Hi All,
motivation of the topic comes from planning to build the little gadget described in these two threads:
https://www.eevblog.com/forum/projects/picoammeter-design/ https://www.eevblog.com/forum/beginners/static-control-requirements-for-picoamp-measurements-using-ucurrent-gold/but is of more general interest. Schematic from the original thread is attached for reference. While I do understand the main workings of the circuit, I wonder about the output protection which as mentioned in the threads "was added as an afterthought". Issues I am trying to sort out are the following:
1) Really the only realistic threat I can imagine to appear there is ESD, or am I overlooking something?.
2) I have hardly ever seen a circuit with ESD protection on the output (at the input yes, at the output really only overcurrent and kickback in case of inductive loads). So is ESD protection at an output really needed? Or are IC output structures by themselves typically strong enough to survive?
3) Now I wouldn't mind putting in a few etra components just to be on the safe side, but then I would want them to really work in case of an ESD event. By IEC-61000-4-2 one would have to expect peak currents up to several 10A depending on the test level, albeit only for durations less than ~100ns. I wonder wether the Zeners would survive this without anything between them and the ESD source?
4) Zener datasheets show thermal resistance only for much longer pulses. Roughly extrapolating these values and doing the thermal calculations gives permissible peak currents of <2A for the BZX55 series and <5A for the ZPD series as examples. A spice simulation shows (despite of all the involved approximations) that adding a current limiting resistor of ~1...3.3k would be needed to bring the current down to that range. With a small shunt capacitor of ~100 ... 300p the resistor value could be reduced (of course stability of the opamp would need to be reconsidered). Is a careful selection of diode type or the inclusion of further components necessary to make the protection work or am I overly concerned?
If anyone with more experience on thse specific issues can help, I would greatly appreciate!
Best regards,
Roland