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).
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?
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
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