Protecting CMOS 4069UB Outputs from ESD When Using Off-Board Controls

I designed a magnetometer, which worked fabulously.  But then it suffered massive blowout.  Twelve out of sixteen amplifier stages.  Everything had been on board -- then I wired controls via cables off board.  I pulled on a jersey and took it all up Table Mountain to test it in 'magnetic silence'.  But with me wearing a jersey, and controls now open to fingers, I blew it (almost) all out with static.  A redeeming grace was that I foresaw that that could happen, and I could plug in new amplifier stages.  Now I want to protect my circuit against static, using series resistors with the off board controls.  But how far would I need to go?  Protection on CMOS inputs, yes.  But CMOS outputs?   What would you advise?  Assuming reasonable protection against static, not extreme.  My twelve amplifier stages are CMOS 4069UB.

Thomas....ah, Table Mountain, I am envious!  Cape Town is I reckon one of the most beautiful cities in the world, wherever you are you can see the mountains, or the sea, or both.  Let me know if you need someone to carry your gear for you....   But you would need a jumper at this time of year...If you're testing, think about wearing a grounding strap between you and the chassis of your equipment (is it in a metal box?)  You can protect just about any output (or input) with a couple of the ubiquitous and cheap 1n4001 (any 1N400x really) diodes wired like this:
The top diode will clamp the output to the + line if it goes 0.6V over the supply, and the bottom diode will do the same if it goes negative.  You can put a small resistance in your output line (the right arrow) to further isolate it but this will depend on what you are doing with your output.  You should also put a Zener diode across your supply rails rated at just over what the maximum possible supply voltage would be.If the frequencies you are using are very high the diode capacitance (given as ~30pf) may interfere with your circuit operation, but CMOS (even UB) is pretty slow so I doubt this would be a problem.  You maybe able to find faster diodes with less capacitance if you need, but they'd be a bit more expensive.Hope this helps / cheers // David

Thank you David, and for the explanation. Here is one example of what I have done which left me with a dead magnetometer. I use European conventions:
All in all, my design was working very nicely, picking up a few nanoteslas in a magnetically noisy city environment -- and happily I have brought it back to life -- for now.
So once P1 (above) and other controls were moved off board, the 4069UBs blew out. I thought of inserting 10k resistors at the CMOS inputs only, of which I would then need four in all. If I put diodes at the inputs and outputs as described, I shall need ten or more. Incidentally this is low frequencies, below 20Hz.I have used series resistors in the past, and they have prevented damage. What do you think? Would I have fair protection with 10k resistors at the inputs only?  Should I really insert the diodes, which would be quite an operation?
I live on the slopes of Table Mountain, just 1½ blocks from a World Heritage park -- so high up that I hear car tyres squealing on the slope outside my door.  Sir Francis Drake called it the fairest cape in all the world.

ThomasAside from David's suggestion there are all kinds of Transient Voltage Suppressors (TVS, Transzorb) and MOVs that you can put on your output lines.  They will add capacitance (although there are low capacitance TVSs ) and perhaps your circuit cannot handle that. Your proposed series resistors are a great idea especially in conjunction with the diodes or TVS, but will also possibly affect your signal with an RC network on the end of it.I would make sure that your circuit has the protection, but I am suspicious that this was a static electricity event. 12 out of 16 stages- are they all outputs or are they stages in a a few amplifiers? Was there any pattern in the units that failed- what I mean is if there were a series of 3 gates as an amplifier, was only the last one blown, last two? That sort of thing.
 Were you aware of any static discharge? Once you had replaced the amplifiers and made your measurements, did you try any more "finger poking" to see if the unit would fail again?I am not saying that it is impossible, but from everything I have seen (although it is a lo-o-o-o-ng time since I worked with unbuffered CMOS) 12 devices at once seems a lot. I would also be looking at the power supply, and wiring including the signal returns and possibly even floating CMOS inputs on unused gates.I appreciate your foresight in preparing for the worst- I have worked in many remote locations (like Ellisras in what was the Northern Transvaal, Mnjoli dam in Swaziland, or even London Ontario) where solving and repairing an issue where there are no resources around can prove problematic. The contents of a good toolbox have been the subject of several blogs from Max and David, amongst others.

Thank you David and Aubrey. I really appreciate your advice.Purely out of curiosity on David's reply, I see that the 4069UB schematic shows similar protection on the inputs. Why then is this IC still so sensitive to ESD?Thank you Aubrey, I ran with resistors only, and Table Mountain tests this morning went fine. Yes it could well be that gates remained intact. The overall result, though, was that two ICs (twelve gates) needed replacing.
The result of my tests: my magnetometer reliably detects 3.1 nT. The electronics still need to be housed, though, before the device is operational.

Thomas, I don't have figures but the diodes on CMOS inputs are not I think capable of absorbing a lot of power.  And I am also unsure of how sensitive the outputs are to static discharge - less than inputs, to be sure, but probably still susceptible. Given your circuit and confirmation that the diode capacitances should not affect the frequencies you are working with, you could probably put resistances up to 10K on inputs and outputs, and then put the diodes after those,  The diodes should catch most of the energy of a static discharge (and bear in mind this can be considerable) and the resistors will limit any residual effects on the ICs.  The total cost will not be much and is probably worth it for peace of mind. If space is at a premium then SMD components will make the best use of it.When you house your creation, consider a metal case and (grounded) screened cabling to the controls,