Lacking such a ground, separate ground paths can be provided. This problem can be eliminated by having a zero-impedance ground. Figure 1(a) shows the effects of a ground loop when stray interference currents divide and flow through signal ground. This condition allows interference currents to mix with signal currents, which may lead to ground interference. What do these terms mean and when are the techniques appropriate?Ī ground loop exists whenever there is more than one conductive path between two points. Whenever grounding is an issue, design engineers inevitably turn to ground loops and single-point grounds. Whatever the condition, however, device designers must provide a way for the interfering current to return to its source, and that rarely involves earth ground. The edge rates and current magnitudes are such that significant voltage bounce will occur across even the smallest length of wire or circuit-board trace. For the specific case of electrostatic discharge (ESD), transients are measured in nanoseconds (giving Fourier frequency components up to 300 MHz), and currents range to 10 A or even higher. The duration of an event can range from nanoseconds, in the case of a transient, to years, in the case of a continuous wave. EMI can cover a very wide range: currents from microamperes to amperes and frequencies from direct current to daylight. These two cases are the most commonly known uses of grounding, but the grounding requirements for EMI control in medical device applications are vastly different. At this frequency level, inductance is not important, so a length of 4/0 wire connected to the nearest building steel works just fine-an earth ground may be present, but is not needed for electrical safety. In contrast, electricians look at a ground as being a return path for fault currents, which may involve tens or hundreds of amperes at 50 or 60 Hz. Because the approximately 1-microsecond rise time produces significant Fourier frequency components up to about 300 kHz, inductance can become an important concern. In this application, the ground needs to be able to handle currents up to 100,000 A for a few milliseconds. Facility engineers, for example, look at a ground as a return for lightning strikes. The only time earth ground is necessary is for lightning.Ĭonfusion arises because the term ground is used for a variety of applications and means different things to different people. If an interference current is diverted successfully into earth ground, it will simply come out elsewhere in order to return to its source. Its purpose is to close the current loop, not to lead it into the earth. Succinctly put, a ground is a return path for current. Since it isn't, device designers need to seek ways of maximizing the effectiveness of the grounds that can be implemented. If it were possible to achieve zero impedance, all other grounding issues would become meaningless. In the overwhelming majority of medical electronic applications, good grounding involves achieving a sufficiently low-impedance return path for the highest interference frequency of interest. As the following discussion makes clear, an earth ground is not essential to EMI control and is almost never needed. Perhaps no topic in electronics is as misunderstood as grounding, which usually evokes an image of a long braid snaking off to a ground post set into a concrete floor. Previous articles have covered such means of achieving EMI control as filtering, cable shields, and enclosure shielding (MD&DI, February, July, and November 1995, respectively). Issues of patient and operator safety must also be addressed. Electronic devices can both emit and be damaged by electromagnetic interference (EMI) and must be protected from its harmful effects.
#Signal path ground bounce portable
Electromagnetic compatibility is an important consideration in the design and operation of today's sophisticated medical electronic equipment, particularly as portable systems proliferate.
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