THS7374 Datasheet, PDF (25/37 Pages) Texas Instruments – 4-Channel SDTV Video Amplifier with 6th-Order Filters and 6-dB Gain

THS7374
www.ti.com....................................................................................................................................................................................................... SLOS590 – JULY 2008
LOW-PASS FILTER
Each channel of the THS7374 incorporates a
sixth-order low-pass filter. These video reconstruction
filters minimize DAC images from being passed onto
the video receiver. Depending on the receiver design,
failure to eliminate these DAC images can cause
picture quality problems as a result of ADC aliasing.
Another benefit of the filter is to smooth out
aberrations in the signal which some DACs can have
if the internal device filtering is not very good. This
technique helps with picture quality and helps ensure
that the signal meets video bandwidth requirements.
Each filter has an associated Butterworth
characteristic. The benefit of the Butterworth
response is that the frequency response is flat, with a
relatively steep initial attenuation at the corner
frequency. The problem is that the group delay rises
near the corner frequency. Group delay is defined as
the change in phase (radians/second) divided by a
change in frequency. An increase in group delay
corresponds to a time domain pulse response that
has overshoot and some possible ringing associated
with the overshoot.
The use of other type of filters, such as elliptic or
chebyshev, are not recommended for video
applications because of the very large group delay
variations near the corner frequency, resulting in
significant overshoot and ringing. While these elliptic
or chebyshev filters may help meet the video
standard specifications with respect to amplitude
attenuation, the group delay is well beyond the
standard specifications. When considering these filter
types, keep in mind that video can go from a white
pixel to a black pixel over and over again, and ringing
can easily occur. Ringing typically causes a display to
have ghosting or fuzziness appear on the edges of a
sharp transition. On the other hand, a Bessel filter
has ideal group delay response, but the rate of
attenuation is typically too low for acceptable image
rejection. Thus, the Butterworth filter is a respectable
compromise for both attenuation and group delay.
The THS7374 filters have 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 standard
definition (SD) NTSC, PAL, and SECAM composite
video (CVBS) signals. It is also useful for s-video
signals (Y'C'), 480i/576i Y'P'_BP'_R, Y'U'V', broadcast
G'B'R' (R'G'B') signals, and computer video 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 appearing around this frequency can appear in
the baseband as a result of aliasing effects of an
ADC found in a receiver.
Keep in mind that images do not stop at 27-MHz;
they continue around the sampling frequencies of
54-MHz, 81-MHz, 108-MHz, etc. Because of these
multiple images that an ADC can fold down into the
baseband signal, the low-pass filter must also
eliminate these higher-order images. The THS7374
has 60-dB attenuation at 54-MHz, 55-dB attenuation
at 81-MHz, and 50-dB attenuation at 108-MHz.
Attenuation above 108-MHz is at least 45-dB, which
makes sure that images do not affect the desired
video baseband signal.
The 9.5-MHz filter frequency was chosen to account
for process variations in the THS7374. To ensure that
the required video frequencies are effectively passed,
the filter corner frequency must be high enough to
allow component variations. The other consideration
is the attenuation must be large enough to ensure the
anti-aliasing/reconstruction filtering is enough to meet
the system demands. Thus, the filter frequencies
were not arbitrarily selected and are a good
compromise that should meet the demands of most
systems.
Benefits Over Passive Filtering
Two key benefits of using an integrated filter system,
such as the THS7374, over a passive system is 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% are
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. This approach can lead to
potential performance and quality issues in
mass-production environments. The THS7374 solves
most of these problems with only 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 nearby video
channels that use inductors for filtering, or it can
come from nearby switch-mode power supplies.
Some other forms of coupling could be from outside
sources with strong EMI radiation which 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 changes. To minimize temperature
effects, the THS7374 uses low temperature
coefficient resistors and high quality—low
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