Author Topic: Best way to protect my voltage reference  (Read 5669 times)

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Offline David Hess

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Re: Best way to protect my voltage reference
« Reply #25 on: July 14, 2018, 01:43:24 pm »
I don't have a great understanding of the noise levels involved, but I would keep in mind noise when you start adding anything that presents itself as a high impedance.

The high impedance series element has to be enclosed inside the feedback loop of the operational amplifier.

Some designs like the diode bridge example I gave are only high impedance under overload although in that case, it still needs to be inside of the feedback loop because of offset.  Series protection using depletion mode FETs might be able to do without that.

The feedback loop removes any added noise within its bandwidth but since it needs to be protected also, care is needed to make sure it does not add its own noise.  This is much easier however since the feedback may feed a high impedance load like the inverting input of an operational amplifier.

« Last Edit: July 14, 2018, 01:45:47 pm by David Hess »
 

Offline IconicPCB

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Re: Best way to protect my voltage reference
« Reply #26 on: July 15, 2018, 07:21:52 am »
A PNP voltage clamp referenced to Vref output plus a reverse biased diode in parallel followed by a fuse provides a bidirectional "zener brier" with potentially adjustable upper voltage limit.

Transistor collector to ground.
transistor emitter to Vref.
The base via a resistor to adjustable  clamping voltage slightly lower ( a few hundred milivolts ) than Vref and independent of Vref.
 

Offline Magnificent Bastard

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Re: Best way to protect my voltage reference
« Reply #27 on: July 15, 2018, 04:43:07 pm »
The P6KE12A TVS diode has an absolute minimum stand-off voltage of 10.2V, and at that voltage will "leak" no more than 5uA.  It's the right TVS diode to use for clamping a 10V reference output that will (very likely) be set to very near 10V.  That said, every zener will create noise when you reverse bias them-- the question is how much LF noise will this diode create when you reverse bias it at 10V.  The data sheet on these devices is silent on this subject-- so it would have to be measured.  Pretty easy to do for someone with a very quiet reference (LTZ1K based) and an LF noise amplifier.  Measure the output of the 10V reference with the noise amp before and after the attachment of the P6KE12A-- and note the difference.  Worrying, pontificating, speculating and theorizing will get you nowhere-- the Real TruthTM will be known after actual measurements.
 

Offline eurofoxTopic starter

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Re: Best way to protect my voltage reference
« Reply #28 on: July 15, 2018, 06:28:36 pm »
The P6KE12A TVS diode has an absolute minimum stand-off voltage of 10.2V, and at that voltage will "leak" no more than 5uA.  It's the right TVS diode to use for clamping a 10V reference output that will (very likely) be set to very near 10V.  That said, every zener will create noise when you reverse bias them-- the question is how much LF noise will this diode create when you reverse bias it at 10V.  The data sheet on these devices is silent on this subject-- so it would have to be measured.  Pretty easy to do for someone with a very quiet reference (LTZ1K based) and an LF noise amplifier.  Measure the output of the 10V reference with the noise amp before and after the attachment of the P6KE12A-- and note the difference.  Worrying, pontificating, speculating and theorizing will get you nowhere-- the Real TruthTM will be known after actual measurements.

I order different voltages including the P6KE12A.
eurofox
 

Offline Brak

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Re: Best way to protect my voltage reference
« Reply #29 on: August 03, 2018, 05:54:12 am »
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 think I'm vaguely understanding this but I'm not sure.

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.

As I understand it, for proper, official ESD protection a series impedance is necessary.

With a voltage reference the series impedance, usually a resistor, will introduce error.  For example, a 470 Ohm resistive series impedance and a voltmeter with a 1 megohm input impedance would introduce an error of about 470 parts per million.  With a 10 megohm voltmeter impedance the error would be about 47 parts per million. Which is not good for a precision voltage reference.

I figured, and Mr. Hess seems to be saying, that shunt protection without a 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?

Would a series inductance help with ESD protection?  A ferrite bead or two, or a larger inductance?

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.

Brak

« Last Edit: August 03, 2018, 05:56:02 am by Brak »
 


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