DESIGN AND DEVELOPMENT OF MEDICAL ELECTRONIC ...

GOOD DESIGN PRACTICES, REMEDIES, AND DUCT TAPE 189To use the test system, increase the variac’s output until you read 5.55 V RMS across R1.At this point, 4.5 A should be circulating through the two loops of L1. Correcting for thedifference in magnetic field produced by a square loop versus that of a circular loop, andapplying the Biot–Savart law, the magnetic field at the center of the loop should be(4.5 A 2)/0.9 m 10 A/m.GOOD DESIGN PRACTICES, REMEDIES, AND DUCT TAPENo cure for EMC problems is better than prevention. Trouble avoidance in EMC is accomplishedby considering the emissions and susceptibility aspects of EMC at every stage ofthe design process. The following important questions must be part of the circuit design,selection of components, and packaging:• Will this part of the design generate or be susceptible to interference?• What are the characteristics of the interference?• At what frequency or frequencies does it occur?• From where is it most likely to originate?• Which radiated and/or conducted path(s) can the interference take from source tovictim?Once potential sources of interference are identified, you must decide what to do toreduce their impact. There are four broad solutions to an individual EMC problem:1. Prevention. Eliminate the sources of potential interference.2. Reflection. Keep internally generated signals inside the device and keep externalinterference outside the device’s enclosure.3. Absorption. Use filter networks and filtering materials to absorb interfering signals.4. Conduction. Divert interfering signals to the device’s RF ground.Fortunately, most of the rules and perils are known in the war against EMI. Designingan instrument to pass EMC testing is, in all likelihood, all that will be needed to ensureproper performance under real-world situations. Avoid overdesign. The authors are notaware of a single medical device malfunction attributed to interference by unknown UFOradiation. All you need to do is figure out the potential level of interference that you mayencounter, and design within these limits.Kendall [1998] proposed a simple way of estimating the amount of protection that maybe needed in a medical device to counteract an EMI threat. His step-by-step proceduredemonstrates how to estimate the protection that needs to be incorporated in the design ofan analog comparator with 5-mV sensitivity.1. Start by identifying the RF threat level. For example, if the applicable standard foryour device establishes immunity against radiated interference at 3 V/m, use thislevel for your calculations.2. Multiply the threat level by the field uniformity of the test chamber in which thedevice will be exposed to EMI. A factor of 2 is appropriate for ferrite-lined chambers,while a factor of 4 is typically used for semianechoic chambers. Assuming a ferritelinedchamber, the uniform field will be 2 3(V/m) 6(V/m) 136(dBµV/m).3. Account for losses between the source and the victim. A minimum theoretical lossof 14 dB would happen in the case in which the source and the victim are both