Datasheet

THS7364
SBOS530 –AUGUST 2010 www.ti.com
achieve BENEFITS OVER PASSIVE FILTERING
54 dB of attenuation at 27 MHz—a common sampling
Two key benefits of using an integrated filter system,
frequency between the DAC/ADC second and third
such as the THS7364, over a passive system are
Nyquist zones found in many video systems. This
PCB area and filter variations. The small TSSOP-20
consideration is important because any signal that
package for six video channels is much smaller over
appears around this frequency can also appear in the
a passive RLC network, especially a six-pole passive
baseband as a result of aliasing effects of an ADC
network. Additionally, consider that inductors have at
found in a receiver.
best ±10% tolerances (normally, ±15% to ±20% is
The THS7364 FHD filters have a nominal corner common) and capacitors typically have ±10%
(–3 dB) frequency at 72 MHz and a –1-dB passband tolerances. Using a Monte Carlo analysis shows that
typically at 60 MHz. This 72-MHz filter is ideal for the filter corner frequency (–3 dB), flatness (–1 dB), Q
1080p50 or 1080p60 component video. It is also ideal factor (or peaking), and channel-to-channel delay
for oversampling systems where the video DAC have wide variations. These variances can lead to
upsamples the video signal such as 720p or 1080i potential performance and quality issues in
upsampled to 148.5 MHz. The benefit is an extremely mass-production environments. The THS7364 solves
flat passband response along with almost no group most of these problems with the corner frequency
delay within the HD video passband. In bypass mode, being essentially the only variable.
these filters can also be used for some computer
Another concern about passive filters is the use of
R’G’B’ video signals including VGA, SVGA, XGA,
inductors. Inductors are magnetic components, and
SXGA, and QXGA.
are therefore susceptible to electromagnetic
Keep in mind that images do not stop at the DAC coupling/interference (EMC/EMI). Some common
sampling frequency, f
S
(for example, 27 MHz for coupling can occur because of other video channels
traditional SD DACs); they continue around the nearby using inductors for filtering, or it can come
sampling frequencies of 2x f
S
, 3x f
S
, 4x f
S
, and so on from nearby switched-mode power supplies. Some
(that is, 54 MHz, 81 MHz, 108 MHz, etc.). Because of other forms of coupling could be from outside sources
these multiple images, an ADC can fold down into the with strong EMI radiation and can cause failure in
baseband signal, meaning that the low-pass filter EMC testing such as required for CE compliance.
must also eliminate these higher-order images. The
One concern about an active filter in an integrated
THS7364 filters are Butterworth filters and, as such,
circuit is the variation of the filter characteristics when
do not bounce at higher frequencies, thus maintaining
the ambient temperature and the subsequent die
good attenuation performance.
temperature changes. To minimize temperature
The filter frequencies were chosen to account for effects, the THS7364 uses low-temperature
process variations in the THS7364. To ensure the coefficient resistors and high-quality, low-temperature
required video frequencies are effectively passed, the coefficient capacitors found in the BiCom3X process.
filter corner frequency must be high enough to allow These filters have been specified by design to
component variations. The other consideration is that account for process variations and temperature
the attenuation must be large enough to ensure the variations to maintain proper filter characteristics.
anti-aliasing/reconstruction filtering is sufficient to This approach maintains a low channel-to-channel
meet the system demands. Thus, the selection of the time delay that is required for proper video signal
filter frequencies was not arbitrarily selected and is a performance.
good compromise that should meet the demands of
Another benefit of the THS7364 over a passive RLC
most systems.
filter is the input and output impedance. The input
One of the features of the THS7364 is that these impedance presented to the DAC varies significantly,
filters can be bypassed. Bypassing the SD filters from 35 Ω to over 1.5 kΩ with a passive network, and
results in an amplifier with 150-MHz bandwidth and may cause voltage variations over frequency. The
100-V/ms slew rate. This configuration can be helpful THS7364 input impedance is 800 kΩ, and only the
when diagnosing potential system issues or when 2-pF input capacitance plus the PCB trace
simply wishing to pass higher frequency signals capacitance impact the input impedance. As such,
through the system. the voltage variation appearing at the DAC output is
better controlled with a fixed termination resistor and
Bypassing the FHD filters results in a amplifier
the high input impedance buffer of the THS7364.
supporting 350-MHz bandwidth and 500-V/ms slew
rate. This configuration supports computer R'G'B' On the output side of the filter, a passive filter again
signals up to UWXGA resolution. 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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