DESIGN AND DEVELOPMENT OF MEDICAL ELECTRONIC ...

152 ELECTROMAGNETIC COMPATIBILITY AND MEDICAL DEVICESFigure 4.2 The characteristic voltage spectrum envelope of emissions by every logic gate is highlydependent on the technology being used.integrated filters are available for dc power lines. For example, Figure 4.3 shows the wayin which muRata BNX002 block filters are used to filter the raw dc power outputs producedby two C&D Technologies’ HB04U15D12 isolating dc/dc converters. In the circuit,each dc/dc (IC1 and IC2) produces unregulated 24 V (12 V if the center-tap common isused), which is isolated from the 15 V dc power input by an isolation barrier rated at3000 V dc (continuous, tested at 8 kV, 60 Hz for 10 s). The outputs of the dc/dc convertersare filtered via filters FILT1 and FILT2, which internally incorporate multiple EMI filtersimplemented with feed-through capacitors, monolithic chip capacitors, and ferrite-beadinductors. Each of these filters attenuates RF by at least 40 dB in the range 1 MHz to1 GHz. C1/C4 and C7/C10 are used to reduce ripple, and the circuits following thesecapacitors are linear regulators that yield regulated 24 V at 50 mA to the applied part forwhich this isolation power supply was designed.Another filter worth mentioning is muRata’s PLTxR53C common-mode choke coil.This family of modules is ideal for suppressing noise from a few megahertz (1 to 5 MHz,depending on the model) to several hundred megahertz (10 MHz to 1 GHz, depending onthe model) from dc power supplies. This module is useful in suppressing noise radiatedfrom the cable connecting a device to an external wall-mounted or “brick” ac adapter.ELECTROMAGNETIC FIELDSEMI standards establish that radiated-emissions test measurements should be performed ata distance of 10 to 30 m, depending on the device’s classification. For compliance testing,the device under test should be placed on a nonconductive table 0.8 m above a ground