THS7374_16 Datasheet, PDF (28/42 Pages) Texas Instruments – 4-Channel SDTV Video Amplifier with 6th-Order Filters and 6-dB Gain

THS7374
SLOS590B – JULY 2008 – REVISED JANUARY 2011
BiCom3X process. The filters have been specified by
design to account for process variations and
temperature variations to maintain proper filter
characteristics. This architecture maintains a low
channel-to-channel time delay, which is required for
proper video signal performance.
Another benefit of the THS7374 over a passive RLC
filter is the input and output impedance. The input
impedance presented to the DAC vary significantly,
from 35-Ω to over 1.5-kΩ, with a passive network and
may cause voltage variations over frequency. The
THS7374 input impedance is 800-kΩ and only the
2-pF input capacitance plus the PCB trace
capacitance impacting 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 THS7374.
On the output side of the filter, a passive filter also
has a large impedance variation over frequency,
again from 35-Ω to over 1.5-kΩ. The THS7374 is an
operational amplifier which approximates an ideal
voltage source. A voltage source 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, this output resistor must maintain a 75-Ω
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impedance. A passive filter impedance variation
cannot ensure this while the THS7374 has about
0.7-Ω of output impedance at 5-MHz. Thus, the
system is matched much better with a THS7374
compared to a passive filter.
One final advantage of the THS7374 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 source impedance
matching resistor next to the DAC to maintain the
source impedance requirement. This approach 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. With
a high input impedance and the capability to drive up
to two video lines, utilizing the THS7374 can reduce
the DAC power dissipation significantly. This occurs
because the resistance the DAC is driving can be
substantially increased. It is common to set this in a
DAC by a current setting resistor on the DAC. Thus,
the resistance can be 300-Ω or more, substantially
reducing the current drive demands from the DAC
and saving a substantial amount of power. For
example, a 3.3-V four-channel DAC dissipates 440
mW just for the steering current capability (four
channels × 33.3 mA × 3.3 V) if it needs to drive
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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