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THS7374 Datasheet, PDF (21/37 Pages) Texas Instruments – 4-Channel SDTV Video Amplifier with 6th-Order Filters and 6-dB Gain
THS7374
www.ti.com....................................................................................................................................................................................................... SLOS590 – JULY 2008
discharge current. If more hum rejection is desired or
there is a loss of sync occurring, simply decrease the
0.1-µF input coupling capacitor. A decrease from 0.1
µF to 0.047 µF increases the hum rejection by a
factor of 2:1. Alternatively, an external pull-down
resistor to ground may be added that decreases the
overall resistance and ultimately increases the
discharge current.
To ensure proper stability of the ac STC control loop,
the source impedance must be less than 1-kΩ with
the input capacitor in place. Otherwise, there is a
possibility for the control loop to ring; this ringing may
appear on the THS7374 output. Because most DACs
or encoders use resistors to establish the voltage,
which are typically less than 300-Ω, meeting the less
than 1-kΩ requirement is easily done. However, if the
source impedance looking from the THS7374 input
perspective is very high, simply adding a 1-kΩ
resistor to GND ensures proper operation of the
THS7374.
INPUT MODE OF OPERATION: AC BIAS
Sync tip clamps are ideal for signals that have
horizontal and/or vertical syncs associated with them.
However, some video signals do not have a sync
embedded within the signal. If ac-coupling of these
signals is desired, then a dc bias is required to
properly set the dc operating point within the
THS7374. This function is easily accomplished with
the THS7374 by simply adding an external pull-up
resistor to the positive power supply, as shown in
Figure 39.
+3.3 V
Input
CIN
0.1 mF
RPU
Input
Pin
+3.3 V
800 kW
Internal
Circuitry
Level
Shift
Figure 39. AC-Bias Input Mode Circuit
Configuration
The dc voltage that appears at the input pin is equal
to Equation 1:
VDC = VS
800 kW
800 kW + RPU
(1)
The THS7374 allowable input range is approximately
0 V to (VS+ – 1.5 V), which allows for a very wide
input voltage range. As such, the input dc bias point
is very flexible; the output dc bias point is the primary
factor. For example, if the output dc bias point is
desired to be mid-rail on a 3.3-V supply, then the
input dc bias point is recommended to be (1.6 V –
300 mV)/2 = 0.65 V. Thus, the pull-up resistor
calculates to a standard 3.3-MΩ resistor, resulting in
0.644 V. If the output dc-bias point is desired to be
1.6 V with a 5-V power supply, then the pull-up
resistor value calculates to be approximately
5.36-MΩ.
Keep in mind that the internal 800-kΩ resistor has a
±20% variance. As such, the calculations should take
this variance into account. For the 0.644-V input bias
voltage example above using an ideal 3.3-MΩ
resistor, the input dc bias voltage is about 0.644 V
(±0.1 V).
The value of the output bias is very flexible and is left
to each individual design. It is important to ensure
that the signal does not clip or saturate the video
signal. Thus, it is recommended to ensure the output
bias voltage is between 0.9 V and (VS+ – 1 V). For
100% color saturated CVBS or signals with
Macrovision, the CVBS signal can reach up to 1.23
VPP input, or 2.46 VPP output. In contrast, other
signals are typically 0.7 VPP input, or 1.4 VPP output.
As such, the output bias voltage must account for a
worst-case situation depending on the potential
signals.
One other issue that must be taken into account is
the dc-bias point as a function of the power supply.
As such, there is an impact on the system PSRR. To
help reduce this impact, the input capacitor combines
with the pull-up resistance to function as a low-pass
filter. Additionally, the time to charge the capacitor to
the final dc bias point is also a function of the pull-up
resistor and the input capacitor. Lastly, the input
capacitor forms a high-pass filter with the parallel
impedance of the pull-up resistor and the 800-kΩ
resistor. In general, it is good to have this high-pass
filter at approximately 3-Hz to minimize any potential
droop on a P’B, P’R, or non-sync B’ or R’ signal. A
0.1-µF input capacitor with a 3.3-MΩ pull-up resistor
equates to a 2.5-Hz high-pass corner frequency.
This mode of operation is recommended for use with
chroma (C’), P’B, P’R, U’, V’, and non-sync R'G'B’
signals. This method can also be utilized with signals
with sync if desired. The benefit of using the STC
function is that it maintains a constant back porch
voltage as opposed to a back porch voltage that
fluctuates depending on the video content. Because
the corner frequency of the input is a very low 2.5 Hz,
then this is still very good performance, but not as
good relative to a STC configuration.
Copyright © 2008, Texas Instruments Incorporated
Product Folder Link(s): THS7374
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