THS7320 Datasheet, PDF (26/35 Pages) Texas Instruments – 3-Channel ED Filter Video Amplifier with 4-V/V Gain

THS7320
SBOS565B – JULY 2011 – REVISED SEPTEMBER 2012
www.ti.com
BENEFITS OVER PASSIVE FILTERING
Two key benefits of using an integrated filter system (such as the THS7320) over a passive system are PCB
area and filter variations. The ultra-small MicrostarCSP 9-ball package is much smaller over a passive RLC
network, especially a three-pole passive network for three channels. Additionally, consider that inductors have at
best ±10% tolerances (normally, ±15% to ±20% is common) and capacitors typically have ±10% tolerances. A
Monte Carlo analysis shows that the filter corner frequency (–3 dB), flatness
(–1 dB), Q-factor (or peaking), and channel-to-channel delay have wide variations. These variances can lead to
potential performance and quality issues in mass-production environments. The THS7320 solves most of these
problems with the corner frequency being essentially the only variable.
Another concern about passive filters is the use of inductors. Inductors are magnetic components, and are
therefore susceptible to electromagnetic coupling/interference (EMC/EMI). Some common coupling can occur
because of other video channels nearby using inductors for filtering, or it can come from nearby switched-mode
power supplies. Some other forms of coupling could be from outside sources with strong EMI radiation and can
cause failure in EMC testing such as required for CE compliance.
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 change. To minimize temperature effects, the
THS7320 uses low-temperature coefficient resistors and high-quality, low-temperature coefficient capacitors
found in the BiCom3X process. These filters have been specified by design to account for process variations and
temperature variations to maintain proper filter characteristics. This approach maintains a low channel-to-channel
time delay that is required for proper video signal performance.
Another benefit of the THS7320 over a passive RLC filter are the input and output impedances. With a passive
filter, the input impedance presented to the DAC varies significantly, from 35 Ω to over 1.5 kΩ, and may cause
voltage variations over frequency. The THS7320 input impedance is 2.4 MΩ, 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 THS7320.
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 use. For a video
system, this requirement implies that the source impedance (which includes the source, series resistor, and the
filter) must be better than 75 Ω, +9 Ω/–8 Ω. The THS7320 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 THS7320 has approximately 1.4 Ω of output impedance, or a return
loss of 41 dB, at 11 MHz. Thus, the system is matched significantly better with a THS7320 compared to a
passive filter.
One final benefit of the THS7320 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 three channels are being driven.
Using the THS7320 with a high input impedance 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, three-channel DAC dissipates 330 mW alone for the steering current capability (three 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 41 mW (three channels × 4.16 mA × 3.3 V), or over eight times lower
power. For overall system power, this scenario must also account for the THS7320 power. The THS7320 only
consumes 3.4-mA total quiescent current. The quiescent power added is then 3.3 V × 3.4 mA = 11.2 mW. The
total system power is then 41 mW + 11 mW = 52 mW, or a factor of six times lower power compared to the DAC
driving the line directly. Saving power by adding the THS7320 in a system is easy to see and accomplish, not to
mention that it incorporates the added benefit of a three-pole filter on each channel.
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