Address: P. O. Box 1457, Los Gatos,
Tidbit - September 2011
PCB-Chassis Ground Connections - Avoiding Pitfalls Due to Unintended System Resonances
PCB With Chassis Ground Connection
Abstract: PCB to 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.
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.
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
take chances with your design, engineer the system resonances involving
PCBs rather than letting system performance fall to chance.
Additional articles on this website related to this topic are:
April 2002 Technical Tidbit: Printed Wiring Board
Coupling to a Nearby Metal Plane
- June 2006, Technical Tidbit: Measuring Structural Resonances
- March 2007, Technical Tidbit: Isolating Board and Chassis Grounds - A Potential Problem
(An ESD Example)
- February 2010, Damping Board Resonances Using Discrete Resistors
- March 2010, Damping Board Resonances Using Discrete Resistors - Part 2
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Copyright © 2011 Douglas C. Smith