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I am looking for a circuit that can, on input, remember a certain voltage and output that voltage indefinitely even after the input has been taken away. The circuit should not change its output until a new input has been provided.

I understand that such a circuit can be made by digitally sampling the input up to some arbitrary resolution, but I would like to know if a simple analog solution is possible.

I would also like to keep this solution purely electronic, as I can also envisage a mechanical solution in which a feedback circuit mehanically controls a potentiometer.

Finally, I would ideally not like the circuit to rely on the passive stability of any floating inputs. The circuit should be stable for at least hours.

Thanks for any help.

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Simply put, it doesn't exist and I know I'm sticking my neck out so, if anyone has a good answer to this I'm ready with the upvote button LOL. – Andy aka Aug 12 '14 at 20:15
The best way to do this is digitally (convert A->D, store, convert D->A). It's possible to do this in an analog fashion, but it will be expensive, have limited accuracy and prone to errors from humidity etc. – Spehro Pefhany Aug 12 '14 at 20:21
This circuit is called "sample-and-hold". Stability for hours will be difficult. – markrages Aug 12 '14 at 20:21
Single polarity? Can I use a relay? Aluminum electrolytics hold charge for a long time... in a temperature controlled box? Why not digital? – George Herold Aug 12 '14 at 22:25
@Andyaka See my comment on Sphero's answer. I think the ISD 256 ANALOG levels in a digital bit may qualify for your upvote :-) - even if it's only a comment. – Russell McMahon Aug 13 '14 at 9:59

This is not a practical answer unless you happen to work for a company with the resources of, say, Intersil, but the technology exists to make this work. Consider the ISL21080 type references which hold charge, hopefully for the life of the equipment they're installed into, based on a tiny capacitance isolated by quantum tunneling effects. Provided they don't get too much in the way of X-rays etc. they'll remain pretty stable for years. See, for example, this application note.

enter image description here

I might add this kind of thing gives me the willies.

For an ordinary application, digital is most likely the way to go.

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This is how cheap analog voice recorders and greeting cards work. Example: kowatec.com/prod/ap/doc/apr6016-v13.pdf – Phil Aug 13 '14 at 2:00
@phil: No, it's not. From your link: "A maximum of 30K bits of digital data can be stored." It also mentiond 256 signal levels -- that's discrete storage, not analog. – Ben Voigt Aug 13 '14 at 2:58
Well I guess all memory is analog storage to some degree. But the key point is that the analog voice recorder IC doesn't even guarantee the signal stays in the same bucket, except for a few "digital-approved" sectors. So it's not holding the value very well. – Ben Voigt Aug 13 '14 at 3:06
@BenVoigt The ISD speech recorder ICs stored 256 analog levels in a "charge well" originally intended to be a digital bit store. That's analog storage in a notionally digital bit. – Russell McMahon Aug 13 '14 at 4:25
@BenVoigt - I didn't explain that clearly enough. In the case of ISD they took a charge well in a technology ORIGINALLY intended to be used as a 1 or 0 bit store and instead in each "1 digital bit" charge well they stored incremental charges in 256 levels (they may use a companding law). They then sensed the amount of charge stored (by specific means unknown to me) and obtained a 256 analog level / 8 bit equivalent word. By doing this they increased the amount of available speech memory by a factor of 8 as what would usually take 8 bits (2^8 = 256) was able to be stored in a single bit space. – Russell McMahon Aug 13 '14 at 9:58

I left a comment and thought about it for a minute and will say with hopeful certainty that it does not exist - some drift away from the "sampled" voltage is not only likely but a certainty. Resolution is critical it seems, (as implied in your question) and this is why I say it doesn't exist. Noise is another factor that will reduce the fidelity of what you have sampled.

Even a digital system (with more than enough resolution) will be inaccurate in reproducing the voltage you apparently "stored". Anything taken to limits will be a problem. The potentiometer idea (suggested in the question) is also flawed because it relies on the reference voltage across its terminals being kept (or reproduced) - you cannot know how these things minutely drift but, again, it's all down to accepting an error or rejecting that error.

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Minute drift and noise is to be expected in all circuits, and I am not concerned with extremely precise reproduction, just something that will be a least slightly more stable than a free floating input. I would be satisfied with a digital memory solution, but was wondering if a more direct solution existed. Thanks for the input. – user2640461 Aug 12 '14 at 20:37

This device does exist although it's not readily available in single unit quantities, its output amplifiers will get in the way and it's very non-linear.

It is a Floating Gate MOSFET, used in Flash memory, EEPRom and the ilk. The programming charge can be variable though somewhat unpredictable as the FN tunnelling (Fowler Nordheim) will be variable across the die. While non-linear it is a proportional effect so you could imagine designing a circuit that linearized the programming effect (of Vth shift). It will be stable over weeks to months so it meets the requirements of hours that you say you'd need.

But a lot depends upon the specifications that you need, how much drift is acceptable etc.

Just to be clear here, I am talking about the individual device/transistor not the complete component as the support circuits of a Flash will prevent you from operating the cells in this way.

Here are 3 references from an EDN article talking about a company called GTronix which was acquired by National Semi (now TI).

Lee, BW, BJ Sheu, and H Yang, “Analog floating-gate synapses for general-purpose VLSI neural computation,” IEEE Transactions on Circuits and Systems, Volume 38, Issue 6, June 1991, pg 654.

Fujita, O, and Y Amemiya, “A floating-gate analog memory device for neural networks,” IEEE Transactions on Electron Devices, Volume 40, Issue 11, November 1993, pg 2029.

Smith, PD, M Kucic, and P Hasler, “Accurate programming of analog floating-gate arrays,” IEEE International Symposium on Circuits and Systems, Volume 5, May 2002, pg V-489.

THere is another class of device that is called a MNOS transistor (Metal Nitride Oxide Semiconductor) in which tehere are two dielectrics in the gate, one of which is Si3N4 which has a lot of traps. This device operates very similarly to the flash cell above.

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