MAX7452 Maxim Integrated Products, MAX7452 Datasheet - Page 6

• Manufacturer Part Number: MAX7452
• Description: (MAX7450 - MAX7452) Video-Signal Conditioners
• Manufacturer: Maxim Integrated Products

Video-Signal Conditioners with AGC and
Back-Porch Clamp
Figure1. MAX7452 Functional Diagram
As shown in Figure 1, the devices include a 2nd-order
lowpass filter intended to reject out-of-band noise. The
MAX7450/MAX7451 clamp the back-porch voltage to
ground, and the MAX7452 clamps to a user-supplied
reference voltage. These devices also include an auto-
matic gain control (AGC), which automatically adjusts
the gain to ensure the sync amplitude is normalized to
a standard video level; an AGC disable function; and
an output driver that drives a standard video load
(150Ω) with a full 2V
P
The clamp and the AGC work concurrently. Interaction
between the two different control loops is eliminated by
the large difference in time constants. The time con-
stant of the clamp settles within 100 lines, while the
AGC loop is digitally stepped so that it settles between
1000–64,000 lines.
The AGC control works independently of the gain set-
ting of the output buffer. The overall gain is the multipli-
cation of the AGC gain and the output buffer gain. The
maximum overall gain is +12dB and the minimum gain
is -6dB.
6
video signal (GSET = 0).
_______________________________________________________________________________________
VIDEO
INPUT
P-P
IN
Detailed Description
video signal (GSET = 1) or 1V
V
CC
GND
FILTER
NOISE
SYNC SEPARATOR
AND AMPLITUDE
DETECTOR
BPLVL
±6dB
P-
GAIN SET (0dB OR +6dB)
MAX7452
0 OR +6dB
The MAX7450/MAX7451/MAX7452 feature a back-
porch clamp to set the output blanking level. The
devices sense the voltage during back porch and feed
back into a control system. The control system provides
the appropriate DC-level shift to clamp the output to
ground (MAX7450/MAX7451) or to a voltage set by
V
ther video processing such as on-screen display (OSD)
insertion and analog-to-digital conversion. The back-
porch clamp to ground also eliminates the need for
large output-coupling capacitors that can introduce
unwanted line-time distortion (tilt) and cost. This can
also reduce board space. The feedback network and
the on-chip capacitors introduce a finite settling time
after power-up or after any dramatic shift in input volt-
age (see the Electrical Characteristics section).
The MAX7452 features an adjustable back-porch level
at the output as shown in Figure 1. V
back-porch clamp level. The back-porch clamp-output
level is defined by the following equations.
BPLVL
GSET = 1 (Gain = 2V / V), V
GSET = 0 (Gain = 1V / V), V
(MAX7452). This restores the DC level for fur-
OUT
LOS
AGCD
75Ω
Back-Porch Level Input (MAX7452)
V
BPLVL
BACKPORCHLEVEL
/ 1.5
BACKPORCHLEVEL
Back-Porch Clamp
75Ω
BPLVL
= V
sets the
BPLVL
=