THS7372 Datasheet, PDF (38/49 Pages) Texas Instruments – 4-Channel Video Amplifier with One CVBS and Three Full-HD Filters with 6-dB Gain

THS7372
SBOS578 – AUGUST 2011
www.ti.com
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
THS7372 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 THS7372 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 THS7372 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 THS7372.
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, series resistor,
and the filter) must be better than 75 Ω, ±9 Ω. The THS7372 is an operational amplifier that approximates an
ideal voltage source, which is desirable because the output impedance is very low and can source and sink
current. To properly match the transmission line characteristic impedance of a video line, a 75-Ω series resistor is
placed on the output. To minimize reflections and to maintain a good return loss meeting EIA/CEA specifications,
this output impedance must maintain a 75-Ω impedance. A wide impedance variation of a passive filter cannot
ensure this level of performance. On the other hand, the THS7372 has approximately 0.7 Ω of output impedance,
or a return loss of 47 dB, at 6.75 MHz for the SD filter and approximately 4 Ω of output impedance, or a return
loss of 32 dB, at 60 MHz for the FHD filters. Thus, the system is matched significantly better with a THS7372
compared to a passive filter.
One final benefit of the THS7372 over a passive filter is power dissipation. A DAC driving a video line must be
able to drive a 37.5-Ω load: the receiver 75-Ω resistor and the 75-Ω impedance matching resistor next to the
DAC to maintain the source impedance requirement. This requirement forces the DAC to drive at least 1.25 V_P
(100% saturation CVBS)/37.5 Ω = 33.3 mA. A DAC is a current-steering element, and this amount of current
flows internally to the DAC even if the output is 0 V. Thus, power dissipation in the DAC may be very high,
especially when four channels are being driven. Using the THS7372 with a high input impedance and the
capability to drive up to two video lines per channel can reduce DAC power dissipation significantly. This
outcome is possible because the resistance that the DAC drives can be substantially increased. It is common to
set this resistance in a DAC by a current-setting resistor on the DAC itself. Thus, the resistance can be 300 Ω or
more, substantially reducing the current drive demands from the DAC and saving significant amounts of power.
For example, a 3.3-V, four-channel DAC dissipates 440 mW alone for the steering current capability (four
channels × 33.3 mA × 3.3 V) if it must drive a 37.5-Ω load. With a 300-Ω load, the DAC power dissipation as a
result of current steering current would only be 55 mW (four channels × 4.16 mA × 3.3 V).
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