We hope you enjoy another guest blog post from electrical engineer Dough Smith.
PCB Chassis Ground Connections
Figure 1. PCB Chassis Ground Connection
Abstract: PCB chassis ground connections are often needed in electronic equipment, but if care is not taken, the PCB and the chassis ground structure can become a hi-Q resonant circuit that may cause problems. Background information is given and techniques to avoid problems in designs are discussed.
Discussion: Connecting PCB signal grounds to equipment chassis ground is a common feature in electronic systems although not necessarily done as shown for the experimental board in Figure 1. Problems can arise when the PCB and nearby metal, whether it is another PCB or chassis metal, form a parallel plate capacitor and the connections between them form an inductor. The combination forms a high-Q resonant circuit as illustrated in Figure 2. If this resonant circuit is tuned to a clock harmonic or other critical system frequeny, there is a risk of an EMC problem or even an operational problem for sensitive circuits. At today’s frequencies, all PCBs have significant capacitance to nearby metal and all connecting wires are inductors.
Figure 2. Equivalent Circuit of PCB and Chassis Connections
There are two ways to approach this potential problem.
Measure the resonant frequencies of the PCB chassis or other PCB structure and adjust the number of ground connections to move the resonant frequency of the structure away from critical system frequencies. See June 2006, Technical Tidbit: Measuring Structural Resonances.
- Leave the option to put a small resistor in one or a few of the ground connections (but not all) to dampen the resonance that occurs. Usually the value of this resistor is on the order of 20 Ohms. The optimum value is equal to the reactance of the PCB to metal capacitance at resonance. For further information see February 2010, Damping Board Resonances Using Discrete Resistors and March 2010, Damping Board Resonances Using Discrete Resistors – Part 2.
In my work, I continue to see problems caused by PCB to metal (including another PCB) capacitance and connections between them on a regular basis.
Summary: Don’t take chances with your design, engineer the system resonances involving PCBs rather than letting system performance fall to chance.
Douglas C. Smith
Address: P. O. Box 1457, Los Gatos, CA 95031
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