Measuring Stuff Digitally Without an A to D Converter - Page 1
Forum section: electronics / general

author: Richard_S
author_rank: Contributor
timestamp: December 22, 2023, 03:53:54 am
content: This is just a jolly question to celebrate the end of the year.Many years ago I was fortunate to have been granted a patent for measuring light intensity. That patent raised a question that I was at that time unable to answer. The invention measured light without the use of an analogue to digital conversion process. Ever since then I wondered whether this was unique, or whether there are other analogue quantities that we can measure numerically without analogue to digital conversion.Then recently it occurred to me that not only is there another such device but I actually own one, a vintage laboratory balance. The balance has an ingenious mechanism where, as you turn a dial, weights of increasing size are placed on one side of the balance arm. These, as you would expect, are used to counter the weight of the subject on the other side until equilibrium is restored. When it all balances, you can read the weight directly from the numeric dial.So, now that I know of two, I am left wondering whether there are any other cases of turning what we would think of as an analogue quantity directly to digital without using the type of device that we would usually call an analogue to digital converter? If there are, then I bet that the community here would know.

author: jbb
author_rank: Super Contributor
timestamp: December 22, 2023, 04:55:46 am
content: Hmm. I’m sure there are things like that around.I’m a technicality: how about a good old fashioned moving coil / needle panel meter?Ooh, a classic: analog ‘spinning disc’ power meter.

author: fourfathom
author_rank: Super Contributor
timestamp: December 22, 2023, 05:11:34 am
content: Quote from: Richard_S on December 22, 2023, 03:53:54 amHow is this not digital? You are manually adding weight in discrete increments, and determining the measured value by observing when a threshold is passed. No electronics are used, instead human stimulus and response, but it's not fundamentally different from many types of A-D converter.

author: Richard_S
author_rank: Contributor
timestamp: December 22, 2023, 05:20:54 am
content: Quote from: fourfathom on December 22, 2023, 05:11:34 amAbsolutely, this is an analogue to digital process. Of course it is. I meant "without what we would conventionally call an analogue to digital converter". I did think about making that explicit but decided everyone would prefer brevity. Happy to clear that up though, without the electronic circuit that we would conventionally label an analogue to digital converter.I hope that helps.

author: Richard_S
author_rank: Contributor
timestamp: December 22, 2023, 05:25:07 am
content: Quote from: jbb on December 22, 2023, 04:55:46 amYes, of course, a basic moving pointer meter is turning an analogue quantity to a numeric one by literally pointing to the values. Good call. I think the added element of this challenge would be to look for cases where the output was still in electronic numerical form and so unlike the balance which is human-readable but not electronic as such.

author: abeyer
author_rank: Frequent Contributor
timestamp: December 22, 2023, 06:28:29 am
content: Seems like a geiger counter or anything similar where your measurement is a count of discrete events would qualify.

author: ledtester
author_rank: Super Contributor
timestamp: December 22, 2023, 06:58:27 am
content: A voltage to frequency converter is another kind of A/D converter.

author: jbb
author_rank: Super Contributor
timestamp: December 22, 2023, 08:26:39 am
content: Perhaps stretching the prompt a bit: a Kelvin-Varley divider plus a null meter (eg galvanometer)

author: wraper
author_rank: Unknown
timestamp: December 22, 2023, 09:24:00 am
content: Quote from: Richard_S on December 22, 2023, 03:53:54 amIt's still an ADC, it's just that comparator that is part of ADC is largely implemented mechanically, rather than electronically.

author: ch_scr
author_rank: Frequent Contributor
timestamp: December 22, 2023, 10:00:29 am
content: So the whole premise of this is pretending an SAR-ADC is "not an conventional ADC" somehow, or did I misunderstand?If an analogue value (a voltage or light intensity or whatever) goes in and an digital one comes out, it's an ADC by definition?First, the question becomes what is a "conventional ADC", then?A KVD+null meter is a kind-of SAR-ADC, just the interface is a bit more convoluted (and even though it's comprised of digits, would stretch the conventional meaning of digital)Edit: Reading the question again, this is aboutdirectly converting analog to digits?

author: tggzzz
author_rank: Super Contributor
timestamp: December 22, 2023, 10:08:41 am
content: Quote from: Richard_S on December 22, 2023, 03:53:54 amAllDVMs used to actexactlylike that; the basic principle dates from c1840. I own one, a Fluke 893a nominally accurate to 0.005%:Principle is that the knobs control a Kelvin Varley Divider with a known voltage applied to its "top". The knob settings determine output voltage, and the meter measures the difference between that voltage and the input voltage. When the meter reads 0, the dials indicate the voltageand zero current flows even if the input voltage is 1kV. Modern DMMs have a 10Mohm impedance at that voltage, so 100uA would be flowing.

