Datasheet

BENEFITS OVER PASSIVE FILTERING
THS7319
www.ti.com .............................................................................................................................................................. SBOS468A – JUNE 2009 – REVISED JULY 2009
The THS7319 filters have a nominal corner ( – 3 dB)
frequency at 20-MHz and a – 1 dB passband typically
Two key benefits of using an integrated filter system,
at 17-MHz. This 20-MHz filter is ideal for enhanced
such as the THS7319, over a passive system are
definition (ED) NTSC or PAL 480p/576p Y ’ P ’
B
P ’
R
or
PCB area and filter variations. The ultra-small
G'B'R'/R'G'B' signals. For oversampled systems, the
MicrostarCSP 9-ball package is much smaller over a
THS7319 works well for passing standard definition
passive RLC network, especially a three-pole passive
(SD) NTSC, PAL, or SECAM composite video
network for three channels. Additionally, consider that
(CVBS), S-Video signals (Y ’ C ’ ), 480i/576i Y ’ P ’
B
P ’
R
,
inductors have at best ± 10% tolerances (normally,
Y ’ U ’ V ’ , broadcast G ’ B ’ R ’ signals, and R'G'B' video
± 15% to ± 20% is common) and capacitors typically
signals. The 20-MHz, – 3-dB corner frequency was
have ± 10% tolerances. A Monte Carlo analysis shows
designed to achieve 27-dB of attenuation at
that the filter corner frequency ( – 3 dB), flatness ( – 1
54 MHz — a common sampling frequency between the
dB), Q factor (or peaking), and channel-to-channel
DAC/ADC second and third Nyquist zones found in
delay have wide variations. These variances can lead
many video systems. This consideration is important
to potential performance and quality issues in
because any signal that appears around this
mass-production environments. The THS7319 solves
frequency can also appear in the baseband as a
most of these problems with the corner frequency
result of aliasing effects of an ADC found in a
being essentially the only variable.
receiver. Another specification ensured for the
THS7319 is attenuation at 43 MHz. This frequency is
Another concern about passive filters is the use of
derived from the fact that the ED Y' signal has an
inductors. Inductors are magnetic components, and
11-MHz bandwidth. Following standard sampling
are therefore susceptible to electromagnetic
theory, this means that the second Nyquist zone
coupling/interference (EMC/EMI). Some common
image starts at 54 MHz – 11 MHz = 43 MHz.
coupling can occur because of other video channels
nearby using inductors for filtering, or it can come
Keep in mind that images do not stop at the DAC
from nearby switched-mode power supplies. Some
sampling frequency, f
S
(for example, 54 MHz for
other forms of coupling could be from outside sources
traditional ED DACs); they continue around the
with strong EMI radiation and can cause failure in
sampling frequency harmonics of 2 × f
S
, 3 × f
S
, 4 × f
S
,
EMC testing such as required for CE compliance.
and so on (that is, 108-MHz, 162-MHz, 216-MHz,
etc.). Because of these multiple images, an ADC can
One concern about an active filter in an integrated
fold down into the baseband signal, meaning that the
circuit is the variation of the filter characteristics when
low-pass filter must also eliminate these higher-order
the ambient temperature and the subsequent die
images. The THS7319 filters are designed to
temperature change. To minimize temperature
attenuate all of these higher frequencies without
effects, the THS7319 uses low-temperature
bounce effect that some filters can allow.
coefficient resistors and high-quality, low-temperature
coefficient capacitors found in the BiCom3X process.
The filter frequencies were chosen to account for
These filters have been specified by design to
process variations in the THS7319. To ensure the
account for process variations and temperature
required video frequencies are effectively passed, the
variations to maintain proper filter characteristics.
filter corner frequency must be high enough to allow
This approach maintains a low channel-to-channel
component variations. The other consideration is that
time delay that is required for proper video signal
the attenuation must be large enough to ensure the
performance.
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.
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