLE

MAX3864

Figure 1. Functional Diagram

AMPLITUDE

OUTPUT (LARGE SIGNALS)

TIME

OUTPUT (SMALL SIGNALS)

Figure 2. Limited Output

AMPLITUDE

INPUT FROM PHOTODIODE

TIME

INPUT (AFTER DC CANCELLATION)

Figure 3. DC Cancellation Effect on Input