+3.0V to +5.5V, 125Mbps to 266Mbps

Limiting Amplifiers with Loss-of-Signal Detector

6

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Detailed Description

The MAX3964 contains a series of limiting amplifiers

and power detectors, offset correction, data-squelch

circuitry, and PECL output buffers for data and loss-of-

signal (LOS) outputs. The MAX3965 is functionally the

same, but it provides TTL buffers on the LOS outputs.

The MAX3968 provides PECL LOS outputs with data

outputs suitable for 266Mbps. Figure 1 shows a func-

tional diagram of the MAX3964/MAX3965/MAX3968.

Limiting Amplifiers

A series of four limiting amplifiers provides gain of

approximately 65dB.

Power Detector

Each amplifier stage contains a full-wave logarithmic

detector (FWD), which indicates the RMS input signal

power. The FWD outputs are summed together at the

FILTER pin where the signal is filtered by an external

capacitor (CFILTER) connected between FILTER and

age, which is proportional to the input power in deci-

following equation:

reduced approximately 120mV when LOS+ is asserted.

PECL Outputs

The data outputs (OUT+, OUT-) and the MAX3964/

MAX3968 loss-of-signal outputs (LOS+, LOS-) are sup-

ply-referenced PECL outputs. Standard PECL termina-

for best performance.

TTL Outputs

The MAX3965 LOS outputs (LOS+, LOS-) are imple-

mented with open-collector Schottky-clamped TTL-

internally with 2kΩ resistors and do not require external

pullup resistors.

Input Offset Correction

A low-frequency feedback loop around the limiting

amplifier improves receiver sensitivity and powerdetec-

tor accuracy. The offset-correction loop’s bandwidth is

determined by an external capacitor (CAZ) connected

between the CZP and CZN pins.

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

CZN

CZP

OUT+/OUT-

LOS+/LOS-

RSSI

SQUELCH

LOS+

LOS

COMPARATOR

FILTER

IN+/IN-

R1

R2

VTR

SUB

GND

GNDO

(MAX3965 ONLY)

INV

FWD = FULL-WAVE DETECTOR

LIMITER

OFFSET

CORRECTION

LIMITER

I

I

O

LIMITER

LIMITER

FWD

FWD

FWD

FWD

1.2V

REFERENCE

MAX3964

MAX3965

MAX3968

Figure 1. Functional Diagram