Datasheet

75 W
CVBS
CVBS
+3.3V
R
DAC/Encoder/SOC
Y’
R
P'
B
R
P'
R
R
1
2
3
4
5
6
7
14
13
12
11
10
9
8
CVBSOUT
HDCH1OUT
HD CH2OUT
HD CH3OUT
V
S+
HDBYPASS
NC
CVBSIN
HDCH1IN
HD CH2IN
HD CH3IN
GND
DISABLE
NC
THS7373
ToGPIOController
orGND
75 W
+3Vto+5V
0.1 Fm
(1)
0.1 Fm
(1)
0.1 Fm
(1)
0.1 Fm
(1)
3.65MW
+3.3V
3.65MW
330 Fm
(2)
+
75 W
Y'/G Out'
75 W
330 Fm
(2)
+
75 W
P' /B Out'
B
75 W
330 Fm
(2)
+
75 W
P' /R Out'
R
75 W
330 Fm
(2)
+
THS7373
SBOS506A –DECEMBER 2009–REVISED AUGUST 2012 www.ti.com
Lastly, because of the edge rates and frequencies of Each filter has an associated Butterworth
operation, it is recommended (but not required) to characteristic. The benefit of the Butterworth
place a 0.1-μF to 0.01-μF capacitor in parallel with response is that the frequency response is flat with a
the large 220-μF to 1000-μF capacitor. These large relatively steep initial attenuation at the corner
value capacitors are most commonly aluminum frequency. The problem with this characteristic is that
electrolytic. It is well-known that these capacitors the group delay rises near the corner frequency.
have significantly large equivalent series resistance Group delay is defined as the change in phase
(ESR), and the impedance at high frequencies is (radians/second) divided by a change in frequency.
rather large as a result of the associated inductances An increase in group delay corresponds to a time
involved with the leads and construction. The small domain pulse response that has overshoot and some
0.1-μF to 0.01-μF capacitors help pass these high- possible ringing associated with the overshoot. The
frequency signals (greater than 1 MHz) with much greater the variation in group delay, the greater the
lower impedance than the large capacitors. pulse response overshoot will be.
Figure 103 shows a typical configuration where the
The use of other type of filters, such as elliptic or
input is dc-coupled and the output is also ac-coupled.
chebyshev, are not recommended for video
applications because of the very large group delay
LOW-PASS FILTER
variations near the corner frequency resulting in
significant overshoot and ringing. While these filters
Each channel of the THS7373 incorporates a sixth-
may help meet the video standard specifications with
order, low-pass filter. These video reconstruction
respect to amplitude attenuation, the group delay is
filters minimize DAC images from being passed onto
well beyond the standard specifications. Considering
the video receiver. Depending on the receiver design,
this delay with the fact that video can go from a white
failure to eliminate these DAC images can cause
pixel to a black pixel over and over again, it is easy to
picture quality problems because of aliasing of the
see that ringing can occur. Ringing typically causes a
ADC. Another benefit of the filter is to smooth out
display to have ghosting or fuzziness appear on the
aberrations in the signal that DACs typically have
edges of a sharp transition. On the other hand, a
associated with the digital stepping of the signal. This
Bessel filter has ideal group delay response, but the
benefit helps with picture quality and ensures that the
rate of attenuation is typically too low for acceptable
signal meets video bandwidth requirements.
image rejection. Thus, the Butterworth filter is a
respectable compromise for both attenuation and
group delay.
(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 Copyright © 2009–2012, Texas Instruments Incorporated