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Digital Circuit Boards: Mach 1 GHz by Ralph Morrison

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5.6 Logic and Voltage Drops

The signal levels that arrive at a logic gate can vary for a number of reasons. We have already considered the voltage drop in the driver logic. In Section 4.12, we discussed how skin effect increases the trace resistance. The voltage drop in traces can best be understood by first considering the resistance of a bar of metal.

The dc resistance of a bar of metal having a cross-section A and a thickness h is ρs/A, where s is the length of the bar and ρ is the resistivity. If the bar of metal is a square then the square has a resistance of

5.1 5.1

When the thickness h of the bar is small compared to width and length, the conductor takes on the form of a sheet of metal. When the sheet of metal is used on a circuit board, it is referred to as a ground or power plane.

A sheet of metal can be characterized by the resistance of a square of that material. The term used is ohms-per-square or Ω2. The assumption that must be made is that the current flows uniformly across the square between opposite edges.

N.B.
A square of material has a resistance between opposite edges that depends on thickness and not on the size of the square. If two squares are placed in series and the current flows in the series connection, the resistance simply doubles. If the current flows across the width the resistance is one half.

The term two-ounce copper means that two ounces of copper ...

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