Rainbow Electronics MAX7452 - Manual (Page 6)

MAX7450/MAX7451/MAX7452
Video-Signal Conditioners with AGC and
Back-Porch Clamp
6 _______________________________________________________________________________________
Detailed Description
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
(150
Ω) with a full 2V
P-P video signal (GSET = 1) or 1V P-
P video signal (GSET = 0).
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.
Back-Porch Clamp
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
BPLVL (MAX7452). This restores the DC level for fur-
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).
Back-Porch Level Input (MAX7452)
The MAX7452 features an adjustable back-porch level
at the output as shown in
Figure 1. V BPLVL sets the
back-porch clamp level. The back-porch clamp-output
level is defined by the following equations.
GSET = 1 (Gain = 2V / V), V BACKPORCHLEVEL = V BPLVL
GSET = 0 (Gain = 1V / V), V BACKPORCHLEVEL =
V BPLVL / 1.5
MAX7452
NOISE
FILTER
V CC
75 Ω
75 Ω
LOS
SYNC SEPARATOR
AND AMPLITUDE
DETECTOR
AGCD
IN OUT VIDEO
INPUT
BPLVL
±6dB 0 OR +6dB
GAIN SET (0dB OR +6dB)
GND
Figure1. MAX7452 Functional Diagram