ADN2807 datasheet(13/21 Pages) AD

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Data Sheet

ADN2807

Rev. B | Page 13 of 21

FUNCTIONAL DESCRIPTION

MULTIRATE CLOCK AND DATA RECOVERY

The ADN2807 recovers clock and data from serial bit streams at

OC-3, OC-12 data rates as well as the 15/14 FEC rates. The

output of the 2.5 GHz VCO is divided down in order to support

the lower data rates. The data rate is selected by the SEL[2..0]

inputs (Table 5).

Table 5. Data Rate Selection

SEL[1..0]

Rate

Frequency (MHz)

OC-12

622.08

OC-3

155.52

OC-12 FEC

666.51

OC-3 FEC

166.63

LIMITING AMPLIFIER

The limiting amplifier has differential inputs (PIN/NIN) that

are internally terminated with 50 Ω to an on-chip voltage

reference (VREF = 0.6 V typically). These inputs are normally

ac-coupled, although dc-coupling is possible as long as the

input common-mode voltage remains above 0.4 V (Figure 24 to

Figure 26 in the Applications Information section). Input offset

is factory trimmed to achieve better than 4 mV typical

sensitivity with minimal drift. The limiting amplifier can be

driven differentially or single-ended.

SLICE ADJUST

The quantizer slicing level can be offset by ±100 mV to mitigate

the effect of ASE (amplified spontaneous emission) noise by

applying a differential voltage input of ±0.8 V to SLICEP/N

inputs. If no adjustment of the slice level is needed, SLICEP/N

must be tied to VCC.

LOSS-OF-SIGNAL (LOS) DETECTOR

The receiver front end level signal detect circuit indicates when

the input signal level has fallen below a user adjustable threshold.

The threshold is set with a single external resistor from THRADJ

(Pin 1) to GND. The LOS comparator trip point versus the

resistor value is illustrated in Figure 4 (this is only valid for

SLICEP = SLICEN = VCC). If the input level to the ADN2807

drops below the programmed LOS threshold, SDOUT (Pin 45)

will indicate the loss-of-signal condition with a Logic 1. The

LOS response time is ~300 ns by design but will be dominated

by the RC time constant in ac-coupled applications. If the LOS

detector is used, the quantizer slice adjust pins must both be tied to

VCC. This is to avoid interaction with the LOS threshold level.

Note that it is not expected to use both LOS and slice adjust at

the same time. Systems with optical amplifiers need the slice

adjust to evade ASE. However, a loss-of-signal in an optical link

that uses optical amplifiers causes the optical amplifier output

to be full-scale noise. Under this condition, the LOS would not

detect the failure. In this case, the loss-of-lock signal indicates

the failure because the CDR circuitry is unable to lock onto a

signal that is full-scale noise.

REFERENCE CLOCK

There are three options for providing the reference frequency to

the ADN2807: differential clock, single-ended clock, or crystal

oscillator. See Figure 15 to Figure 17 for example

configurations.

100k

Ω 100kΩ

BUFFER

ADN2807

VCC/2

REFCLKN

REFCLKP

CRYSTAL

OSCILLATOR

XO1

XO2

VCC

REFSEL

Figure 15. Differential REFCLK Configuration

OUT

100k

Ω 100kΩ

BUFFER

ADN2807

VCC/2

REFCLKN

REFCLKP

CRYSTAL

OSCILLATOR

XO1

XO2

VCC

REFSEL

CLK

OSC

VCC

Figure 16. Single-Ended REFCLK Configuration

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