THS7315 Datasheet, PDF (16/26 Pages) Texas Instruments – 3-Channel SDTV Video Amplifier with 5th-Order Filters and 5.2-V/V Gain

THS7315
SLOS532 – JUNE 2007
www.ti.com
APPLICATION INFORMATION (continued)
ADVANTAGES OVER PASSIVE FILTERING
Two key benefits of using an integrated filter system such as the THS7315 over a passive system are PCB area
and filter variations. For overall board area, the small SOIC-8 package for 3-video channels is much smaller over
a passive RLC network, especially a 5-pole passive network. As for filter variations, consider that inductors
generally have 10% tolerances (normally 15% to 20%) and capacitors typically have 10% tolerances. A Monte
Carlo analysis shows that the desired filter corner frequency (–3 dB), flatness (–1 dB), Q-factor (or peaking), and
channel-to-channel delay will have wide variations. These variations can lead to potential performance and
quality issues in mass-production environments. The THS7315 solves most of these problems with the corner
frequency being the only variable.
One concern about using an active filter in an integrated circuit is the variation of the filter characteristics when
the ambient temperature and the subsequent die temperature change. To minimize temperature effects, the
THS7315 uses low temperature coefficient resistors and high quality/low temperature coefficient capacitors
found in the BiCom3 process. The filters have been specified by design to account for process and temperature
variations to maintain proper filter characteristics. This design guideline maintains a low channel-to-channel time
delay that is required for proper video signal performance.
The input and output impedances are another benefit of the THS7315 over a passive RLC filter. The input
impedance presented to the DAC varies significantly with a passive network and may cause voltage variations
over frequency. The THS7315 input impedance is 800 kΩ; only the 2-pF input capacitance plus the PCB trace
capacitance affect this value. As such, the voltage variation appearing at the DAC output is better controlled with
the THS7315.
On the output side of the filter, a passive filter again has an impedance variation over frequency. The THS7315
is an operational amplifier that 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 impedance must maintain a 75-Ω impedance. A passive filter
impedance variation cannot specify this condition, while the THS7315 has about 0.8 Ω of output impedance at 1
MHz. Thus, the system is matched much better with a THS7315 when compared to a passive filter.
One final benefit of the THS7315 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 design requirement forces the DAC to drive at least
1.25 V_PP (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 THS7315, with a high input impedance and the
capability to drive up to two video lines, can reduce the DAC power dissipation significantly. This reduction
occurs because the resistance that the DAC is driving can be substantially increased. It is common to set this
increase in a DAC by a current-setting resistor on the device. Thus, the resistance can be 300 Ω or
more—significantly reducing the current drive demands from the DAC and saving a substantial amount of power.
For example, a 3.3-V, six-channel DAC dissipates 660 mW just for the steering current capability (6 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 82.5 mW (6 channels × 4.16 mA × 3.3 V).
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