200Mbps SFP Limiting Amplifier

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5

Pin Description

PIN

NAME

FUNCTION

17, 18

Supply Voltage

19

SQUELCH

Squelch Input. The squelch function disables the data outputs by forcing OUT- low and OUT+ high

when the signal is below the power-detect threshold. Connect to GND or leave unconnected to

20

Output of Internal Op Amp that Sets Power-Detect Threshold Voltage (Figure 1). Connect a resistor

Ω), to program the desired threshold voltage.

EP

Exposed

Pad

Ground. The exposed pad must be soldered to the circuit board ground for proper thermal and

electrical performance.

OFFSET

CORRECTION

1

1

O

1.2V

REFERENCE

IN+

FILTER

IN-

FWD

FWD

FWD

CZN

CZP

R1

100k

Ω

R2

INV

GND

OUT-

OUT+

SQUELCH

RSSI

LOS

LOS

SD

MAX3969

PECL

TTL

PECL

Figure 1. Functional Diagram

Detailed Description

The MAX3969 contains a series of limiting amplifiers

and power detectors, offset correction, data-squelch

circuitry, TTL buffers for LOS outputs, and PECL output

buffers for signal detect (SD) and data outputs. See

Figure 1 for the functional diagram.

Gain Stages and Offset Correction

A cascade of limiting amplifiers provides approximately

65dB of combined small-signal gain. The large gain

makes the amplifier susceptible to small DC offsets in

the signal path. To correct DC offsets, the amplifier has

an internal feedback loop that acts as a DC autozero

circuit. By correcting the DC offsets, the limiting amplifi-

er sensitivity and power-detector accuracy are

improved.

The offset correction is optimized for data streams with

a 50% duty cycle. A different average duty cycle results

in increased pulse-width distortion and loss of sensitivi-

ty. The offset-correction circuitry is less sensitive to vari-

ations of input duty cycle (for example, the 40% to 60%

duty cycle encountered in 4B/5B coding) when the

The data inputs must be AC-coupled for the offset cor-

rection loop to function properly. Differential input

impedance is >5k

Ω.