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How do I implement ESD protection for a voltage reference?

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Brak:
I knew something about electronics thirty years ago, but I've forgotten everything I once knew.  Re-learning at my age is going slow.  I may have some of the terminology wrong, or even be misunderstanding something basic.  Everything below is on an "as I understand it" basis.

I'm going to build some sort of semi-precision voltage reference that I'll have to carry around and find someone or someplace to measure the voltage precisely.  I'd like as robust ESD protection as possible but am not sure how to go about it.

It will quite likely use an op amp to butter the output.  In a Metrology thread ( https://www.eevblog.com/forum/metrology/best-way-to-protect-my-voltage-reference/msg1667924/?topicseen#msg1667924 ) about ESD protection for a voltage reference David Hess replied, in part:


--- Quote from: David Hess on July 12, 2018, 11:51:43 pm ---You protect an output the same way that you protect an input.  Add series impedance to limit the current and shunt protection.

[snip]

For protection against ESD, a shunt capacitor and shunt diodes may be sufficient.

--- End quote ---

Any resistance in the series impedance would introduce an error of about a microvolt per ohm for a voltmeter with a 1 megohm input impedance if my math is correct.

I figured, and Mr. Hess seems to be saying, that shunt protection (capacitors, zener diodes, transient voltage suppressors) with no series impedance is better than nothing and might protect a device from a low energy ESD event.  Any series impedance you can fit in within your error budget would would be even better.

Am I understanding this correctly?

And finally my question.  Would an inductor in the series impedance help?  A ferrite bead or two, a larger inductor?  Inductors resist current change, ESD events seem to have a fast rise time which an inductor might slow down, maybe.  I can find a lot about driving capacitive loads with an op amp, but little about driving inductive loads.

Brak

T3sl4co1l:
Series resistance seems paradoxical, no?



You do it like this -- inside the feedback loop. ;)

Ferrite beads aren't much impedance compared to ESD, and saturate quickly besides, but yes, that's a help, especially around bipolar circuits like TL431s (if that's the sort of thing you were thinking).  Chokes work best with caps -- don't exceed the C-load limit of the amp, or, more generally, avoid the region of instability for the device (e.g., TL431 is okay for C < 10nF or whatever, and >4uF or whatever).

The above circuit also works well for dealing with C-load while keeping the active device happy: note the out-to-in cap. :)

Also also, note that TL431 is basically an opamp with a horrendous (but suspiciously stable) input offset voltage, unidirectional output (it can only pull down, not push up), and +in tied to GND internally as it were.  Other refs vary (the LM4040 series is similar, but "PNP", i.e., referenced to the more-positive voltage node, AFAIK; a lot of fancier (REFxxx, AD, LT) parts have sense inputs that basically generalize this even further), but the same method applies. :)

Tim

Brak:

--- Quote from: T3sl4co1l on August 09, 2018, 03:44:42 am ---Series resistance seems paradoxical, no?

--- End quote ---

Paradoxical, yes.

I think "inside the feedback loop" is what I was looking for.  I don't really understand the schematic you provided, it's been a long time.  But given the hint I should be able figure it out.

I haven't decided what to use.  I've got some new LM4040, other than that it's 25 year old stuff.  ICL8069, REF02, 1N829A.  And some OP177 op amps which might be suitable.  I'm thinking an "Old fashioned zener 10V reference".  https://www.eevblog.com/forum/metrology/old-fashioned-zener-10v-reference/

Thank you for your help.

Brak

Andreas:

--- Quote from: T3sl4co1l on August 09, 2018, 03:44:42 am ---The above circuit also works well for dealing with C-load

--- End quote ---

Against ESD you could have a (low inductive) capacitor (>= 10 nF) at the output.
This gives a capacitive voltage divider with the ESD capacitance (typically 150 pF).

of course you can also put a varistor or transient zener parallel to the output.

with best regards

Andreas

T3sl4co1l:
Just out of frame, actually, there are clamp diodes -- another good way to deal with that. :)  Or a zener/TVS.

Tim

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