Hi, I normally connect Zener diodes for ESD protection to ground so that the ESD event will be shunted to ground. However i have noticed a design that actually connects the ESD zener diode to the power rail (3.3V). Does anyone know why it is done this way instead of shunting it to ground? What would be the advantages and disadvantages of doing it this way?
Thanks in advance, Kean
1) you said "zener". Reasonably abrupt knee zeners are above 5.6V which is not too good for such low voltage devices. 2) tied to the supply line, the forward region would prevent the voltage from going about 0.6V more positive, and the negative zener region would prevent the input from going (6.0V-3.3V =) 2.7V below ground (ASSuMEing the power is on). 3) i am assuming the devices can withstand short negative input energy bursts without damage. CMOS cannot withstand energy bursts above the supply rail.
On 27 Dec 2004 23:55:14 -0800, kean_a snipped-for-privacy@yahoo.com (kean) wroth:
Voltages are always two terminal sources. In other words, a voltage at one point must be specified with respect to another point somewhere.
If you expect ESD voltages to be referenced to ground, then it makes sense to return the diode to ground. If, on the other hand, the ESD is expected to be referenced to the power rail, it makes sense to return the diode to the rail.
ESD protection design is no more or less important and worthy of careful consideration than any other part of a design.
Jim
And another
Solve my problem
post from a google information leech.
Well..... it would avoid 'ground noise' that might cause malfunctioning.
As others have noted, zeners aren't really good at this. Small signal diodes to ground and supply make more sense. You'll need a smallish value limiting resistor before them too. Most chips now actually have ESD specs for their I/O but the above gives extra 'security'.
Graham
Thanks for the answers. They were really helpful.
I was wondering also another possible problem with connecting a zener diode to ground would be that if the ground is too noisy, the diode may actually turn on causing noise signals to travel from ground into the signal line.
Noisy with respect to what?
If the signal ground is noisy the noise is already injected.
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