THS7365 Datasheet, PDF (40/53 Pages) Texas Instruments – 6-Channel Video Amplifier with 3-SD and 3-HD Sixth-Order Filters and 6-dB Gain

THS7365
SBOS467 – MARCH 2009.................................................................................................................................................................................................. www.ti.com
Figure 121 shows a typical configuration where the
input is ac-coupled and the output is also ac-coupled.
AC-coupled inputs are generally required when
current-sink DACs are used or the input is connected
to an unknown source, such as when the THS7365 is
used as an input device.
LOW-PASS FILTER
Each channel of the THS7365 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 in the receiver. Another benefit
of the filter is to smooth out aberrations in the signal
that some DACs can have if the internal filtering is
not very good. 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
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 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
S-Video Y’
S-Video C’
Y'/G'
P'_B/B'
P'_R/R'
0.1 mF⁽¹⁾
R
+V
0.1 mF⁽¹⁾
R
+V
0.1 mF⁽¹⁾
R
+V
0.1 mF⁽¹⁾
R
+V
0.1 mF⁽¹⁾
R
+V
0.1 mF⁽¹⁾
+2.7 V to
+5 V
THS7365
1 SD1 IN
SD1 OUT 20
2 SD2 IN
SD2 OUT 19
3 SD3 IN
SD3 OUT 18
R_{PU 4 NC}
Disable SD 17
5 V_S+
6 NC
GND 16
Disable HD 15
7 HD1 IN
HD1 OUT 14
8 HD2 IN
HD2 OUT 13
9 HD3 IN
HD3 OUT 12
10 Bypass SD Bypass HD 11
Disable SD
To GPIO or
GND/V_S+
Disable HD
75 W ^{330 mF(2)}
75 W ^{330 mF(2)}
Y' Out
75 W ^{330 mF(2) P’B Out}
75 W ^{330 mF(2)}
Y' Out
Bypass
SD LPF
Bypass
HD LPF
75 W ^{330 mF(2)}
P'_B Out
To GPIO or
GND/V_S+
75 W ^{330 mF(2)}
P'_R Out
R
+V
R_PU
R_PU
+2.7 V to +5 V
75 W
75 W
75 W
75 W
75 W
75 W
(1) AC-coupled input is shown in this example. DC-coupling is also allowed as long as the DAC output voltage is within the allowable linear
input and output voltage range of the THS7365. To apply dc-coupling, remove the 0.1-µF input capacitors and the R_PU pull-up resistors along
with connecting the DAC termination resistors (R) to ground.
(2) This example shows an ac-coupled output. DC-coupling is also allowed by simply removing these capacitors.
Figure 121. Typical AC Input System Driving AC-Coupled Video Lines
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