THS7364_10 Datasheet, PDF (36/47 Pages) Texas Instruments – 6-Channel Video Amplifier with 3 SD and 3 Full-HD Filters with 6-dB Gain

THS7364
SBOS530 – AUGUST 2010
www.ti.com
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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 THS7364 FHD filters have a nominal corner
(–3 dB) frequency at 72 MHz and a –1-dB passband
typically at 60 MHz. This 72-MHz filter is ideal for
1080p50 or 1080p60 component video. It is also ideal
for oversampling systems where the video DAC
upsamples the video signal such as 720p or 1080i
upsampled to 148.5 MHz. The benefit is an extremely
flat passband response along with almost no group
delay within the HD video passband. In bypass mode,
these filters can also be used 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
THS7364 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 THS7364. 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 THS7364 is that these
filters can be bypassed. Bypassing the SD filters
results in an amplifier with 150-MHz bandwidth and
100-V/ms 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 FHD filters results in a amplifier
supporting 350-MHz bandwidth and 500-V/ms slew
rate. This configuration supports computer R'G'B'
signals up to UWXGA resolution.
BENEFITS OVER PASSIVE FILTERING
Two key benefits of using an integrated filter system,
such as the THS7364, 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 THS7364 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 THS7364 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 THS7364 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
THS7364 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 THS7364.
On the output side of the filter, a passive filter again
has a large impedance variation over frequency. The
EIA/CEA-770 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 source impedance (which includes the source,
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