THS7373 Datasheet, PDF (39/49 Pages) Texas Instruments – 4-Channel Video Amplifier with 1-SD and 3-HD Sixth-Order Filters and 6-dB Gain

THS7373
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The THS7373 CVBS CVBS filter has a nominal
corner (–3 dB) frequency at 9.5-MHz and a –1-dB
passband typically at 8.2 MHz. This 9.5-MHz filter is
ideal for CVBS NTSC, PAL, and SECAM composite
video (CVBS) signals. The 9.5-MHz, –3-dB corner
frequency was designed to achieve 54 dB of
attenuation at 27 MHz—a common sampling
frequency between the DAC/ADC second and third
Nyquist zones found in many video systems. This
consideration is important because any signal that
appears around this frequency can also appear in the
baseband as a result of aliasing effects of an ADC
found in a receiver.
The THS7373 HD filters have a nominal corner
(–3-dB) frequency at 36MHz and a –1-dB passband
typically at 33 MHz. This 36-MHz filter is ideal for HD
720p, 1080i, up to 1080p30 Y’/P’_B/P’_R, broadcast
G’B’R’ signals, and computer R’G’B’ video signals.
The 36-MHz, –3-dB corner frequency was designed
to achieve 40 dB of attenuation at 74.25 MHz—a
common sampling frequency between the DAC/ADC
second and third Nyquist zones found in many video
systems.
Keep in mind that images do not stop at the DAC
sampling frequency, f_S (for example, 27 MHz for
traditional CVBS DACs); they continue around the
sampling frequencies of 2x f_S, 3x f_S, 4x f_S, and so on
(that is, 54-MHz, 81-MHz, 108-MHz, etc.). Because of
these multiple images, an ADC can fold down into the
baseband signal, meaning that the low-pass filter
must also eliminate these higher-order images. The
THS7373 filters are Butterworth filters and, as such,
do not bounce at higher frequencies, thus maintaining
good attenuation performance.
The filter frequencies were chosen to account for
process variations in the THS7373. To ensure the
required video frequencies are effectively passed, the
filter corner frequency must be high enough to allow
component variations. The other consideration is that
the attenuation must be large enough to ensure the
anti-aliasing/reconstruction filtering is sufficient to
meet the system demands. Thus, the selection of the
filter frequencies was not arbitrarily selected and is a
good compromise that should meet the demands of
most systems.
HD FILTER BYPASS MODE
The THS7373 has an HD filter bypass mode that
bypasses the HD channels internal filters, thus the
THS7373 effectively becomes a fixed gain 2-V/V
operational amplifier. Bypassing the HD filters results
in an amplifier supporting a 350-MHz bandwidth and
SBOS506 – DECEMBER 2009
450-V/µs slew rate. This bypass supports 1080p60
signals along with computer R’G’B’ signals up to
QXGA or UWXGA resolution. This mode still uses the
dc + shift functionality along with the transparent
sync-tip-clamp function. Essentially, the only
difference in this mode is that the HD filters are
bypassed.
BENEFITS OVER PASSIVE FILTERING
Two key benefits of using an integrated filter system,
such as the THS7373, over a passive system are
PCB area and filter variations. The small TSSOP-14
package for four video channels is much smaller over
a passive RLC network, especially a six-pole passive
network. Additionally, consider that inductors have at
best ±10% tolerances (normally, ±15% to ±20% is
common) and capacitors typically have ±10%
tolerances. Using a Monte Carlo analysis shows that
the filter corner frequency (–3 dB), flatness (–1 dB), Q
factor (or peaking), and channel-to-channel delay
have wide variations. These variances can lead to
potential performance and quality issues in
mass-production environments. The THS7373 solves
most of these problems with the corner frequency
being essentially the only variable.
Another concern about passive filters is the use of
inductors. Inductors are magnetic components, and
are therefore susceptible to electromagnetic
coupling/interference (EMC/EMI). Some common
coupling can occur because of other video channels
nearby using inductors for filtering, or it can come
from nearby switched-mode power supplies. Some
other forms of coupling could be from outside sources
with strong EMI radiation and can cause failure in
EMC testing such as required for CE compliance.
One concern about an active filter in an integrated
circuit is the variation of the filter characteristics when
the ambient temperature and the subsequent die
temperature change. To minimize temperature
effects, the THS7373 uses low-temperature
coefficient resistors and high-quality, low-temperature
coefficient capacitors found in the BiCom3X process.
These filters have been specified by design to
account for process variations and temperature
variations to maintain proper filter characteristics.
This approach maintains a low channel-to-channel
time delay that is required for proper video signal
performance.
Copyright © 2009, Texas Instruments Incorporated
Product Folder Link(s): THS7373
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