MAX3760(1998) データシートの表示（PDF） - Maxim Integrated

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MAX3760
(Rev.:1998)

622Mbps, Low-Noise Transimpedance Preamplifier for LAN and WAN Optical Receivers

Maxim Integrated 

622Mbps, Low-Noise Transimpedance

Preamplifier for LAN and WAN Optical Receivers

IN

INREF

D1

RF

VCC

TRANSIMPEDANCE

AMPLIFIER

Q1

Q4

GND

DC

CANCELLATION

AMPLIFIER

VCC

1k

MAX3760

VCC

Q2

R1

PARAPHASE

AMP

R3

VCC

Q3

R2

R4

(FILTER)

OUT-

OUT+

COMP

Figure 1. Functional Diagram

Detailed Description

The MAX3760 is a transimpedance amplifier designed

for 622Mbps fiber optic applications. As shown in the

Functional Diagram (Figure 1), it comprises a transim-

pedance amplifier, a paraphase amplifier with emitter-

follower outputs, and a DC cancellation circuit.

Transimpedance Amplifier

The signal current at the input flows into a high-gain

amplifier’s summing node. Shunt feedback through RF

converts this current to a voltage with 6.5kΩ gain.

Diode D1 clamps the output voltage for large input cur-

rents. INREF is a direct connection to the input transis-

tor’s emitter, and must be connected directly to the

photodetector AC ground return for best performance.

Paraphase Amplifier

The paraphase amplifier converts single-ended signals to

differential signals and introduces a voltage gain of 2.

This signal drives a pair of internally biased emitter follow-

ers, Q2 and Q3, which form the output stage. Resistors

R1 and R2 provide back-termination at the output, deliv-

ering a 100Ω differential output impedance. The output

emitter followers are designed to drive a 100Ω differential

load between OUT+ and OUT-. The MAX3760 can also

GND

( ) INDICATE PINS AVAILABLE ON DIE ONLY.

be terminated with higher output impedances for

increased gain and output voltage swing. The MAX3760

will not drive a 50Ω load to ground. For best noise rejec-

tion, terminate the MAX3760 with differential loads.

DC Cancellation Circuit

The DC cancellation circuit removes the input signal’s

DC component by employing low-frequency feedback.

This feature centers the input signal within the transim-

pedance amplifier’s dynamic range, thereby reducing

pulse-width distortion on large input signals.

The paraphase amplifier’s output is sensed through

resistors R3 and R4, then filtered, amplified, and fed

back to the base of transistor Q4. The transistor draws

the input signal’s DC component away from the trans-

impedance amplifier’s summing node.

The MAX3760 DC cancellation loop is internally com-

pensated and does not require external capacitors in

most 622Mbps applications. Add external capacitance

at the COMP pin to reduce the DC cancellation circuit’s

frequency response and improve data-dependent jitter.

Connecting the COMP pin directly to GND disables the

circuit. The DC cancellation circuit can sink up to 1mA

at the input.

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