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ESD protection to chassis gnd or isolated gnd?

Hi,

If I have an opamp that runs from an isolated DC/DC, do I protect the
inputs from esd damage by connecting the protection device between the
input line and ISOLATED ground?
Or between the input line and chassis ground? Chassis ground is
connected to the metal casing which in turns connects to ground on the
input side of the DC/DC converter.

Any ideas?
MJ
 
W

whit3rd

Jan 1, 1970
0
Hi,

If I have an opamp that runs from an isolated DC/DC, do I protect the
inputs from esd damage by connecting the protection device between the
input line and ISOLATED ground?
Or between the input line and chassis ground? Chassis ground is
connected to the metal casing which in turns connects to ground on the
input side of the DC/DC converter.

Any ideas?
MJ

I'd connect to chassis ground. In the case of lightning-strike kinds
of
transients, it's the Faraday cage that protects the semiconductors.
But, that's protection against COMMON MODE transients. Do you
also want protection against differential mode? The differential-mode
protection would, of course, have to connect to the isolated
ground (because that's one of the terminals of the differential
signal).
 
I'd connect to chassis ground. In the case of lightning-strike kinds
of
transients, it's the Faraday cage that protects the semiconductors.
But, that's protection against COMMON MODE transients. Do you
also want protection against differential mode? The differential-mode
protection would, of course, have to connect to the isolated
ground (because that's one of the terminals of the differential
signal).

Ok, I wasn't thinking about lightning kind of strikes, more like, walk
across the carpet and zap the input with your finger senario.
Yes, now that you mention it, common AND differential mode protection
would be good. Do you suggest I use 2 protection devices to the
different grounds?
 
E

Eeyore

Jan 1, 1970
0
Ok, I wasn't thinking about lightning kind of strikes, more like, walk
across the carpet and zap the input with your finger senario.
Yes, now that you mention it, common AND differential mode protection
would be good. Do you suggest I use 2 protection devices to the
different grounds?

Using 'different grounds' is a recipe for disaster.

Graham
 
R

Rich Grise

Jan 1, 1970
0
Actually, that is a "lightning kind of" strike. :)
Using 'different grounds' is a recipe for disaster.

I think he's talking about chassis/safety ground (earth) vs. circuit
common.

Cheers!
Rich
 
I think he's talking about chassis/safety ground (earth) vs. circuit
common.

Cheers!
Rich

Hi Rich,

I am talking about ISOLATED ground vs NON-ISOLATED ground (the non-
isolated ground is also connected to a metal chassis which may connect
to mains/safety earth). To which one should the protection device
connect and why?
 
D

David Starr

Jan 1, 1970
0
Hi,

If I have an opamp that runs from an isolated DC/DC, do I protect the
inputs from esd damage by connecting the protection device between the
input line and ISOLATED ground?
Or between the input line and chassis ground? Chassis ground is
connected to the metal casing which in turns connects to ground on the
input side of the DC/DC converter.

Any ideas?
MJ
I would return your ESD protection circuit to the op amp power supply
ground, which ever one that is. What pops the semiconductor is high
voltages between pins. The ESD circuit is supposed to switch ON and
crowbar the excessive voltage to ground. If the ESD circuitry returns
to a distant ground, voltage drop alone the ground run will reduce its
effectiveness.

David Starr
 
R

Rich Grise

Jan 1, 1970
0
I am talking about ISOLATED ground vs NON-ISOLATED ground (the non-
isolated ground is also connected to a metal chassis which may connect to
mains/safety earth). To which one should the protection device connect and
why?

The protection should go to the NON-ISOLATED ground, because the ISOLATED
one doesn't have any current path to real ground to drain off the charge.

Hope This Helps!
Rich
 
The protection should go to the NON-ISOLATED ground, because the ISOLATED
one doesn't have any current path to real ground to drain off the charge.

Hope This Helps!
Rich

Still not clear - what if the circuit was powered from a battery (and
there is no other ground available), where to must the current then
flow? Also, what is 'real ground', I suppose you mean mains earth
connection?


Thanks for the help though!
MJ
 
W

whit3rd

Jan 1, 1970
0
Still not clear - what if the circuit was powered from a battery

If one applies shunt-type overvoltage protection to an isolated
ground, the problem isn't solved- your isolation is usually good
to a few hundred volts, so ISOLATION WILL FAIL. So, the
recommendation to shunt to chassis ground makes sense.

In the case of a battery powered circuit, isolation can be in the
high kV range, so maybe that would be OK. The useful properties of
DC/DC converter isolation are generally in the line-voltage range,
and that's fine for disabling ground-loop problems.

So if there are two wires coming in with a signal, one being pseudo-
ground, one should shunt from pseudo-ground to chassis, and from
signal to pseudo-ground, to protect both from common mode and
differential spikes. If neither is pseudo-ground, usual practice
(look at Ethernet) is to use a transformer and apply the shunt
from the center tap of the transformer winding to chassis ground.
 
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