THS7360 Datasheet, PDF (32/41 Pages) Texas Instruments – 6-Channel Video Amplifier with 3-SD and 3-SD/ED/HD/Full-HD Filters and High Gain

THS7360
SLOS674 – JUNE 2010
the source impedance (which includes the source,
series resistor, and the filter) must be better than
75 Ω, +9/–8 Ω. The THS7360 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 THS7360 has approximately 0.7 Ω of output
impedance, or a return loss of 46 dB, at 6.75 MHz for
the SD filters and approximately 1.7 Ω of output
impedance, or a return loss of 39 dB, at 30 MHz for
the SF-HD filters. Thus, the system is matched
significantly better with a THS7360 compared to a
passive filter.
One final benefit of the THS7360 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
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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 six
channels are being driven. Using the THS7360 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, six-channel
DAC dissipates 660 mW alone for the steering
current capability (six 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 82 mW (six channels ×
4.16 mA × 3.3 V).
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