For a few different categories of interconnection from a PCB containing inputs and outputs, how much protection from ESD, overvoltage, and electrical surge should be applied?
Here are my standards:
- BNC or other physical connection logic/analog signal input from the external world on something that warrants being called an "instrument" (not some development board on my bench, but a final product that needs to impress a customer). This connection will be frequently connected and disconnected, and is likely to have bare wire adapters attached by the ordinary user while taking no ESD precautions.
This should have the maximum protection. Inputs should have a resistor sized for acceptable balance between input bandwidth and current limiting during ESD events and overvoltage application, dual diodes to the rails for primary ESD shunting away from the device, and another resistor between that diode and the device. The power rails need shunt protection from slower transients via a TVS and DC overvoltage protection via a zener or SCR crowbar.
Input network should be modeled with an RLC pulse source with component values and initial voltage conforming to the IEC 1000-4-2 ESD models, and verified that the protection and protected devices do not have their ratings exceeded. Actual testing should be performed as well.
- Multi-pin connectors such as D-sub and others that are intended to connect some other sensor or instrument to the "instrument", and that will not be changed frequently.
This one's a little more difficult. It might be very costly and take a lot of board area to put the full suite of protections on every pin in this case.
What I commonly see for these situations, is a single RC network. I wonder if the designers of these RC networks are certain that they can actually protect against standard ESD models? In my recollection from my recent SPICE experiments with ESD protection networks, RC networks just tended to shuffle charge from the external capacitor to the internal one, and causing the (presumed) protection diodes in the device to be protected to bear an excessively large surge current.
Thus, this protection usually can't meet the tougher 8kV and 15kV contact and HBM ESD models. In fact, I am not certain it is really intended to protect against ESD at all, but rather intended just to provide some noise filtering.
So perhaps most designers forgo thorough ESD protection on these multi-pin connectors, assuming that since they are not to be changed frequently, that there is little change of ESD damage occurring here?
Is this a wise practice on robust instrumentation?
- Connections from one board to another inside a chassis, assuming that reasonable protections are in place on each of the boards.
Here I think it is acceptible to provide no protections.
- Outputs from devices such as op-amps (analog) and logic chips (logic levels).
I rarely see ESD protections applied to these. Though I have tended to apply some protections to these as well. I have put SMD 0.2A fuses, to a pair of diodes to the rails. Then a resistor between my diodes and the device output pin. That way if there is an overvoltage applied, at least just the fuse blows instead of the whole output device.
Your input is of interest.