DESIGN AND DEVELOPMENT OF MEDICAL ELECTRONIC ...

190 ELECTROMAGNETIC COMPATIBILITY AND MEDICAL DEVICESconnected to perfectly tuned dipole antennas. However, more severe losses can beassumed for less perfect situations, such as when the length of the signal line underconsideration is much shorter than one-fourth wavelength of the offending spectralcomponent for which the threat analysis is conducted. Assume for this example thatthe line of interest presents an impedance of 100 Ω at the frequency of interest andthat the coupling between the offending source and the signal line under analysis is10 dB below ideal.4. Calculate the coefficient to be used for the offending signal at the victim circuit. Inthe example 136(dB) 14(dB) 10(dB) yields 112 dBµV across the signal line’sload impedance of 100 Ω.5. Convert this level back to linear units: 112 dBµV 400 mV. This will be the voltageinduced by the offending source at the frequency of interest on the signal line underanalysis. Note that if the load impedance increases, so will the induced voltage. Forexample, for a 100-kΩ load, the induced voltage will be as high as 2 V.6. Compare the induced voltage levels against typical circuit threshold values at all susceptiblefrequencies. For example, if 26 MHz is within the bandwidth of the processingcircuit connected to the line under analysis, and since the threshold value forthis example was chosen to be 5 mV, the protection level required for a load impedanceof 100 Ω would be 20 log(400 mV/5 mV) 38 dB. For a 100-kΩ load impedance,the protection need would increase to 52 dB.With this approximation in hand, it is possible to select shielding, grounding, and filteringcomponents that will afford a combined protection that surpasses the estimate by a certainsafety margin.Shields Up!Shielding and grounding (reflection and conduction) are the primary methods of guardingagainst EMI entry and exit to and from a circuit. Chances are that you will not build yourown enclosure. Rather, you will probably use an off-the-shelf case or hire an enclosuremanufacturer to supply you with custom-made enclosures. In either case, look at the enclosure’sdata sheets for EMC specifications. The authors’ preference is to use enclosureswhich have a conductive cage that is contained completely inside a plastic enclosure withoutany exposed metallic parts.If a conductive enclosure is chosen, ensure that the conductive surface is as electricallycontinuous as possible. For a split enclosure, ensure as good an electrical contact as possiblebetween the parts. Openings in the case that are required for display windows, coolingslots, and so on, must be kept as small as possible. If the size of the opening is largerthan 1 of a potential offending EMI component, use transparent grilles to close the RF gap.20Finally, ensure that unshielded lines that carry offending signals do not pass directlythrough a shielded enclosure. Use shielded cables for high-sensitivity inputs.EMI grounding requires different, sometimes conflicting considerations from thoseused to protect low-frequency low-level signal lines. The first difference is the issue of single-pointversus distributed grounding. Single-point grounding of circuits is a commonpractice in the design of low-noise electronic circuits because it eliminates ground loops.This assumption is valid only up to a few megahertz. At higher frequencies in the radiospectrum, line inductances and parasitic capacitances become significant elements, voidingthe effectiveness of single-point grounding. For example, for the 300-MHz components ofan ESD event, a 0.25-cm length of wire or PCB track acts as a one-fourth wavelengthantenna, providing maximum voltage at the ungrounded end. As such, any cable that islonger than 1 to 10 1 of offending spectral components should be grounded at both ends. If20