THS7373 Datasheet, PDF (38/49 Pages) Texas Instruments – 4-Channel Video Amplifier with 1-SD and 3-HD Sixth-Order Filters and 6-dB Gain

THS7373
SBOS506 – DECEMBER 2009
Lastly, because of the edge rates and frequencies of
operation, it is recommended (but not required) to
place a 0.1-μF to 0.01-μF capacitor in parallel with
the large 220-μF to 1000-μF capacitor. These large
value capacitors are most commonly aluminum
electrolytic. It is well-known that these capacitors
have significantly large equivalent series resistance
(ESR), and the impedance at high frequencies is
rather large as a result of the associated inductances
involved with the leads and construction. The small
0.1-μF to 0.01-μF capacitors help pass these
high-frequency signals (greater than 1 MHz) with
much lower impedance than the large capacitors.
Figure 103 shows a typical configuration where the
input is dc-coupled and the output is also ac-coupled.
LOW-PASS FILTER
Each channel of the THS7373 incorporates a
sixth-order, low-pass filter. These video
reconstruction filters minimize DAC images from
being passed onto the video receiver. Depending on
the receiver design, failure to eliminate these DAC
images can cause picture quality problems because
of aliasing of the ADC. Another benefit of the filter is
to smooth out aberrations in the signal that DACs
typically have associated with the digital stepping of
the signal. This benefit helps with picture quality and
ensures that the signal meets video bandwidth
requirements.
Each filter has an associated Butterworth
characteristic. The benefit of the Butterworth
response is that the frequency response is flat with a
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relatively steep initial attenuation at the corner
frequency. The problem with this characteristic is that
the group delay rises near the corner frequency.
Group delay is defined as the change in phase
(radians/second) divided by a change in frequency.
An increase in group delay corresponds to a time
domain pulse response that has overshoot and some
possible ringing associated with the overshoot. The
greater the variation in group delay, the greater the
pulse response overshoot will be.
The use of other type of filters, such as elliptic or
chebyshev, are not recommended for video
applications because of the very large group delay
variations near the corner frequency resulting in
significant overshoot and ringing. While these filters
may help meet the video standard specifications with
respect to amplitude attenuation, the group delay is
well beyond the standard specifications. Considering
this delay with the fact that video can go from a white
pixel to a black pixel over and over again, it is easy to
see that ringing can occur. Ringing typically causes a
display to have ghosting or fuzziness appear on the
edges of a sharp transition. On the other hand, a
Bessel filter has ideal group delay response, but the
rate of attenuation is typically too low for acceptable
image rejection. Thus, the Butterworth filter is a
respectable compromise for both attenuation and
group delay.
CVBS
Y’
P'_B
P'_R
0.1 mF⁽¹⁾
R
0.1 mF⁽¹⁾
R
+3.3 V
0.1 mF⁽¹⁾
3.65 MW
R
+3.3 V
0.1 mF⁽¹⁾
3.65 MW
R
THS7373
1 CVBS IN
CVBS OUT 14
2 HD CH1 IN HD CH1 OUT 13
3 HD CH2 IN HD CH2 OUT 12
4 HD CH3 IN HD CH3 OUT 11
5 GND
6 DISABLE
V_S+ 10
HD BYPASS 9
7 NC
NC 8
To GPIO Controller
or GND
+3 V to +5 V
75 W 330 mF⁽²⁾
CVBS
75 W 330 mF^{(2) Y'/G' Out}
75 W 330 mF^{(2) P'B/B' Out}
75 W 330 mF^{(2) P'R/R' Out}
75 W
75 W
75 W
75 W
(1) This example shows an ac-coupled input. DC-coupling is also allowed as long as the DAC output voltage is within the allowable linear
input and output voltage range of the THS7373. To achieve dc-coupling, remove the 0.1-μF input capacitors and the 3.65-MΩ pull-up
resistors.
(2) This example shows ac-coupled outputs. DC-coupled outputs are also allowed by simply removing the series capacitors on each output.
Figure 103. Typical AC Input System Driving AC-Coupled Video Lines
38
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