Datasheet

THS7374
www.ti.com
SLOS590B JULY 2008REVISED JANUARY 2011
LOW-PASS FILTER Keep in mind that images do not stop at 27-MHz;
they continue around the sampling frequencies of
Each channel of the THS7374 incorporates a
54-MHz, 81-MHz, 108-MHz, etc. Because of these
sixth-order low-pass filter. These video reconstruction
multiple images that an ADC can fold down into the
filters minimize DAC images from being passed onto
baseband signal, the low-pass filter must also
the video receiver. Depending on the receiver design,
eliminate these higher-order images. The THS7374
failure to eliminate these DAC images can cause
has 60-dB attenuation at 54-MHz, 55-dB attenuation
picture quality problems as a result of ADC aliasing.
at 81-MHz, and 50-dB attenuation at 108-MHz.
Another benefit of the filter is to smooth out
Attenuation above 108-MHz is at least 45-dB, which
aberrations in the signal which some DACs can have
makes sure that images do not affect the desired
if the internal device filtering is not very good. This
video baseband signal.
technique helps with picture quality and helps ensure
that the signal meets video bandwidth requirements. The 9.5-MHz filter frequency was chosen to account
for process variations in the THS7374. To ensure that
Each filter has an associated Butterworth
the required video frequencies are effectively passed,
characteristic. The benefit of the Butterworth
the filter corner frequency must be high enough to
response is that the frequency response is flat, with a
allow component variations. The other consideration
relatively steep initial attenuation at the corner
is the attenuation must be large enough to ensure the
frequency. The problem is that the group delay rises
anti-aliasing/reconstruction filtering is enough to meet
near the corner frequency. Group delay is defined as
the system demands. Thus, the filter frequencies
the change in phase (radians/second) divided by a
were not arbitrarily selected and are a good
change in frequency. An increase in group delay
compromise that should meet the demands of most
corresponds to a time domain pulse response that
systems.
has overshoot and some possible ringing associated
with the overshoot.
Benefits Over Passive Filtering
The use of other type of filters, such as elliptic or
Two key benefits of using an integrated filter system,
chebyshev, are not recommended for video
such as the THS7374, over a passive system is PCB
applications because of the very large group delay
area and filter variations. The small TSSOP-14
variations near the corner frequency, resulting in
package for four video channels is much smaller over
significant overshoot and ringing. While these elliptic
a passive RLC network, especially a six-pole passive
or chebyshev filters may help meet the video
network. Additionally, consider that inductors have at
standard specifications with respect to amplitude
best ±10% tolerances (normally ±15% to ±20% are
attenuation, the group delay is well beyond the
common) and capacitors typically have ±10%
standard specifications. When considering these filter
tolerances. Using a Monte Carlo analysis shows that
types, keep in mind that video can go from a white
the filter corner frequency (–3 dB), flatness (–1 dB), Q
pixel to a black pixel over and over again, and ringing
factor (or peaking), and channel-to-channel delay
can easily occur. Ringing typically causes a display to
have wide variations. This approach can lead to
have ghosting or fuzziness appear on the edges of a
potential performance and quality issues in
sharp transition. On the other hand, a Bessel filter
mass-production environments. The THS7374 solves
has ideal group delay response, but the rate of
most of these problems with only the corner
attenuation is typically too low for acceptable image
frequency being essentially the only variable.
rejection. Thus, the Butterworth filter is a respectable
compromise for both attenuation and group delay. Another concern about passive filters is the use of
inductors. Inductors are magnetic components and
The THS7374 filters have a nominal corner (–3 dB)
are therefore susceptible to electromagnetic
frequency at 9.5-MHz and a –1 dB passband typically
coupling/interference (EMC/EMI). Some common
at 8.2-MHz. This 9.5-MHz filter is ideal for standard
coupling can occur because of other nearby video
definition (SD) NTSC, PAL, and SECAM composite
channels that use inductors for filtering, or it can
video (CVBS) signals. It is also useful for s-video
come from nearby switch-mode power supplies.
signals (Y'C'), 480i/576i Y'P'
B
P'
R
, Y'U'V', broadcast
Some other forms of coupling could be from outside
G'B'R' (R'G'B') signals, and computer video signals.
sources with strong EMI radiation which can cause
The 9.5-MHz –3 dB corner frequency was designed
failure in EMC testing such as required for CE
to achieve 54-dB of attenuation at 27-MHz—a
compliance.
common sampling frequency between the DAC/ADC
second and third Nyquist zones found in many video One concern about an active filter in an integrated
systems. This consideration is important because any circuit is the variation of the filter characteristics when
signal appearing around this frequency can appear in the ambient temperature and the subsequent die
the baseband as a result of aliasing effects of an temperature changes. To minimize temperature
ADC found in a receiver. effects, the THS7374 uses low temperature
coefficient resistors and high quality—low
temperature coefficient capacitors found in the
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