This so-called "zero drift" opamp droops .001V/sec, at temp 85C with a 1 uF cap. If I'm reading the spec correctly, that's 3.6V/hour!
http://www.ti.com/lit/ds/symlink/lf398-n.pdf
Is there a method to store a low current V for up to about 5 hours with drift or droop within my desired resolution of approx 250 ppm, or equivalent 12 bits?
"Low current" meaning mA or uA scale.
The sample rate is between once per second and once per 5 hours.
Prefer to stay in analog domain, because I want to explore and expand my analog knowledge.
The solution should be practical, and use commonly-available components
Digital solutions are ok, but should be code-free, so it's accessible to non coders, and not require a computer to implement, so it's accessible to people who don't own a computer (such as the economically-disadvantaged teens i mentor).
Not asking for specific part numbers, just the basic method.
update:
The manufacturer confirmed that my estimate droop-per-hour is correct. According to the manufacturer, droop is largely influenced by the input bias current of the buffer amplifier and whatever leakage may occur through the switch – not just normal capacitor leakage.
https://e2e.ti.com/support/amplifiers/precision_amplifiers/f/14/p/641041/2365384#2365384
Best Answer
Well, there seem to be solutions, although it's a bit of a blast from the past...
A Survey of Analog Memory Devices (from 1962)
"The transpolarizer, an electrostatic analog of the more widely known transfiuxor..."
For a more modern solution, a micro with ADC and DAC seems the way to go. Also, unlike the analog solutions, it is much more likely to be stable with temperature, which is always a nice bonus...
About large caps: there are several problems.
Capacitor value depends on temperature, thus with a constant amount of charge in your cap, the voltage will vary with temperature. The effect will be tiny, or huge, depending on cap type.
Capacitor leakage depends a lot on temperature (for electrolytics)
X7R is a piezoelectric microphone.
Dielectric absorption means that you charge your cap, then disconnect it, wait a bit, and then the voltage on it is now different! And it depends on the voltage that was there before you charged (or discharged it). Also, for large caps intended for supply decoupling, the effect is absolutely harmless, so no-one cares about it, and therefore there is no specification. I don't know if it depends on temperature and aging but there is no reason why it would not. You will only get a useful specification for caps that are intended for high precision integrators and stuff like that.
I remember measuring leakage on a 470µF 6V3 Panasonic FR cap. I charged it to 5V for a few minutes, then measured it every few minutes. The voltage would drop quickly due to DA, then it stabilized at around 4V. I left the cap on a shelf for a week, and measured again. Calculated leakage was in the nanoamps, but you'd have to keep it at the target voltage for a while (like at least several hours if not days) in order to overcome dielectric absorption... so it would be completely useless in this application.