AD9981SLASHPCB datasheet(16/44 Pages) AD

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AD9981

Preliminary Technical Data

Rev. 0 | Page 16 of 44

ACTIVITY

DETECT

AD9981

SYNC

PROCESSOR

PLL CLOCK

GENERATOR

HSYNC

MUX

COAST

MUX

EQ PULSE

FILTER

HSYNC0

HSYNC1

SOGOUT

VSYNCOUT

VSYNC

HSYNC

COAST

DATACK

SOGIN0

SOGIN1

COAST

POLARITY

DETECT

CHANNEL

SELECT

HSYNC

SELECT

FILTERED,

RECONSTRUCTED

HSYNC

ACTIVITY

DETECT

POLARITY

DETECT

ACTIVITY

DETECT

VSYNC0

VSYNC1

POLARITY

DETECT

ACTIVITY

DETECT

POLARITY

DETECT

ACTIVITY

DETECT

SYNC

SLICER

HSYNC/VSYNC

ACTIVITY

DETECT

SET POLARITY

H/V COUNT

R26, R27

Figure 8. Sync Processing Block Diagram

Sync Processing

The inputs of the sync processing section of the AD9981 are

combinations of digital Hsyncs and Vsyncs, analog sync-on-

green, or sync-on-Y signals, and an optional external Coast

signal. From these signals it generates a precise, jitter-free (9%

or less at 95 MHz) clock from its PLL; an odd-/even-field signal;

Hsync and Vsync out signals; a count of Hsyncs per Vsync; and

a programmable SOG output. The main sync processing blocks

are the sync slicer, sync separator, Hsync filter, Hsync

regenerator, Vsync filter, and Coast generator.

The sync slicer extracts the sync signal from the green graphics

or luminance video signal that is connected to the SOGIN input

and outputs a digital composite sync. The sync separator’s task

is to extract Vsync from the composite sync signal, which can

come from either the sync slicer or the Hsync input. The Hsync

filter is used to eliminate any extraneous pulses from the Hsync

or SOGIN inputs, outputting a clean, low-jitter signal that is

appropriate for mode detection and clock generation. The

Hsync regenerator is used to recreate a clean, although not low

jitter, Hsync signal that can be used for mode detection and

counting Hsyncs per Vsync. The Vsync filter is used to elimi-

nate spurious Vsyncs, maintain a stable timing relationship

between the Vsync and Hsync output signals, and generate the

odd/even field output. The Coast generator creates a robust

Coast signal that allows the PLL to maintain its frequency in the

absence of Hsync pulses.

Sync Slicer

The purpose of the sync slicer is to extract the sync signal from

the green graphics or luminance video signal that is connected

to the SOGIN input. The sync signal is extracted in a two step

process. First, the SOG input is clamped to its negative peak,

(typically 0.3 V below the black level). Next, the signal goes to a

comparator with a variable trigger level (set by Register 0x1D,

Bits [7:3]), but nominally 0.128 V above the clamped level. The

sync slicer output is a digital composite sync signal containing

both Hsync and Vsync information (see Figure 9).

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