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 0.1µF 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 — 0.1µF
capacitors presents no problem. Surface mount capacitors
meeting these requirements are both inexpensive and
plentiful.
The ML6428-1 also requires four 220µF output coupling
capacitors if all outputs are to be used. See Figure 1.
However, some users may use values up to 1000 µF 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 µF’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 220µF 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, 220µF,
6V to 10V, aluminum electrolytic capacitor with ESR less
than 1Ω 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
220µF
220µF
1µF0.1µF
220µF
0.1µF
0.1µF
220µF
VIDEO CABLES
75Ω
75Ω
75Ω
C*
R*
75Ω
75Ω
75Ω
75Ω
75Ω
ML6428-1
Y
OUT
CV
OUT
CV
OUT
ON-CHANNEL
MODULATOR,
VCR, AND TV
C
OUT
Y
IN
C
IN
5V
Σ
+
+
4th-ORDER
FILTER
4th-ORDER
FILTER
1
4
8
5
372
6
* C AND R DEPEND ON THE INPUT IMPEDANCE OF LOAD
REV. 1.0 10/25/2000