The Circuit Designer's Companion - diagramas.diagram...

288 The Circuit Designer’s Companionmetallisationbulkheadceramic dielectricinner conductorferritesimple capacitorπ-sectionFigure 8.18 The feedthrough capacitormilitary equipment) then a feedthrough construction is essential.Here, the ground connection is made by the outer body of the capacitor beingscrewed or soldered directly to the metal screening or bulkhead (Figure 8.18). Becausethe current spreads out through 360˚ around it, there is effectively no inductanceassociated with this terminal and the capacitor performance is maintained well into theGHz region. The inductance of the through lead can be increased, thereby creating a π-section filter, by separating the ceramic metallisation into two parts and incorporatinga ferrite bead within the construction.Feedthrough capacitors are available in a wide range of voltage and capacitanceratings but their cost increases with size. Cheap solder-in types between 100pF and1000pF may be had for a few tens of pence, but larger screw mounting components willcost £1−£2. If you need good performance down to the low-MHz region then you willhave to pay even more for it. A cheaper solution is to parallel a small feedthroughcomponent with a larger, cheaper conventional unit which suppresses the lowerfrequencies at which physical construction is less critical.Circuit considerationsWhen using any form of capacitive filtering, you have to be sure that your circuit canhandle the extra capacitance to ground. This factor can be particularly troublesomewhen you need to filter an isolated circuit at radio frequencies. The RF filtercapacitance provides a ready-made ac path to ground for the signal circuit and willseriously degrade the ac isolation, to such an extent that an RF filter may actuallyincrease susceptibility to lower frequency common mode interference. This is afunction of the capacitance imbalance between the isolated signal and return lines(Figure 8.19), and it may restrict your allowable RF filter capacitance to a few tensof pF.capacitance imbalance allowscommon mode signal to develop adifferential mode interference currentFigure 8.19 Unbalance between feedthrough capacitors