author: guenthert
author_rank: Frequent Contributor
timestamp: December 22, 2023, 10:22:12 am
content: I don't know what we would conventionally call an analogue to digital converter. There are various schemes and they look quite different to me.One measurement just done digitally (on a high level) is perhaps electron counting, recently made possible:https://www.ptb.de/cms/en/presseaktuelles/journals-magazines/ptb-news/ptb-news-ausgaben/archivederptb-news/ptb-news-2012-2/current-standard-based-on-single-electrons-in-the-offing.htmlI believe astronomers counted photons for a while already.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 22, 2023, 10:24:03 am
content: You'll have to cite the patent, because it doesn't sound like you're hinting at anything original?For example, if your light intensity is measured by photodiode, and say you cascode the photocurrent through a BJT just to keep it more stable (wide compliance voltage range), and say you counterbalance it with another current source, this time a binary weighted series: the bit value of that "digital" current source, when bounded on one side with a gain-node voltage that saturates to one side, and the other side on the adjacent code, has performed exactly a digital conversion of the input current, to a digital value between those two bounds.SAR ADC is the same thing, comparing an incrementally-more-accurate synthetic voltage to the input reference; S-D could be considered the same as well but time-averaged through an integrator; Flash ADC is doing the same thing but the bounds are explicitly spans of myriad window comparators; etc.Or if we put a capacitor on the gain node, the current difference gives a dV/dt, which we can measure with a clock and counter, and, same thing: the instant the voltage crosses the reference threshold, occurs at some instant between two edges of the clock; the analog time is bounded.Tim

author: tggzzz
author_rank: Super Contributor
timestamp: December 22, 2023, 10:53:23 am
content: Quote from: T3sl4co1l on December 22, 2023, 10:24:03 amPatents are like Schreodinger's Cat: you don't know whether they are valid until they are challenged in court.A patent that applies a known technique in a new context can be valid, provided it isn't "obvious to one skilled in the art". Obviously "obvious" is the key word thereRecursion: see recursion.

author: Richard_S
author_rank: Contributor
timestamp: December 22, 2023, 10:57:32 am
content: Quote from: tggzzz on December 22, 2023, 10:08:41 amWonderful!

author: Richard_S
author_rank: Contributor
timestamp: December 22, 2023, 11:11:19 am
content: Quote from: T3sl4co1l on December 22, 2023, 10:24:03 amWow, amazing.My patent was a lot simpler and less clever. It was just a photon counter. Well, to be honest it was a way of using up CCD sensors that had failed inspection. A CCD from a (then) high-resolution camera would be put beneath a diffuser. The diffuser, and so the CCD, would be exposed to the light source. The contents of the device would then be clocked out into a capacitor. The voltage on the capacitor would be read by a comparator. The process would continue until the threshold was passed at which point the count would stop and provide the digital value. A value that was essentially a count of the photons. Not individual photons granted, given the noise performance of the CCD sensors, but still that is the essence. Dodgy cells could be skipped which was why reject CCDs could be used. The patent has long since expired. No idea if the company ever made one. It is irrelevant now, there aren't surplus reject high-quality CCDs anymore I would assume.

author: Richard_S
author_rank: Contributor
timestamp: December 22, 2023, 11:14:46 am
content: Quote from: guenthert on December 22, 2023, 10:22:12 amGoodness! That is incredible technology and spot on for the challenge. A very elegant solution to the issues they have in losing electrons too. Thanks for sharing that.

author: Richard_S
author_rank: Contributor
timestamp: December 22, 2023, 11:16:02 am
content: Quote from: abeyer on December 22, 2023, 06:28:29 amA geiger counter is exactly the type of thing yes. Great suggestion. Thank you.

author: coppice
author_rank: Super Contributor
timestamp: December 22, 2023, 11:17:51 am
content: Quote from: ledtester on December 22, 2023, 06:58:27 amMost voltage to frequency converters are purely analogue, converting one analogue measure to another. If you then quantise the frequency you have an overall analogue to digital converter.

