Datasheet

THS7374
SLOS590B –JULY 2008–REVISED JANUARY 2011 www.ti.com
BiCom3X process. The filters have been specified by impedance. A passive filter impedance variation
design to account for process variations and cannot ensure this while the THS7374 has about
temperature variations to maintain proper filter 0.7-Ω of output impedance at 5-MHz. Thus, the
characteristics. This architecture maintains a low system is matched much better with a THS7374
channel-to-channel time delay, which is required for compared to a passive filter.
proper video signal performance.
One final advantage of the THS7374 over a passive
Another benefit of the THS7374 over a passive RLC filter is power dissipation. A DAC driving a video line
filter is the input and output impedance. The input must be able to drive a 37.5-Ω load—the receiver
impedance presented to the DAC vary significantly, 75-Ω resistor and the 75-Ω source impedance
from 35-Ω to over 1.5-kΩ, with a passive network and matching resistor next to the DAC to maintain the
may cause voltage variations over frequency. The source impedance requirement. This approach forces
THS7374 input impedance is 800-kΩ and only the the DAC to drive at least 1.25 V
P
(100% saturation
2-pF input capacitance plus the PCB trace CVBS)/37.5 Ω = 33.3 mA. A DAC is a current
capacitance impacting the input impedance. As such, steering element and this amount of current flows
the voltage variation appearing at the DAC output is internally to the DAC even if the output is 0-V. Thus,
better controlled with a fixed termination resistor and power dissipation in the DAC may be very high,
the high input impedance buffer of the THS7374. especially when four channels are being driven. With
a high input impedance and the capability to drive up
On the output side of the filter, a passive filter also
to two video lines, utilizing the THS7374 can reduce
has a large impedance variation over frequency,
the DAC power dissipation significantly. This occurs
again from 35-Ω to over 1.5-kΩ. The THS7374 is an
because the resistance the DAC is driving can be
operational amplifier which approximates an ideal
substantially increased. It is common to set this in a
voltage source. A voltage source is desirable
DAC by a current setting resistor on the DAC. Thus,
because the output impedance is very low and can
the resistance can be 300-Ω or more, substantially
source and sink current. To properly match the
reducing the current drive demands from the DAC
transmission line characteristic impedance of a video
and saving a substantial amount of power. For
line, a 75-Ω series resistor is placed on the output. To
example, a 3.3-V four-channel DAC dissipates 440
minimize reflections and to maintain a good return
mW just for the steering current capability (four
loss, this output resistor must maintain a 75-Ω
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).
28 Submit Documentation Feedback Copyright © 2008–2011, Texas Instruments Incorporated
Product Folder Link(s): THS7374