THS7314 Datasheet, PDF (20/23 Pages) Texas Instruments – 3-Channel SDTV Video Amplifier With 5th-Order Filters and 6-dB Gain

THS7314
SLOS513 – DECEMBER 2006
www.ti.com
APPLICATION INFORMATION (continued)
BENEFITS OVER PASSIVE FILTERING
Two key benefits of using an integrated filter system, such as the THS7314, over a passive system is PCB area
and filter variations. The small SOIC-8 package for 3-video channels is much smaller over a passive RLC
network, especially a 5-pole passive network. Add in the fact that inductors have at best ±10% tolerances
(normally ±15% to ±20% is common) and capacitors typically have ±10% tolerances. Using a Monte Carlo
analysis shows that the filter corner frequency (–3 dB), flatness (–1 dB), Q factor (or peaking), and
channel-to-channel delay will have wide variations. This can lead to potential performance and quality issues in
mass-production environments. The THS7314 solves most of these problems with only the corner frequency
being essentially the only variable.
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
THS7314 uses low temperature coefficient resistors and high quality – low temperature coefficient capacitors
found in the BiCom-3 process. The filters have been specified by design to account for process variations and
temperature variations to maintain proper filter characteristics. This maintains a low channel-to-channel time
delay which is required for proper video signal performance.
Another benefit of a THS7314 over a passive RLC filter are the input and output impedances. The input
impedance presented to the DAC will vary significantly with a passive network and may cause voltage variations
over frequency. The THS7314 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 the THS7314.
On the output side of the filter, a passive filter will again have a impedance variation over frequency. The
THS7314 is an op-amp which approxiamates 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 impedance must maintain a 75-Ω impedance. A passive filter
impedance variation cannot guarantee this while the THS7314 has about 0.6-Ω of output impedance at 5.1-MHz.
Thus, the system is matched much better with a THS7314 compared to a passive filter.
One last benefit of the THS7314 over a passive filter is power dissipation. A DAC driving a video line must be
able to drive a 37.5-Ω load - the reciever 75-Ω resistor and the 75-Ω impedance matching resistor next to the
DAC to maintain the source impedance requirement. This forces the DAC to drive at least 1.25Vpeak (100%
Saturation CVBS) / 37.5Ω = 33.3mA. 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 6-channels are being driven. Using the THS7314, with a high input impedance and the capability to drive
up to 2-video lines, can reduce the DAC power dissipation significantly. This is 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 substantial amount of power. For example, a 3.3-V 6-Channel DAC dissipates 660mW just for
the steering current capability (6ch X 33.3mA X 3.3V) if it needs to drive 37.5-Ω load. With a 300-Ω load, the
DAC power dissipation due to current steering current would only be 82.5mW (6ch X 4.16mA X 3.3V).
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