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WHAT'S YOUR ENGINEERING QUOTIENT?

Problem 1—Johnny is tasked with designing the trigger circuit for the free world's latest super-secret weapon. This circuit must be built on a double-sided PCB to reduce cost and it must operate in an extremely noisy environment with considerable ESD and EMI.

The device is triggered by an edge-triggered signal called "Fire" between U1 and U2. Johnny is concerned about ground loops in the PCB layout so he made sure there are no "loops" in the ground copper on the PCB by leaving an opening in the ground trace surrounding the PCB (see the figure).

Explain why we are all doomed.

The term "Ground Loop" does not refer to physical loops in the ground circuit but to a voltage differential across two points which should be at the same potential (ground).

Physical loops in the ground circuit can be beneficial in that they provides multiple return paths for the various signals and can reduce the area enclosed by the signal. Since the current induced by a changing electromagnetic field is directly proportional to the magnetic flux enclosed by the conductor and, therefore the area, reducing the enclosed area will reduce noise susceptibility.

The area enclosed by the signal "Fire" in Johnny's PCB layout is the total area to the left of the "Fire" trace and ground, which is nearly the entire area of the PCB. This results in high noise susceptibility. Closing the gap in the ground so the ground trace completely surrounds the PCB would decrease the area enclosed by the Fire signal and its return path. The area would be the amount to the right of the "Fire" trace for the signal low and the area through the bypass capacitors and to the right of the "Fire" trace for the signal high.

Termination of the "Fire" signal should take into account the area circumscribed by the signal and return path along with the magnitude of the interference expected. The wisdom of using an edge-triggered signal for Fire is a different issue.

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