THS7320 Datasheet, PDF (23/35 Pages) Texas Instruments – 3-Channel ED Filter Video Amplifier with 4-V/V Gain

THS7320
www.ti.com
SBOS565B – JULY 2011 – REVISED SEPTEMBER 2012
The input impedance of the THS7320 is dictated by the internal high-impedance gain of 2 V/V initial amplifier, as
shown in Figure 57. This buffer has a very high 2.4-MΩ || 2-pF input impedance that is effectively transparent to
the source with no interactions. Unlike other products where the filter elements are tied directly to the input pin
without buffering, there are no filter performance changes or interaction with the DAC termination resistance.
Note that the internal voltage shift does not appear at the input pin; it only shows at the output pin.
+V_S
Input
Pin
Level
Shift
x2
Figure 57. Equivalent Video Section DC Input Mode Circuit
While ac-coupling with dc-biasing using external resistor dividers can be done, it is generally not recommended
because of the large resistor values required. These large resistor values coupled with the input bias current of
the THS7320 input can cause a significant voltage shift to appear on the input. If ac-coupling is necessary for a
system, several elements must be taken into account for a proper design: the high-pass corner frequency
(typically desired to be approximately 2.5 Hz); the size of the input capacitor value; the parallel input resistance
of the voltage divider; and the input bias current. Contact Texas Instruments for design support if ac-coupling is
necessary in the design.
DC-COUPLED OUTPUT
The THS7320 video section incorporates a rail-to-rail output stage that can drive the line directly without the need
for large ac-coupling capacitors. This design offers the best line tilt and field tilt (droop) performance because no
ac-coupling occurs. Keep in mind that if the input is ac-coupled, then the resulting tilt as a result of the input ac-
coupling continues to be seen on the output, regardless of the output coupling. The 70-mA output current drive
capability of the THS7320 is designed to drive the video line while keeping the output dynamic range as wide as
possible.
One concern of dc-coupling, however, arises if the line is terminated to ground. If an ac-bias input configuration
is used or if a dc reference from the DAC is applied, such as S-Video C'/component P'_B/or component P'_R
signals, the output of the THS7320 then has a dc bias on the output, such as 1 V. This configuration allows a dc
current path to flow, such as 1 V/150 Ω = 6.67 mA. The result of this configuration is a slightly decreased high
output voltage swing and an increase in power dissipation of the THS7320. While the THS7320 was designed to
operate with a junction temperature of up to +125°C, care must be taken to ensure that the junction temperature
does not exceed this level or else long-term reliability could suffer. Using a 5-V supply, this configuration can
result in an additional power dissipation of (5 V – 1 V) × 6.67 mA = 26.7 mW per channel. With a 3.3-V supply,
this dissipation reduces to 15.3 mW per channel. The overall low quiescent current of the THS7320 design
minimizes potential thermal issues even when used at high ambient temperatures, but power and thermal
analysis should always be examined in any system to ensure that no issues arise. Be sure to use RMS power
and not instantaneous power when evaluating the thermal performance.
Note that the THS7320 can drive the line with dc-coupling regardless of the input mode of operation. The only
requirement is to make sure the video line has proper termination in series with the output (typically 75 Ω). This
requirement helps isolate capacitive loading effects from the THS7320 output. Failure to properly isolate
capacitive loads may result in ringing or oscillation. The stray capacitance appearing directly at the THS7320
output pins should be kept below 18 pF. One method to ensure this condition is valid is to verify that the 75-Ω
source resistor is placed next to each THS7320 output pin.
Copyright © 2011–2012, Texas Instruments Incorporated
Product Folder Links: THS7320
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