ESD Zener diode shunt to ground or power rail?

K

kean

Guest
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
 
kean wrote:

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?
Click to expand...
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.
 
<kean_a_teoh@yahoo.com> wrote in message
news:1104376608.748149.269100@c13g2000cwb.googlegroups.com...
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.
Click to expand...
Noisy with respect to what?

If the signal ground is noisy the noise is already injected.
 
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