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Capacitor Selection for the FMS6410B S-Video Filter

The FMS6410B requires 0.1µF input coupling capacitors, but may be AC or DC coupled out. 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 FMS6410B, when AC coupled out, also requires four 220µF output coupling capacitors if all outputs are to be used as in the following diagram :

However, some users may use values up to 1000µF 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 FMS6410B 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 highquality, high-cost tantalums. 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 1 ? at 6MHz and with less than 2% capacitance change from 20Hz to 6MHz is a candidate for use in FMS6410B circuits. Many capacitors meet these requirements. With the right capacitors, their cost is no barrier to using the FMS6410B. [Source : Fairchild Semiconductor Application Notes]