THS7375 Datasheet, PDF (25/36 Pages) Texas Instruments – 4-Channel SDTV Video Amplifier with 6th-Order Filters and 5.6-V/V Gain

THS7375
www.ti.com......................................................................................................................................................................................... SBOS449 – SEPTEMBER 2008
coefficient capacitors found in the BiCom3X process.
These filters have been specified by design to
account for process and temperature variations to
maintain proper filter characteristics. This architecture
maintains a low channel-to-channel time delay, which
is required for proper video signal performance.
Another benefit of the THS7375 over a passive RLC
filter is the input and output impedance. The input
impedance presented to the DAC may vary
significantly with a passive network and may cause
voltage variations over frequency. The THS7375
input impedance is 800 kΩ, and only the 2-pF input
capacitance plus the PCB trace capacitance impacts
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 THS7375.
On the output side of the filter, a passive filter also
has a large impedance variation over frequency. The
EIA770 specifications requires the return loss be at
least 25 dB over the video frequency range of usage.
For a video system, this condition implies the source
impedance—which includes the source and the
series resistor and the filter—must be better than 75
+9/–8 Ω. The THS7375 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 resistance must maintain
a 75-Ω impedance. A passive filter impedance
variation cannot ensure this consistent performance
while the THS7375 has about 1.4-Ω of output
impedance at 5-MHz. Thus, the system is much
better matched with a THS7375 as compared to a
passive filter.
One final advantage of the THS7375 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 source impedance
matching resistor next to the DAC to maintain the
source impedance requirement. This approach 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 four channels are being driven. With
a high input impedance and the capability to drive up
to two video lines, utilizing the THS7375 can reduce
the DAC power dissipation significantly. This
reduction occurs because the resistance the DAC is
driving can be substantially increased. It is common
to set this driving resistance in a DAC by a
current-setting resistor on the device. Thus, the
resistance can be 300 Ω or more, substantially
reducing the current drive demands from the DAC
and saving a substantial amount of power. For
example, a 3.3-V four-channel DAC dissipates 440
mW for the steering current capability alone (four
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
55 mW (four channels × 4.16 mA × 3.3 V).
Reducing System Gain
The THS7375 has a built-in gain of 5.6 V/V, or 14.95
dB. While this gain matches the needs of many of
Texas Instruments' video processors, including the
DaVinci family of products, the gain can be easily
reduced to meet other needs. The easiest and most
effective method of adjusting the gain lower is to
simply use a resistor divider on the input to the
THS7375, as shown in Figure 52. This solution uses
resistors R1 and R2 to accomplish two requirements:
1. Terminate the video DAC with the proper
resistance, and
2. Form a resistor divider in before the THS7375
input.
DAC
V_INPUT
0.44 V_PP
R1
21.5 W
R2
53.6 W
V_DAC
0.615 V_PP
100% Saturated CVBS
5.6 V/V
THS7375
2.46 V_PP
Video Out
75 W
1.23 V_PP
75 W
Figure 52. Configuring the THS7375 with 4-V/V (12dB) Gain
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