THS7375 Datasheet, PDF (24/36 Pages) Texas Instruments – 4-Channel SDTV Video Amplifier with 6th-Order Filters and 5.6-V/V Gain

THS7375
SBOS449 – SEPTEMBER 2008......................................................................................................................................................................................... www.ti.com
LOW-PASS FILTER
Each channel of the THS7375 incorporates a
sixth-order low-pass filter. These video reconstruction
filters minimize the passing of DAC images to 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 that 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 with this characteristic 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 that results 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 slow for acceptable image
rejection. Thus, the Butterworth filter is a respectable
compromise for both attenuation and group delay.
The THS7375 filters have a nominal corner (–3 dB)
frequency at 9.5 MHz and a –1-dB passband typically
at 8 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 that appears around this frequency can also
appear in the baseband as the result of aliasing
effects of an ADC found in a receiver.
Keep in mind that images do not stop at the DAC
sampling frequency, f_S (for example, 27 MHz for
traditional SD DACs); they continue around the
sampling frequencies of 2X f_S, 3X f_S, 4X f_S, etc. (54
MHz, 81 MHz, 108 MHz). 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 THS7375 filters are
Butterworth filters and as such, do not bounce at
higher frequencies and maintain good attenuation
performance.
The 9.5-MHz filter frequency was chosen to account
for process variations in the THS7375. 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 that the attenuation must be large enough to
ensure the anti-aliasing/reconstruction filtering is
adequate 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 THS7375 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% 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 THS7375 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 nearby video
channels that use 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 that 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 THS7375 uses low temperature
coefficient resistors and high-quality/low-temperature
24
Submit Documentation Feedback
Product Folder Link(s): THS7375
Copyright © 2008, Texas Instruments Incorporated