Electronic systems require careful design consideration due to a multitude of product constraints. The system must work flawlessly in hostile environments of thermal and electromagnetic energy in medical, industrial and automotive applications. Electronics failures can cause a safety concern due to device failure, incorrect readings, lockup and latchup. In non-redundant systems, the failure rates may not be acceptable and need to be improved.
In these applications, sources of interference include noisy power lines, noise from motors and solenoids, and harness induced noise. The typical solution to this problem is the use of EMI (electromagnetic interference) suppression devices (e.g. ferrites and filters). This article focuses on the use of the faraday cage or "picket fence" approach to PCB design and layout to eliminate costly EMI protection components.
EMI Design Challenge
Electronic failures can be caused by component failure, poor interconnection, incorrect readings, lockup and latchup. Naturally, the end user could experience a wide variety of system errors. Misleading or incorrect "in-range" readings are particularly dangerous as the operator, usually trusting the instrument, is now faced with a reading that appears valid but is truly erroneous.
Other problematic situations involve lockup or latchup. Lockup involves an output voltage, for instance, that is fixed at a seemingly random value. Often lockup can be cleared by a system power reset. Latchup involves a more serious failure occurrence as the fixed output voltage is accompanied by a destructive current flow. In typical non-redundant systems, the failure rates may not be acceptable and will need to be improved.
In these applications, sources of interference include noisy power lines, noise induced into power lines, noise from motors and solenoids, and signal harness induced noise. These interference sources may "awaken" latent lockup or latchup failure modes; therefore it is imperative to protect these circuits from disruption or damage.
Using the illustrative example of industrial automation, we can readily see that electromagnetic interference can come from a number of sources. Industrial EMI treats can appear in the form of: 1) miswired electronics after preventative maintenance, 2) corroded ground connections, 3) signal / voltage loss from long factory cables, and 4) induced current spikes from motors and solenoids.
What is unique in the industrial case is the dynamic, movement filled setting that requires strict compliance to EMI safe guidelines. To make matters worse, often the most difficult electrical noise issues take a great deal of time and expertise to correct, often requiring an outside consultant. In an industrial setting, this type of problem can be very expensive if, for example, it results in a "line down" situation or production halt.
EMI Prevention Techniques
There are many standard principles for designing EMI safe circuit boards and systems. The concepts are generally divided into two camps; reduce emissions or noise at the source (from high speed clock signals and oscillators) and enhancing immunity via attenuation of the interfering signal.
Fortunately, there are many techniques for EMI suppression that engineers have at their disposal. For factory automation or industrial markets, the following methods can be used: 1) minimize cable lengths especially for small signal voltages and use digital communication protocols where ever possible (keep your signals in the "digital domain"), 2) replace shielded wiring with fiber optic systems to boost immunity, 3) protect against differential mode transients with input shunting capacitors, 4) question all grounds and make sure that ground is really "ground" by tracing it back to its source (for example, 60 volt ground differentials have been reported in poorly setup industrial facilities).
Another recommended approach is to co-locate, for example, in the same module, the application sensor with the mating sensor signal conditioning. See Figure 1.
Figure 1. Typical industrial applications.
One of the most cost effective and powerful design techniques used for EMI suppression is the "picket fence" or grounded via approach. It has been observed that this approach can attenuate greater than 80 dB of noise from 100 KHz to 1 GHz. In fact, the picket fence design layout can isolate sensitive analog circuits up to 5 GHz, if used properly. The picket fence is essentially a faraday cage of PCB vias around your device that serves to trap emitted noise and sink away induced EM noise from external sources.
In this dual protective role, the picket fence can isolate analog inputs from the hazards of radiated emissions, voltage dropout, coupled and RF interference. In layout, the fence consists of typically 0.2mm (spaced 2.5mm) vias for the device interconnects and 0.8mm (spaced 2.5mm) for the grounded "fence" that surrounds the design components. See specification SAE J1752/3 for more details of the PCB layout. See Figure 2.
Figure 2. Example of picket fence layout.
Another excellent EMI design tip is to always use a low impedance ground plane for the PCB design and to select the most optimal component grounding scheme, that is , single point parallel (the most effective and practical method) grounding.
Single-point series. Do not use because the output of each circuit is referenced to different points (due to differing supply currents).
Single-point parallel. Good for low frequency EMI protection because this method overcomes the problems with the Single point series grounds. Use for all circuits operating below clock of 10MHz.
Multi-point parallel. Good for high frequency EMI protection because it allows for a lower ground impedance. Use for circuits operating above a clock frequency of 10MHz. This design may not be practical in applications where you cannot connect your grounds to a large grounded substrate.