Shields and guards

One of the most frequently mishandled details of capacitive sensors is shielding or guarding. Suppose, for instance, you want to know when a truck pulls up to 3m from the metal loading door of your shipping dock. You connect a capacitive sensor to the door and find to your happiness that a truck changes the capacitance to ground from 1000 pF to 1200 pF. You set a trigger at 1100 pF and successfully detect trucks.

But you also get a trigger when people stand next to the indoors side of the door. What to do?

Simple. You add a metal guard plate, the same size as the shipping door and insulated from it, on the inside. It gets connected to the output of the high-impedance amplifier in your capacitive sensor, so that its voltage is driven to be the same as the loading door. Since the guard plate is the same voltage as the door, no current can flow, and the door "sees" no capacitance in that direction.

Now the sensor reads 500 pF instead of 1000 pF (since it only looks in one direction, the capacitance is halved), increasing to 700 pF with a local truck but totally ignoring the inside personnel. More signal, less noise. With some circuits that use a virtual ground amplifier (read the book), a grounded shield instead of a driven guard is needed, but this won't work for proximity (capacitance to ground) detectors.

This important effect, invented by Lord Kelvin more than a century ago, is useful for many accurate applications of capacitive sensors.

A similar technique is bootstrapping. For example, you may find out that your high-impedance truck detector circuit blows its input operational amplifier every time somebody hits it with a fork lift. So you add a transient voltage suppressor, but that part adds 500 pF, so your signal worsens from 700/500 pF to 1200/1000 pF. But you can recover your signal swing by bootstrapping the transient voltage suppressor, that is, connecting the "cold" side of the supressor to the same amplifier output as you used to drive the metal guard plate.

If you contemplate an application where shielding, guarding, or bootstrapping is needed, make sure the IC or circuit you choose allows this use. Not all do.