Must I protect absolutely everything with ESD/EMI filters?

[torinwalker]

Hi Everyone.

I'm presently designing in ESD/EMI protection ICs for all the interfaces I suspect may be subject to ESD and/or will be susceptible to noise, or contribute to noise propagation. The design is a simple PDA device contained by an ABS/PC shell with the following interfaces:

- resistive touchscreen
- on/off button (insulated from user by mould-injected plastic pushbutton)
- reset button (also insulated by a pushbutton)
- receiver (i.e. telephone earpiece)
- loudspeaker
- microphone
- audio/mic (headset) jack
- micro SD
- SDIO
- USB

Of all the interfaces, I see six items that may not require ESD/EMI protection, but then I'll take any reasonable argument to the contrary. The resistive touchscreen is plastic, the loudspeaker, receiver and mic are separated from the user by the plastic case through holes that (I believe) would be unlikely to permit an ESD jump from placing it near one's ear, etc. - primarily because one must pick up the device to do so, and the buttons are insulated by several mm of plastic. Also not likely to conduct.

I'm not so sure about the touchscreen though. It is quite likely that someone will shuffle across the floor, reach over, and touch the screen before picking up the device, in which case ESD transients would be suppressed by a protection device.

All the others - particularly the USB, Audio, and SD interfaces should obviously have ESD built in. Luckily, ESD is built into the USB transceiver I'm using.

Secondly, since many devices combine ESD with EMI filtering, perhaps it makes sense to protect all interfaces that may pick up on noise with the added security of ESD protection. This may help reduce errors induced by noise. Could it help with EMI testing later on? What about Federal EMI certification (Canada/US)?

What are your thoughts on this? Every chip costs between 0.30 cents up to more than a buck per IC. At this rate, I'll have added nearly $10 to the base cost which translates to added cost the user will likely not perceive as value add.

I will add these devices, but not merely because the people who manufacture them managed to find ways to describe every imaginable situation where their devices are a must-have in one's electronic design. Surely there must be a line drawn somewhere between devices that actually are necessary, and ones where -- perhaps -- the manufacturer is stretching the ESD/EMI protection story a little bit.

Thanks, guys (and gals). I appreciate any help you can offer.


Torin...

 

[kender]

Hi Torin,

My rule for ESD is to protect only the high impedance inputs. By this rule, for example, the speaker would not need ESD protection, because it has low impedance output(s) and a passive speaker itself.

- Nick

 

[Iouri]

It is really difficult question, If you have pass approval agencies, then you must, otherwise it is up to designer if he wants to make equipment robast, and customer who will pay for it.. Also make sure if you need to do surge test, you might need to add MOV/TVS devices

 

[humber]

Re: Must I protect absolutely everything with ESD/EMI filter

It is difficult to be sure when such measures are necessary, but fear of the unknown can often lead the designer to solve the problem by reaching for expensive off-the-shelf solutions.
Many ESD problems can be solved by nothing more than a resistor or R/C network.
If you have the opportunity, experiment with an ESD gun - you will learn a lot.
Foils or PCB tracks can be designed to shield, or to redirect currents, almost for free.

This report shows how even simple devices can meet EMC requirements, and how often much effort are materials are wasted in a "me too" fashion.
York EMC **broken link removed**

 

[cyberrat]

You may find that it is better & less expensive to design in protection against everything than it is to have it fail EMC testing & need a redesign & retest.

IF you need a product testing for EMC to European standards to it can be sold in Europe then the testing is extremely expensive so you only want it done once.