EL4501 datasheet(15/20 Pages) RENESAS

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EL4501

FN7327 Rev 3.00

Page 15 of 20

November 12, 2010

Block Diagram

Applications Information

Product Description

The EL4501 is a video front-end sub-system comprised of a

video amplifier with DC-restore, an adjustable threshold data

slicer, and an advanced sync separator. The prime function of

the system is to DC-stabilize and buffer AC-coupled analog

video signals and to extract timing reference signals

embedded in the video signal. An adjustable threshold data

slicer incorporated into the EL4501 may be used to extract

data embedded within the active video or VBI regions of a

video signal.

Theory of Operation

DC-RESTORE LOOP

When video signals are distributed, it is common to employ

capacitive coupling to prevent DC current flow due to

differences in local grounds or signal reference levels.

However, the coupling capacitor causes the DC level of the

signal post capacitor to be dependent on the video (luminance)

content of the waveform. A DC-restore loop is used to correct

this behavior by moving a portion of the video waveform to a

DC reference level in response to a control signal. When the

loop is operating, DC drift accumulates over a single line only,

before it is corrected. The peak value of drift is limited by the

rate of the control signal (typically video line rate) and the AC

coupling time constant.

The restore loop is comprised of a 100MHz forward video

amplifier, combined with a nulling amplifier and sample and

hold circuit. For maximum flexibility the hold capacitor is placed

off-chip, allowing the loop response rate to be tailored for

specific applications and minimizing hold-step problems. The

loop provides a restore current peak of ±20µA at room

temperature. Figure 36 shows the amplifier and S/H

connection. During normal operation the internally generated

DC-restore control signal is timed to the back porch of the

video waveform. Figure 37 shows an NTSC video signal, along

with the EL4501 BACK PORCH output. In operation, BACK

PORCH activates the S/H switch, completing the nulling

feedback loop and driving the video amplifier output towards

the reference voltage. At the end of BACK PORCH, the

external capacitor holds the correction voltage for the

remainder of the video line. In the absence of a valid input

signal, the chip generates a repetitive, arbitrary restore control

signal at the line rate set by the external resistor RFREQ.

Although uncorrelated to the input, the pulse prevents the

amplifier output drifting significantly from the DC-restore

FILTER

SYNC

SEPARATOR

GND

GNDD

RFREQ

LOS

SYNC AMP

FSEL

SYNC IN

VIDEO IN

0.1µF

INPUT

VIDEO

VSD

DS REF

ODD/EVEN

VERTICAL

HORIZONTAL

COMPOSITE

BACK PORCH

REF OUT

REF IN

VFB

VIDEO OUT

DS OUT

DS ENABLE

DS MODE

CHOLD

CREF

1.3V

TRACK/

HOLD

SLICE

MODE

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