Rainbow Electronics MAX7473 - Manual (Page 8)

MAX7472/MAX7473
Component/Composite Selection
The MAX7472/MAX7473 accept component or com-
posite inputs. The sync detection path provides an
additional selectable color burst filter to improve sync
detection.
External Sync Detection
When filtering a video signal without embedded sync
information, such as computer formats (RGBHV) with
separate sync signals, use the external sync mode (see
Table 3) and apply the horizontal sync source to the
SYNCA, SYNCB, or SYNCC pin. The sync detector
determines when the clamp circuit is turned on.
The MAX7472/MAX7473 can detect positive or nega-
tive polarity external syncs with TTL logic levels. Use
the I
2 C interface to program the polarity of the external
sync signal.
Filter
The internal video filter delivers an optimized response
with a steep transition band to achieve a wide pass-
band along with excellent stopband rejection. In addi-
tion, the filter is optimized to provide an excellent time-
domain response with low overshoot.
Setting the Filter Frequency
The frequency response (-3dB cutoff frequency) of the
filter in the MAX7472/MAX7473 can be varied from less
than the SD passband to beyond the HD passband in
256 linear steps through the I
2 C interface. Use the com-
mand byte to write to the Frequency register followed
by the 8-bit data word that corresponds to the desired
frequency. See Table 6.
The Frequency register sets the -3dB point. Set this fre-
quency accordingly to achieve the desired flat pass-
band response.
Optimizing the Frequency Response
Select the frequency according to the resolution of the
video-signal format. High-definition signals require
higher bandwidth and standard-definition signals
require less bandwidth. The actual bandwidth con-
tained in the video signal is a function of the visual res-
olution of the signal. This bandwidth is typically less
than what is indicated by the format resolution (1080i,
720p, 480p, and 480i). For more information on this
topic, see Application Note 750:
Bandwidth vs. Video
Resolution on the Maxim website (www.maxim-ic.com).
See Table 6.
The frequency response can be optimized to improve
the overall performance. There are a number of consid-
erations, one of the most important being the sampling
rate of the subsequent ADC or video decoder in the
system. In oversampled systems, the sampling rate is
significantly more than the desired passband response.
The extra frequency span between the passband and
the sampling rate contains noise that can be eliminated
by setting the corner frequency of the filter to just pass
the desired bandwidth. This results in a higher signal-
to-noise ratio of the overall system.
Filter Bypass
The MAX7472/MAX7473 offer selectable filter bypass-
ing that allows the input video signals to bypass the
internal filters reaching the output buffers unfiltered.
The filter-bypass mode is enabled/disabled through the
command byte (see Table 3).
Output Buffer
Each output buffer can drive a 2V P-P signal into a 150Ω
video load. The MAX7472/MAX7473 can drive a DC- or
AC-coupled load. The output DC level is controlled to
limit the DC voltage on the cable so that the blanking
level of the video signal is always less than 1V, meeting
the digital TV specification. As a result, output AC-cou-
pling capacitors can be eliminated when driving a
cable, thus eliminating the normal adverse effects
caused by these capacitors such as line- and field-time
distortion, otherwise known as droop. See the
Output
Considerations section for more information.
Gain Options
The MAX7472 features an overall gain of 0dB, while the
MAX7473 features an overall gain of +6dB. Use the
+6dB option (MAX7473) when driving a back-matched
cable. Use the 0dB option (MAX7472) when driving an
ADC or video decoder with an input range the same as
the input to the MAX7472. To add flexibility, the
MAX7472 accepts input signals up to 2V
P-P , twice the
standard video-signal range.
Output Clamp Level
The MAX7472/MAX7473 output can be DC- or AC-cou-
pled. The nominal output clamp level in the DC-cou-
pled case depends on the clamp voltage setting and
can be determined according to Table 2.
HDTV Anti-Aliasing Filters with Triple-Input Mux
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Table 2. Output Clamp Level
CLAMP SETTING
OUTPUT CLAMP LEVEL (mV)
(typ)
Low ±100
High ±100