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Show 13 which affects the noise: the less the resistance on the line then the less the noise, but the termination on the line needs to be there for impedance matching purposes. Further, in GaAs circuits or in other circuits with an insulating substrate, the effect on the conductor-to-conductor capacitance is increased as there is effectively no substrate to cut the lines of flux that cause coupling capacitance and inductance. This RC coupling acts in a fashion that is analogous to a filtering circuit; when the dv/dt of the signal edges represents a high enough frequency, and the frequency response of the coupling capacitances within the circuit interconnect acts as a high-pass filter at or below that frequency, the circuit is in trouble. The Empirical Determination of Capacitance Three empirical approaches to the determination of capacitance given circuit dimensions are presented; they complement each other by providing different relationships. The first two will be illustrated in the examples of Chapter 5, along with the results of a commercial parasitic extraction program. The first comes from a 1981 book by Arpad Barna of Hewlett-Packard Laboratories. 12 The two equations presented here are his solutions for two coplanar strips where one is above the other and where one is beside the other. First is the eql:Jation for two plates or paths where one is above the other. C is the capacitance in farads when the dimensions are in meters. L is length, W is width (both paths have equal widths) and S is the separation between the paths. e0 is 8.85 x 10-12 farads I meter and er is the relative dielectric constant (Si02 = 3.9). |
