Static electricity can be one of those things that can leave both analog and digital circuits out of commission. Static electricity often occurs when dissimilar materials rub against each other causing charge to build up on the surface of the objects. When this discharges into an object this is known as electro-static-discharge (ESD).
ESD can be upwards of 25KV with low amounts of stored charge, and high source impedances. There are many different methods for qualifying equipment against ESD. Some include the ubiquitous CE mark that is common on consumer goods. Other standards exist for automotive, aircraft, medical, industrial, and other types of equipment.
Protecting against ESD depends on the level of ESD exposure expected, and the market and industry constraints. Devices and methods include:
Spark-gap: In this method there is a gap between two electrodes in free-air, PC board traces, connector contacts, two or three electrodes in a gas-discharge tube (GDT), or other conductors. This air or GDT gap allows the bulk of the ESD discharge to jump to ground. "Ground" may refer to an actual galvanically bonded connection to the earth or simply to circuit common. PC board gaps are common in low-cost consumer goods. GDTs are more common in telecommunications, computer, and industrial equipment. GDTs are filled with an inert gas so that they will have a very predictable breakdown voltage.
As might be guessed regarding the PC board method, layout techniques are often quite critical in getting good ESD performance in devices. Additionally, it is often a good idea to put a perimeter trace around the board edge flashed with non-corrosive metal tied to ground to minimize problems from shop handling.
Series resistance: In this method, series resistance is added to an I/O line to impede the ESD discharge. The result is a voltage drop and reduced current flow. This often is combined with Schottky diode clamps to the power supply rail and ground. Sometimes the clamping diodes are internal to the device being protected.
MOV: The metal oxide varistor (MOV) is another low cost method for ESD and for surge protection. These devices are voltage variable resistors that exhibit a sharp drop in resistance as voltage increases past a certain point. Functionally, they act similar to a Zener diode with a few caveats: They are bipolar; they have very loose voltage tolerances; they wear out with exposure to overvoltage conditions. When they finally wear out, they fail open. They can also add significant capacitive loading to signal lines. But they are low cost.
TVS/TVS arrays: The transient voltage suppressor (TVS) is another frequently used method of ESD protection. These devices are functionally Zener diodes, but with a much larger P-N junction area. As a result, they will tolerate significant reverse current with only minor degradation. These will fail short circuit in the event of overstress. The TVS comes in a wide variety of sizes and styles ranging from miniature BGA arrays for cellphone I/O protection to massive devices used for power input and mains input protection.
Good decoupling: This prevents ESD from "ringing the power rails" and causing analog or digital control circuit malfunctions. Such malfunctions can manifest as sporadic and unpredictable resets or toggling of digital circuits.
Shielding: Shielding around sensitive circuitry can help. Circuitry that may benefit from shielding includes high-gain, wide bandwidth amplifier stages, voltage references, and the inputs to data converters. The ADCs' analog input is sensitive of course, but the clock and data lines to DACs and multiplexers (muxes) can also suffer. If you have control over set-up time, that can prevent ESD glitches from doing things like confusing the DACs and switching muxes to an incorrect channel.
Good power and ground planes: These can shunt ESD into the decoupling network and prevent damage or upsets in circuits.
What ESD-related phenomenon have you seen? Are your designs fairly robust or do you find yourself with a few head scratchers in qualification? Any other methods you use with sensitive analog circuits?