THS7368 Datasheet, PDF (29/39 Pages) Texas Instruments – 6-Channel Video Amplifier with 3-SD and 3-SD/ED/HD/Full-HD Filters and 6-dB Gain

THS7368
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–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 a result of aliasing effects of an ADC
found in a receiver.
The THS7368 SF filters have a nominal corner
(–3 dB) frequency at 9.5 MHz, 18 MHz, 36 MHz, or
72 MHz and a –1-dB passband typically at 8.2 MHz,
16 MHz, 30 MHz, or 60 MHz. The 9.5-MHz filter is
ideal for component 480i or 576i video. The 18-MHz
filter is ideal for component 480p or 576p component
video. The 36-MHz filter is ideal for HD 720p, 1080i,
1080p24, or 1080p30 component video. The 72-MHz
filter is ideal for 1080p50 or 1080p60 component
video. These filters can also be utilized for some
computer R’G’B’ video signals including VGA, SVGA,
XGA, SXGA, and QXGA.
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, and so on
(that is, 54-MHz, 81-MHz, 108-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
THS7368 filters are Butterworth filters and, as such,
do not bounce at higher frequencies, thus maintaining
good attenuation performance.
The filter frequencies were chosen to account for
process variations in the THS7368. 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.
One of the features of the THS7368 is that these
filters can be bypassed. Bypassing the SD filters
results in an amplifier with 150-MHz bandwidth and
125-V/μs slew rate. This configuration can be helpful
when diagnosing potential system issues or when
simply wishing to pass higher frequency signals
through the system.
Bypassing the SF filters results in a amplifier
supporting 375-MHz bandwidth and 600-V/μs slew
rate. This configuration supports computer R'G'B'
signals up to UWXGA resolution.
SBOS497 – DECEMBER 2009
BENEFITS OVER PASSIVE FILTERING
Two key benefits of using an integrated filter system,
such as the THS7368, over a passive system are
PCB area and filter variations. The small TSSOP-20
package for six 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% is
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. These variances can lead to
potential performance and quality issues in
mass-production environments. The THS7368 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 changes. To minimize temperature
effects, the THS7368 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.
Another benefit of the THS7368 over a passive RLC
filter is the input and output impedance. The input
impedance presented to the DAC varies significantly,
from 35 Ω to over 1.5 kΩ with a passive network, and
may cause voltage variations over frequency. The
THS7368 input impedance is 800 kΩ, and only the
2-pF input capacitance plus the PCB trace
capacitance impact the input impedance. As such,
the voltage variation appearing at the DAC output is
better controlled with a fixed termination resistor and
the high input impedance buffer of the THS7368.
On the output side of the filter, a passive filter again
has a large impedance variation over frequency. The
EIA770 specifications require the return loss to be at
least 25 dB over the video frequency range of usage.
For a video system, this requirement implies the
Copyright © 2009, Texas Instruments Incorporated
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