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ESD protection to chassis gnd or isolated gnd?

Discussion in 'Electronic Design' started by [email protected], Jun 6, 2007.

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  1. Guest

    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
     
  2. whit3rd

    whit3rd Guest

    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).
     
  3. Guest

    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?
     
  4. Eeyore

    Eeyore Guest

    Using 'different grounds' is a recipe for disaster.

    Graham
     
  5. Rich Grise

    Rich Grise Guest

    Actually, that is a "lightning kind of" strike. :)
    I think he's talking about chassis/safety ground (earth) vs. circuit
    common.

    Cheers!
    Rich
     
  6. Guest

    I think he's talking about chassis/safety ground (earth) vs. circuit
    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?
     
  7. David Starr

    David Starr Guest

    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
     
  8. Rich Grise

    Rich Grise Guest

    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
     
  9. Guest

    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
     
  10. whit3rd

    whit3rd Guest

    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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