June 1999
Application Brief
42018
Capacitor Selection for the ML6428-1 S-Video Filter
METHOD
The ML6428 comes in two versions, the -1 and the -2. All
outputs must be AC coupled into their loads for the -1
version. The -2 version is DC coupled. Both versions
require 0.1F capacitor input coupling. Finding video
quality -- i.e., low ESR, equivalent series resistance, low
inductance, low leakage, and constant capacitance over
a frequency range from 20Hz to 6MHz -- 0.1F
capacitors presents no problem. Surface mount capacitors
meeting these requirements are both inexpensive and
plentiful.
The ML6428-1 also requires four 220F output coupling
capacitors if all outputs are to be used. See Figure 1.
However, some users may use values up to 1000 F to
pass "TILT" specifications in the two-field test. In the past,
when wideband signals such as video needed coupling
capacitors in the hundreds of F's, tantalums were the
only choice.
This presents a problem. The ML6428 was designed for
low-cost applications. In the quantities needed, the cost
of the tantalums is greater than the cost of the filter and
the other associated components. There is a solution:
modern, surface mount, aluminum electrolytic capacitors
designed for use in high-frequency switching power
supplies have all the necessary qualities. For a given
capacitance and voltage rating these electrolytics are
about the same size as the tantalums. They also have low
equivalent series resistance, low inductance, and low
leakage. Since they are intended for use as bypass
capacitors in switching power supplies operating up to
1MHz, they have an adequate frequency response as
video coupling capacitors.
Several types from several manufacturers have been
investigated. Some 220F surface mount aluminum
electrolytic capacitors from Cornell Dubilier, Elna, and
United Chemi-Con were indistinguishable from high-
quality, high-cost tantalums, as can be seen by comparing
Figures 2 and 4 with Figures 3 and 5. It should be pointed
out that these are not the only suppliers of usable
capacitors. Any physically small, surface mount, 220F,
6V to 10V, aluminum electrolytic capacitor with ESR less
than 1
at 6MHz and with less than 2% capacitance
change from 20Hz to 6MHz is a candidate for use in
ML6428-1 circuits. Many capacitors meet these
requirements. With the right capacitors, their cost is no
barrier to using the ML6428-1.
Figure 1. AC Coupled S-Video and Composite Video Line Driver for NTSC
220F
220F
1F
0.1F
220F
0.1F
0.1F
220F
VIDEO CABLES
75
75
75
C*
R*
75
75
75
75
75
ML6428-1
YOUT
CVOUT
CVOUT
ON-CHANNEL
MODULATOR,
VCR, AND TV
COUT
YIN
CIN
5V
+
+
4th-ORDER
FILTER
4th-ORDER
FILTER
1
4
8
5
3
7
2
6
* C AND R DEPEND ON THE INPUT IMPEDANCE OF LOAD
REV. 1.0 10/25/2000
Application Brief 15
2
REV. 1.0 10/25/2000
Figure 4. Wideband Response with Tantalum Capacitor
Figure 5. Wideband Response with Aluminum Electrolytic
Capacitor
Figure 2. 3dB Response with Tantalum Capacitor
Figure 3. 3dB Response with Aluminum Electrolytic
Capacitor
0
10
20
30
40
50
60
70
80
AMPLITUDE (dB)
FREQUENCY (Hz)
10
100
1k
10k
100k
1M
10M 100M
0
10
20
30
40
50
60
70
80
AMPLITUDE (dB)
FREQUENCY (Hz)
10
100
1k
10k
100k
1M
10M 100M
0
1
2
3
4
5
6
7
8
9
10
AMPLITUDE (dB)
FREQUENCY (MHz)
1
2
10
3
4
6
8
0
1
2
3
4
5
6
7
8
9
10
AMPLITUDE (dB)
FREQUENCY (MHz)
1
2
10
3
4
6
8
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user.
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