Datasheet

THS7360
www.ti.com SLOS674 –JUNE 2010
9.5-MHz, –3-dB corner frequency was designed to BENEFITS OVER PASSIVE FILTERING
achieve
Two key benefits of using an integrated filter system,
54 dB of attenuation at 27 MHz—a common sampling
such as that found in the THS7360, over a passive
frequency between the DAC/ADC second and third
system are PCB area and filter variations. The small
Nyquist zones found in many video systems. This
TSSOP-20 package for six video channels is much
consideration is important because any signal that
smaller over a passive RLC network, especially a
appears around this frequency can also appear in the
six-pole passive network. Additionally, consider that
baseband as a result of aliasing effects of an ADC
inductors have at best ±10% tolerances (normally,
found in a receiver.
±15% to ±20% is common) and capacitors typically
The THS7360 SF filters have a nominal corner have ±10% tolerances. A Monte Carlo analysis shows
(–3 dB) frequency at 9.2 MHz, 17 MHz, 35 MHz, or that the filter corner frequency (–3 dB), flatness (–1
70 MHz and a –1-dB passband typically at 8 MHz, 15 dB), Q factor (or peaking), and channel-to-channel
MHz, 30 MHz, or 60 MHz. The 9.2-MHz filter is ideal delay have wide variations. These variances can lead
for component 480i or 576i video. The 17-MHz filter is to potential performance and quality issues in
ideal for component 480p or 576p component video. mass-production environments. The THS7360 solves
The 35-MHz filter is ideal for HD 720p, 1080i, most of these problems with the corner frequency
1080p24, or 1080p30 component video. The 70-MHz being essentially the only variable.
filter is ideal for 1080p50 or 1080p60 component
Another concern about passive filters is the use of
video. These filters can also be utilized for some
inductors. Inductors are magnetic components, and
computer R’G’B’ video signals including VGA, SVGA,
are therefore susceptible to electromagnetic
XGA, SXGA, and QXGA.
coupling/interference (EMC/EMI). Some common
Keep in mind that images do not stop at the DAC coupling can occur because of other video channels
sampling frequency, f
S
(for example, 27 MHz for nearby using inductors for filtering, or it can come
traditional SD DACs); they continue around the from nearby switched-mode power supplies. Some
sampling frequencies of 2x f
S
, 3x f
S
, 4x f
S
, and so on other forms of coupling could be from outside sources
(that is, 54-MHz, 81-MHz, 108-MHz, etc.). Because of with strong EMI radiation and can cause failure in
these multiple images, an ADC can fold down into the EMC testing such as required for CE compliance.
baseband signal, meaning that the low-pass filter
One concern about an active filter in an integrated
must also eliminate these higher-order images. The
circuit is the variation of the filter characteristics when
THS7360 filters are Butterworth filters and, as such,
the ambient temperature and the subsequent die
do not bounce at higher frequencies, thus maintaining
temperature changes. To minimize temperature
good attenuation performance.
effects, the THS7360 uses low-temperature
The filter frequencies were chosen to account for coefficient resistors and high-quality, low-temperature
process variations in the THS7360. To ensure the coefficient capacitors found in the BiCom3X process.
required video frequencies are effectively passed, the These filters have been specified by design to
filter corner frequency must be high enough to allow account for process variations and temperature
component variations. The other consideration is that variations to maintain proper filter characteristics.
the attenuation must be large enough to ensure the This approach maintains a low channel-to-channel
anti-aliasing/reconstruction filtering is sufficient to time delay that is required for proper video signal
meet the system demands. Thus, the selection of the performance.
filter frequencies was not arbitrarily selected and is a
Another benefit of the THS7360 over a passive RLC
good compromise that should meet the demands of
filter is the input and output impedance. The input
most systems.
impedance presented to the DAC varies significantly,
One of the features of the THS7360 is that these from 35 Ω to over 1.5 kΩ with a passive network, and
filters can be bypassed. Bypassing the SD filters may cause voltage variations over frequency. The
results in an amplifier with 60-MHz bandwidth and THS7360 input impedance is 800 kΩ, and only the
150-V/ms slew rate. This configuration can be helpful 2-pF input capacitance plus the PCB trace
when diagnosing potential system issues or when capacitance impact the input impedance. As such,
simply wishing to pass higher frequency signals the voltage variation appearing at the DAC output is
through the system. better controlled with a fixed termination resistor and
the high input impedance buffer of the THS7360.
Bypassing the SF filters results in a amplifier
supporting 280-MHz bandwidth and 800-V/ms slew On the output side of the filter, a passive filter again
rate. This configuration supports computer R'G'B' has a large impedance variation over frequency. The
signals up to UWXGA resolution. EIA/CEA770 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
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