Rainbow Electronics MAX7447 - Manual (Page 5)

Filter
Filter Response
The reconstruction filter consists of two 2nd-order Sallen-
Key stages. The Butterworth-type response features a
maximally flat passband for NTSC and PAL bandwidths.
The stopband offers at least 43dB (typ) of attenuation at
a video-encoder DAC sampling frequency of 27MHz and
above (see the Typical Operating Characteristics).
High-Frequency Boost
Each channel has a +1.2dB high-frequency boost that
increases image sharpness by compensating for signal
degradation and rolloff in the video encoder.
Output Buffers
Each output buffer has a fixed gain of +6dB and can
drive two 150
Ω video loads with a 2V
P-P signal. The
MAX7447 can drive an AC load or drive the video load
directly without using a large output capacitor. The out-
put buffers drive DC loads with an output blanking level
of less than 1V.
Output Clamp Level
When a sync pulse is detected on channel A (CVBS),
the DC restore loop is activated for channels A, B, and
C. Channel D’s DC restore loop is activated by the sync
pulse on channel D.The function of the loop is to set the
DC level of the video signal to a specified voltage. See
Table 1 for clamp levels and sync source.
Applications Information
Input Considerations
Use 0.1µF ceramic capacitors to AC-couple the inputs.
These input capacitors store a DC level so the outputs
are clamped to an appropriate DC voltage level.
Output Considerations
The outputs are typically connected to a 75
Ω series
back-match resistor followed by the video cable.
Because of the inherent divide-by-two of this configura-
tion, the voltage on the video cable is always less than
1V, complying with industry-standard video require-
ments such as the European SCART standard (which
allows up to 2V of DC on the video cable). The video
buffer can also drive an AC-coupled video load. Good
video performance is achieved with an output capacitor
as low as 220µF.
Power-Supply Bypassing and Layout
The MAX7447 operates from a single +5V supply.
Bypass V
CC to GND with a 0.1µF capacitor. Place all
external components as close to the device as possible.
Exposed Pads
The TSSOP-EP package has an exposed pad on the
bottom of the package. This pad is electrically connect-
ed to GND and should be connected to the ground
plane for improved thermal conductivity. Do not route
signals under this package.
MAX7447
4-Channel S-Video, CVBS Video Filter with
Asynchronous CVBS Input
_______________________________________________________________________________________ 5
14
13
12
11
10
9
8
1
2
3
4
5
6
7
OUTA
OUTB
OUTC
OUTD IND
INC
INB
INA
TOP VIEW
MAX7447
V CC
GND
GND GND
GND
DISABLE
TSSOP
Pin Configuration
Chip Information
TRANSISTOR COUNT: 6300
PROCESS: BiCMOS
Table 1. Output Clamp Level and Sync
Source
CHANNEL CLAMP LEVEL (V) SYNC SOURCE (V)
A 0.8 Channel A
B 0.8 Channel A
C 1.6 Channel A
D 0.8 Channel D