THS7319 Datasheet, PDF (21/34 Pages) Texas Instruments – 3-Channel, Very Low Power Video Amplifiers with EDTV Filters and 6-dB Gain

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
at 17-MHz. This 20-MHz filter is ideal for enhanced
definition (ED) NTSC or PAL 480p/576p Y’P’_BP’_R or
G'B'R'/R'G'B' signals. For oversampled systems, the
THS7319 works well for passing standard definition
(SD) NTSC, PAL, or SECAM composite video
(CVBS), S-Video signals (Y’C’), 480i/576i Y’P’_BP’_R,
Y’U’V’, broadcast G’B’R’ signals, and R'G'B' video
signals. The 20-MHz, –3-dB corner frequency was
designed to achieve 27-dB of attenuation at
54 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. Another specification ensured for the
THS7319 is attenuation at 43 MHz. This frequency is
derived from the fact that the ED Y' signal has an
11-MHz bandwidth. Following standard sampling
theory, this means that the second Nyquist zone
image starts at 54 MHz – 11 MHz = 43 MHz.
Keep in mind that images do not stop at the DAC
sampling frequency, f_S (for example, 54 MHz for
traditional ED DACs); they continue around the
sampling frequency harmonics of 2× f_S, 3× f_S, 4× f_S,
and so on (that is, 108-MHz, 162-MHz, 216-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 THS7319 filters are designed to
attenuate all of these higher frequencies without
bounce effect that some filters can allow.
The filter frequencies were chosen to account for
process variations in the THS7319. 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.
BENEFITS OVER PASSIVE FILTERING
Two key benefits of using an integrated filter system,
such as the THS7319, over a passive system are
PCB area and filter variations. The ultra-small
MicrostarCSP 9-ball package is much smaller over a
passive RLC network, especially a three-pole passive
network for three channels. Additionally, consider that
inductors have at best ±10% tolerances (normally,
±15% to ±20% is common) and capacitors typically
have ±10% tolerances. 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 THS7319 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 THS7319 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
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