Questions regarding ESD protection

[FreshmanNewbie]

I have a couple of questions regarding the Basics of ESD.
Usually, when we employ ESD protection, say placing a TVS diode on the signal line, the other end of the ESD diode terminal:

a) Will be connected to the ground if the signal is a single-ended signal.
b) If the signal is a differential signal, should the other end of the diode should be connected to the other signal pair of the differential signal or should the other end of the diode has to be connected to ground? What happens if connected to other signal pair or ground?

Can someone clarify this point?

Also, is there a concept of differential mode ESD protection and common mode ESD protection (similar to common mode and differential mode noise?)?

My basic understanding question is that when an ESD hits our PCB through a connector pin, should the ESD energy be dissipated through our board ground or is it better if the ESD energy is made return back to its source? Can it be returned back to its source?

 

[KlausST]

Hi,

a) b) it depends. There is no general rule.
Usually you want to protect a device. So read it´s datasheet. It tells you the limits.

Some devices specify limits w.r.t. GND. (example: 74AHC familiy specifies upper input voltage limit with 7V w.r.t. GND)
Some specifiy the limits w.r.t. VCC. (many other 74xxx family specifies upper input voltage "VCC + 0.5V")
Some specify the limits w.r.t. another pin (some OPAMP input pins, one input w.r.t. the other input)

Klaus

 

[FvM]

Ground connection is always required to clamp common mode transients.

 

[FreshmanNewbie]

Ground connection is always required to clamp common mode transients.

Thank you for your answer. Could you please help me understand what do you mean by common mode transients? Any image would help?
Also, are there any differential mode transients too?

 

[dick_freebird]

Clamping one differential-pair pin to the other will protect only
against an ESD threat applied to one and returned from the other.
That is very specific, not the comprehensive "handling ESD" that
manufacturers try to incorporate.

Now your line standard might include specific cross-line threats
besides basic ESD, that comes down to the equipment and
application.

Current loop exists when the threat source is applied between any
two pins. The qualities, capabilities and victims all vary with that
loop (as a portion of the whole) construction.

So besides your across-pair clamps you need line-Gnd (likeliest
threat-loop) and line-VDD (all pins - all pins) figuring the rest of it,
belongs to the other I/Os themselves.