Analog voltage hold circuit to create a volatile analog voltage memory cell - Page 1
Forum section: electronics / projects

author: Pferd
author_rank: Contributor
timestamp: June 24, 2025, 05:52:42 pm
content: Hey guys,I need your professional help again for an idea I had lately.Imagine you have a piece of voltage controlled equipment, for example something like an old synthesizer that's controlled using various control voltages in the range of 0-10 V. Now imagine you want to switch between multiple different settings just by the push of a button. One way would be to use multiple potentiometer-banks with pots setting the voltage that you switch over from one bank to another.Now the idea I had is to only use one bank of pots and some buttons to store the set voltage into some sort of analog voltage memory cell with a capacitor.I know they used something like that in the 70s for some TV-controls, but even they didn't manage to stop the capacitor from self-discharging.My question to you analog circuit proffesionals out there: Is there any way to build something like an active analog voltage hold circuit using OP-amps to stop the capacitor from self-discharging?I purposely want to keep everything free from digital circuitry in this idea, I do well know you could just microcontroller this problem out of the world with ease:).Thanks a lot in advance, every help is welcome!

author: Doctorandus_P
author_rank: Super Contributor
timestamp: June 24, 2025, 06:03:22 pm
content: There will always be drift and leakage with analog electronics.As an experiment, you can use a simple FET opamp as a buffer (such as TL072) and also put a capacitor on it's input. This is very easily built on a breadboard, and you can use your own body to either discharge the capacitor to GND or to the power supply voltage. Then you can go get a cuppa coffee and watch how quickly it drifts without touching it.For lower drift, you can use an opamp with an even higher input impedance, or attempt to compensate the drift with extra external circuitry (such as a resistor multiplier). But the amount of leakage and drift may be dependent on the capacitor voltage and other factors (air humidity, cleanness of the PCB etc).

author: Alex Eisenhut
author_rank: Super Contributor
timestamp: June 24, 2025, 06:23:36 pm
content: You can, but as other have noted, it will drift. How about motorized potentiometers? That will keep the spirit of the thing.

author: David Hess
author_rank: Super Contributor
timestamp: June 24, 2025, 08:49:39 pm
content: With good capacitors like polystyrene, polypropylene, and maybe C0G/NPO ceramic, and a low input bias current CMOS operational amplifier like the LMC6061, LMC6081, or LMC6001, drift significantly below 1 microvolt per second is possible.

author: PCB.Wiz
author_rank: Super Contributor
timestamp: June 25, 2025, 05:14:22 am
content: Quote from: Pferd on June 24, 2025, 05:52:42 pmThe ones I recall all used low cost multi-turn pots.Quote from: Pferd on June 24, 2025, 05:52:42 pmSure, you can cobble a compromise, but itwill always have a drift spec.Quote from: Pferd on June 24, 2025, 05:52:42 pmThe modern high-volume solutions are Digital POTS with NV memory and smart DACs with NV memory, so the MCU is not mandatory, but can often simply be a cheaper NV-DAC.

author: EggertEnjoyer123
author_rank: Frequent Contributor
timestamp: June 25, 2025, 07:38:47 am
content: The HP 436A uses a similar circuit to do zeroing for the power meter.See page 183 here:https://www.sphere.bc.ca/oldsite/test/2022-9-equipment/hp-436a-service.pdfYou will need to select the FET and capacitor for low leakage. Not sure how HP did it...

author: Pferd
author_rank: Contributor
timestamp: June 25, 2025, 03:28:49 pm
content: Thanks a lot for all the good answers!In the meantime I have been simulating an active circuit to try keeping the voltage stable. So far in the simulation it does, but even the slightest drift of the resistor values makes the whole thing continuously rise or fall.I think building it this way would need the circuit to be fine adjusted using the trim pot I included. Also temperature drift would be a problem.What do you think?

author: TimFox
author_rank: Super Contributor
timestamp: June 25, 2025, 03:39:15 pm
content: The closest thing that I actually used in practice was slightly digital.A tracking sample-hold used an up-down counter controlled by a good comparator (up/down), where the counter drove a DAC whose analog output was compared continuously with the input. Stopping the counter’s clock held the state and the analog output (used for part of a tuning circuit) until the clock was re-enabled.

