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

Electromagnetic compatibility 287off characteristics are becoming available to address this problem.I/O filters may also be required to clamp transients to a safe level, determined bythe overvoltage capability of the circuitry inboard of the filter. This is invariablyachieved by a combination of low-pass filtering and transient suppression components,such as zener diodes (section 4.1.7) or varistors. A discrete component approach maysuffice for many applications, but where fast rising transients are expected the lead andwiring inductance can have a significant effect on the circuit’s ability to clamp the edgeof the pulse. In these cases a combined capacitor/varistor component, in which thedielectric has a predictable low-voltage breakdown characteristic, can offer a solution.8.6.4 Feedthrough and 3-terminal capacitorsAny low-pass filter configuration except for the simple inductor uses a capacitor inparallel with the signal path. A perfect capacitor would give an attenuation increasingat a constant 20dB per decade as the frequency increased, but a practical wire-endedcapacitor has some inherent lead inductance which in the conventional configurationputs a limit to its high frequency performance as a filter. The impedance characteristicsgiven in Figure 3.19 show a minimum at some frequency and rise with frequency abovethis minimum. This lead inductance can be put to some use if the capacitor is given athree-terminal construction (Figure 8.17).Z S Z LZ S Z Lstray inductancelimits hf attenuationTwo-terminal capacitorThree-terminal capacitorsurface mount 3-terminal capacitor or filterinputgroundoutputFigure 8.17 Two versus three terminalsground planeThe lead inductance now forms a T-filter with the capacitor, greatly improving itshigh-frequency performance. Lead inductance can be enhanced by incorporating aferrite bead on each of the upper leads. The ground connection, of course, must be asshort as possible to maintain low inductance. The 3-terminal configuration can extendthe effectiveness of a small ceramic capacitor from below 50MHz to upwards of200MHz, which is particularly useful for interference in the vhf band. The constructioncan easily be implemented in surface mount form as also shown in Figure 8.17, againwith the essential proviso that the component’s central ground terminal must beconnected directly to a good ground plane.FeedthroughsAny leaded capacitor is still limited in effectiveness by the inductance of the connectionto the ground point. For the ultimate performance, and especially where penetration ofa screened enclosure must be protected at uhf and above (this is more often the case for