Battery powered device ESD protection - Page 1
Forum section: electronics / general

author: michaelym
author_rank: Regular Contributor
timestamp: March 24, 2013, 10:27:29 am
content: I connected all my ESD protection components to a chassis ground layer.How and if should I connect the Chassis ground layer to the analog, digital and other ground layers?

author: AndyC_772
author_rank: Super Contributor
timestamp: March 24, 2013, 10:40:23 am
content: Does this device connect to anything else? Is there a path to ground via any external cables?During an ESD test, a completely floating battery powered product is likely to simply charge up. There won't be any discharge events to worry about, because there's nowhere for the charge to go.If there's a path to ground, then the challenge is to ensure that the ESD current doesn't give rise to any voltage which will appear across any sensitive components. A single solid, low inductance ground plane that's connected to everything can be a good way to achieve this, and this means you don't actually need separate chassis, analogue and digital grounds at all (unless they're required for other reasons). Such a topology can be better for radiated emissions too.

author: hardsoft
author_rank: Newbie
timestamp: June 06, 2013, 01:48:33 am
content: Ever drag your feet on carpet, touch a metal doorknob in the middle of a wooden door and get zapped? It is certainly possible to damage battery powered devices with ESD events. ESD edge rates are so fast that they are commonly discharged through a capacitive coupling to earth ground than through a cable to ground (and its large associated inductance at such high rates).So the goal is to avoid conductive paths to your sensitive circuits (e.g. with a plastic enclosure). Usually there are some connectors or switches or something that makes this completely possible. ESD protection right at the inputs of the connectors, switches or whatever... grounded to the local return offers some protection. Sometimes engineers will put a thin conductive material under the PCB (I've seen in plastic calculators) which may be used as a 'chassis' ground. Even though it is not tied to earth ground it provides the lowest capacitive coupling to earth ground, and then ESD protection can be grounded to this. It essentially provides the lowest impedance path for ESD to keep it off your PCB.The other option is a full metal enclosure, which is very effective at protecting internal circuitry from ESD damage due to its low impedance to earth ground at high edge rates (regardless if it is physically connected to earth ground). During ESD tests, you may be discharging to a metal enclosure easily, but the stuff inside will be safe.As far as multiple ground layers (analog, digital, power, etc.). It is a total waste of time that more often then not will make things worse, greater noise, more susceptible to radiation, etc. Unless you are an EMC guru, the PCB layout is entirely too difficult for anything but extremely simple circuits. EMC consultants make a living off this. They get paid hundreds of dollars an hour to go in and tell the engineers to tie all their grounds together all over the board (or simply re-spin with a single ground plane) and... all of the sudden, their failing PCBs can pass EMC tests (after much fighting of course as it seems gospel to needlessly use extra planes for some reason). Just use smart partitioning of analog and digital components and you will be fine. High speed current (digital signal return) returns on adjacent power plane, regardless if that be a digital ground, digital power, analog ground, power ground, or whatever. So typically, multiple ground PCBs will have high speed digital traces running adjacent to all types of different planes, and radiating all types of stuff when the trace vias from one layer to another (and therefore references a different plane). This same effect destroys any supposed benefit even if the layout was done very carefully. Digital noise isn't like waves in a wave pool bouncing around the ground plane. The return current (most of it anyways) stays under the trace, so you can use a single plane and keep digital lines from crossing analog lines. There CAN be some benefit to a proper multi ground layout but a) it must be done very carefully and b) you need to be looking at some extremely small signal or using 16 to 24 bit ADCs to even start caring.