r on an I/O line has to be more closely tailored to individual applications, and consequently ready Figure 23.30 gives the equivalent circuits for differential mode made filters are not normally available. A major variable is the and common mode filtering. signal bandwidth of the I/O line. If the signal bandwidth extends into the r.f. range, as for example 10Mbit/s digital interfaces or 23.14.2.1 Safety requirements video lines, then a simple low pass filter cannot be used except for u.h.f. and above. Conversely, a slow signal such as from a transducer The values of CY1,2 are nearly always limited by the allowable earth or switch can easily be filtered with a simple capacitor. return current, which is set by safety considerations. This current is A low pass filter may affect the signal waveshape even if its cut due to the operating voltage at mains frequency developed across off frequency is higher than the signal bandwidth. More complex the capacitors. Several national safety authorities define maximum filter components with very steep cut off characteristics are becom- earth current levels and these depend on the safety class of the ing available to address this problem. equipment and on the actual application. Values range from 0.25mA I/O filters may also be required to clamp transients to a safe level, to 5mA. determined by the over voltage capability of the circuitry inboard of cm(L) cm(N) L Ikg Figure 23.30 Filtering the mains port: (a) differential mode currents; (b) common mode currents; (c) equivalent circuit for differential mode; (d) equivalent circuit for common mode 23/24 Safety and EMC the filter. This is invariably achieved by a combination of low pass lead can be increased, thereby creating a -section filter, by separa- filtering and transient suppression components, such as zener diodes ting the ceramic metallization into two parts and incorporating a or varistors. A discrete component approach may suffice for many ferrite bead within the construction. applications, but where fast rising transien