Stray current and parasitic capacitance in circuits at RF

Strange things can happen at high frequencies. It begins with stray current in your printed circuit board (PCB). What is stray current? Stray current starts with stray voltage, relating through Ohm’s Law (V=IR). Stray voltage refers to a situation where between two points, there exists a voltage potential that should not be there (and wouldn’t be there in ideal conditions). In many cases, a small voltage potential will exist between areas in a circuit (e.g., between two points on a printed circuit board) because the natural flow of current can induce a voltage potential (V=IR).

Stray voltage can also be the result of a dangerous fault, however. A large voltage potential can occur between equipment and the equipment chassis (enclosure) if there’s improper grounding or worn out wiring insulation that’s causing the wire conductor to come in contact with the chassis, for instance. Another seriously dangerous example of stray voltage is often portrayed in OSHA training courses, where a ground fault caused by a transmission wire or other high voltage conductor can come in contact with the earth. Dirt is especially conductive when wet or moist and can conduct current. If someone walks into the area, they can get electrocuted just by standing with their feet apart (most shoes are not sufficiently insulated), since the current will have found a path through the human body and back to the earth.

Figure 1: A typical DC-powered rail system’s running rail is also the return conductor. It’s nearly impossible to isolate running rails, however, which can easily connect to earth through debris or water, creating stray currents that can damage infrastructure and endanger passengers. (Image Source: WITT IndustrieElektronik)

Stray current, then, is the existence of a voltage potential between objects that should not be hosting a voltage potential. Stray current is a problem in railway transportation systems because it’s difficult to isolate the running rail (the return path rail for the electric train) from producing stray voltage with a current path to another element such as a pipe underground. The stray current that’s produced can cause corrosion in underground piping, for example (see Figure 1.)

In PCBs stray current can be a nuisance because it can skew expected results as it flows through the path of least resistance, wherever that may be. To make things worse, the problem of stray current changes at radio frequencies. At radio frequencies (RF), the flow of stray current no longer depends on the resistance of a circuit, but capacitance and inductance now play a role, and capacitors begin to act like a short circuit. Recall that X_C=1/(2πfC), where X_C is the reactance of the capacitor in ohms, f is the frequency, and C is the value of the capacitor. As frequency increases in value, the reactance between the capacitance-creating structures gets lower in value, eventually creating a virtual short circuit. This creates issues with respect to ground when designing circuits because any parasitic capacitance that you might have inadvertently created can then connect the reference plane of your PCB to the chassis. If this happens, mutual inductance (coupling) will create connections in separate circuits that are located near to each other and yet don’t touch each other. For an excellent, detailed review of grounding issues at high frequencies (above 10MHz), you can rely on Dr. Howard W. Johnson’s book “High Speed Digital Design: A Handbook of Black Magic” as an excellent reference.