THS7365 Datasheet, PDF (42/53 Pages) Texas Instruments – 6-Channel Video Amplifier with 3-SD and 3-HD Sixth-Order Filters and 6-dB Gain

THS7365
SBOS467 – MARCH 2009.................................................................................................................................................................................................. www.ti.com
Another benefit of the THS7365 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
THS7365 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 THS7365.
On the output side of the filter, a passive filter again
has a large impedance variation over frequency. The
EIA770 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/–8 Ω. The THS7365 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 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 THS7365 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 HD filters. Thus, the system is matched
significantly better with a THS7365 compared to a
passive filter.
One final benefit of the THS7365 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 six
channels are being driven. Using the THS7365 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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