author: Siwastaja
author_rank: Super Contributor
timestamp: June 25, 2025, 04:42:28 pm
content: This ispossiblebecause modern multi-level flash as used in SSD drives already can store like 16 different voltage levels without drifting into the adjacent level even when powered down for years (except when they fail to do that and require to be powered-on for self-maintenance like happened with some brands). IIRC, some ISDxxx voice recorder ICs also were this kind of "analog flash memory".Now the different question will be, is any analog flash memory IC available now, and can you build it from discretes - probably not going to be easy, unless you are fine with relatively short storage or poor accuracy.

author: David Hess
author_rank: Super Contributor
timestamp: June 25, 2025, 06:00:25 pm
content: Quote from: Siwastaja on June 25, 2025, 04:42:28 pmThere used to be a company which made floating gate MOSFETs for analog trimming, but I do not see them in searches anymore. They also made IC operational amplifiers and other parts.Update:Advanced Linear Devices:https://www.aldinc.com/ald_mosfetarrays.phpReprogramming is limited so not suitable for this application.

author: mawyatt
author_rank: Super Contributor
timestamp: June 25, 2025, 06:18:54 pm
content: Quote from: David Hess on June 25, 2025, 06:00:25 pmIntersil made floating gate references awhile back. Met the guys that invented this way back at the ISSCC, interesting story on how this came about!! Don't want to clutter this thread up unless folks are interested.Best

author: mawyatt
author_rank: Super Contributor
timestamp: June 25, 2025, 07:05:18 pm
content: Quote from: TimFox on June 25, 2025, 03:39:15 pmAlso used this way back, recall was called a "Tracking ADC" back then. Very handy for tracking and digitizing slowly moving signals. Some versions had a jump up/down mode to improve the very slow slew rate where the count would jump up/down much more than 1 bit at a time. This was activated when the input magnitude exceeded the previous value by some preset values on the jump up and down comparators.Best

author: Terry Bites
author_rank: Super Contributor
timestamp: June 25, 2025, 07:39:44 pm
content: Analog will always droop becase of impossible to eradicate leakage paths. The cap will leak too. As much as you try to compensate it will not meet your ambitions.See LF398 for example. It specifies a worst leakage of 100pA. T=CV/I with a 100n (really good) cap that will give you a droop of 1mV/s. Volatile.There is way to create an infinite T&H using an ADC and a DAC back to back.Seehttps://www.analog.com/en/resources/technical-articles/infinite-sample-and-hold-outperforms-many-legacy-sample-and-hold-amplifiers.htmlExpensive! Ther are many serial ADCs an DACs you could consider here. Still VolatileOf course you can implement a multi chanel 12bit system inside an Arduino.If you want you can get it to write back to it's non-volatile memory.Job done. Cheap!I'm sure GPT will help with the code....

author: Doctorandus_P
author_rank: Super Contributor
timestamp: June 25, 2025, 08:52:52 pm
content: Have you already set a goal for the amount of drift you want it to have? And also for the maximum goal of the drift that would be acceptable?Also, your circuit probably does not look very good. Input of an opamp can easily be in the Giga Ohm range, and now you're putting an 50k Ohm impedance on it. (1x 100k parallel). That's about 5 orders of magnitude less. As a result accuracy drift and temperature effects on the resistors easily become a huge factor in real life.Doing this with a uC and some DAC's would be so much simpler. What's your reason for not wanting to go this way?

author: mawyatt
author_rank: Super Contributor
timestamp: June 25, 2025, 09:16:43 pm
content: Here's the Infinite Hold Zero Drift Analog Memory Function mentioned above. Stop the clock and the last Sampled Analog Input is held with no drift, start the clock and the tracking resumes from the previous "analog stored" value, has the added feature of creating a digital result (ADC) also.You can also write to the counter for a digital input starting value, as well as read the present value.Best

author: Terry Bites
author_rank: Super Contributor
timestamp: June 28, 2025, 09:59:59 am
content: I did something similar along while back when you could still find a counter ADC combo.Ferranti/ Plessey ZNA??? I think....those were the days!Its still a great idea but its hard to find a parallel load DAC with more than 8bits of resolution.