155Mbps Low-Noise Transimpedance

Amplifier

6

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Detailed Description

The MAX3657 transimpedance amplifier is designed for

155Mbps fiber-optic applications. The functional dia-

gram of the MAX3657 comprises a transimpedance

amplifier, a voltage amplifier, a DC-cancellation circuit,

and a CML output buffer.

Transimpedance Amplifier

The signal current at the input flows into the summing

node of a high-gain amplifier. Shunt feedback through

diodes clamp the output signal for large input currents

(Figure 1).

Voltage Amplifier

The voltage amplifier provides additional gain and con-

verts the transimpedance amplifier single-ended output

signal into a differential signal.

Output Buffer

The output buffer provides a reverse-terminated volt-

age output and is designed to drive a 200Ω differential

load between OUT+ and OUT-. For optimum supply-

noise rejection, the MAX3657 should be terminated with

a differential load. The MAX3657 single-ended outputs

do not drive a DC-coupled grounded load. The outputs

ended output is required, both the used and the unused

outputs should be terminated in a similar manner.

DC-Cancellation Circuit

The DC-cancellation circuit uses low-frequency feed-

back to remove the DC component of the input signal

(Figure 2). This feature centers the input signal within

the transimpedance amplifier’s linear range, thereby

reducing pulse-width distortion.

The DC-cancellation circuit is internally compensated

and does not require external capacitors. This circuit

minimizes pulse-width distortion for data sequences

that exhibit a 50% mark density. A mark density signifi-

cantly different from 50% causes the MAX3657 to gen-

erate pulse-width distortion. Grounding the FILT pin

disables the DC-cancellation circuit. For normal opera-

tion, the DC-cancellation circuit must be enabled.

The DC-cancellation current is drawn from the input and

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

component, the added noise is insignificant. However,

amplifier noise increases for signals with significant DC

component (see the Typical Operating Characteristics).

VOLTAGE

AMPLIFIER

OUTPUT

BUFFER

OUT+

OUT-

MON

LOWPASS

FILTER

FILT

IN

DC-CANCELLATION

CIRCUIT

TRANSIMPEDANCE

AMPLIFIER

ENABLE

+1.0V

MAX3657

Functional Diagram