author: wraper
author_rank: Unknown
timestamp: December 22, 2023, 11:33:19 am
content: Quote from: wraper on December 22, 2023, 09:24:00 amBTW most of modern high precision balances are made in similar way, the difference is them using electromagnet instead of weights and optical sensor to detect equilibrium. Load cells are used in less precise stuff. Also there is tuning-fork type.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: December 22, 2023, 11:35:29 am
content: Quote from: Richard_S on December 22, 2023, 11:11:19 amAhh, I see. So, there's a way in which it could be said to be -- skipping an A-to-D by always being D in the first place -- but, since it's still going through a capacitor, it's not, really, but "wouldn't it be cool if it were?", I guess is more where you were going with this thread?Yeah, I can see that.Such systems would be better called "quantum", since QM isn't really digital itself, but is stochastic in practice, so, it's not sharply timed digital, it's noisy, it's uncertain, and really these are all just alternative ways to say "analog noise" in the limiting (smallest signal) case, give or take exact physics of the system in question of course (not all systems exhibit full shot noise, for example).Which, put another way: digital is a strict subset of analog where we define thresholds for '1' and '0' (or any other values), but if we dial those thresholds down into the noise floor, it doesn't really matter, does it, we're counting statistics of bits equally as well as analog spectra down there. Quantum is a superset of analog, I suppose is the point then.There are indeed methods nowadays, where for example, an avalanche photodiode array can be paired with individual (per-pixel) counters, thus resolving photon counts (assuming illumination low enough that chance of overlapping events is arbitrarily small) directly. This gives an image nearly identical to a regular one, in the long-exposure limit, but the interesting part is what's enabled by more involved statistical methods, like L1-norm filtering, which, I still have to read up on, but loosely speaking and as I understand it, it's not something that's been done much before because it's computationally expensive, but in the mean time we've trivialized that, so, we can now get shockingly good image formation from rather modest photon counts.Tim

author: TUMEMBER
author_rank: Regular Contributor
timestamp: December 22, 2023, 12:27:54 pm
content: Quote from: tggzzz on December 22, 2023, 10:53:23 amThe perfect pick for every possibility.This is an interesting example of a natural A/D converter. It reacts with a change of state upon stimulation and secondary emission. Until it is stimulated by UV-A, it has a "weak green" color, but a small amount of 365 nm radiation causes an intense "YAG glow". Synthetic rubies (corundum) also have this (red).

author: coppice
author_rank: Super Contributor
timestamp: December 22, 2023, 12:46:50 pm
content: Quote from: tggzzz on December 22, 2023, 10:53:23 amIts more accurate to say its not valid until it is no longer being challenged in court. These days pretty much every court outcome is challenged until someone's spirit or cash runs out.

author: MK
author_rank: Regular Contributor
timestamp: December 22, 2023, 02:34:38 pm
content: Well photomultiplier tubes can be used both ways, count the pulses of electrons, or turn the pulsed current into an average voltage across a burden resistor at the bottom of the chain, or with a current to voltage converter at the output of the PM tube. see AofE for both modes of use.

author: joeqsmith
author_rank: Super Contributor
timestamp: December 22, 2023, 05:02:31 pm
content: I have a ruler.

author: fourfathom
author_rank: Super Contributor
timestamp: December 22, 2023, 06:39:00 pm
content: Quote from: joeqsmith on December 22, 2023, 06:19:45 pmI suppose using a gray-coded optical or electrical-contact disc? Or just a quadrature coded disc for rotation direction and speed but not absolute position. No D-A there, just multiple on/off sensors (still digital, though).

author: julian1
author_rank: Frequent Contributor
timestamp: December 23, 2023, 12:43:39 am
content: I want to say a pendulum clock/or write-watch balance, where the timing of the swinging arm/mechanism is gated and counted by ratchet gears, to provide a count that measures analog time.Although maybe it is an example of a mechanical ADC.

author: David Hess
author_rank: Super Contributor
timestamp: December 23, 2023, 02:15:35 am
content: Quote from: ledtester on December 22, 2023, 06:58:27 amIt is if you add a counter, but the output is pulse density modulation and can be used directly. Run the output into a charge pump along with another voltage, and you have a high accuracy voltage multiplier.Analog sampling oscilloscopes draw a series of dots on the screen, but they are not digital either and produce a 2D pulse density modulated image.I do not remember what it is called, but there is another converter similar to a voltage-to-frequency converter which produces two outputs, and the value is the ratio between the two frequencies rather than the absolute frequency. Some universal frequency counters have an A/B mode which can read their output directly. Maybe that is the reason A/B mode exists on old frequency counters?

author: Nominal Animal
author_rank: Super Contributor
timestamp: December 23, 2023, 02:37:30 am
content: Many old analog joysticks used rheostats AKA variable potentiometers, a capacitor, and a comparator; measuring the delay between the supply side falling to zero, to the RC-filtered signal falling below a fixed threshold. Unlike most other methods, this is not analog to digital conversion, because the variable analog resistance is converted to a variable analog delay!The actual conversion to digital was usually done by providing a fixed-duration pulse to the rheostat, and counting the period of output being high in clock cycles.Quote from: guenthert on December 22, 2023, 10:22:12 amIn optimal conditions, the human eye is sensitive to individual photons. Our brain does some filtering, so a few (three to ten) photons within a short interval (0.1 seconds) are needed for theperceptionto form.Quote from: T3sl4co1l on December 22, 2023, 10:24:03 amYup, very similar to how the old-timey analog joysticks I mentioned above worked. The time interval is still analog; measuring the interval with a digital clock is the actual analog-to-digital conversion.

author: Nominal Animal
author_rank: Super Contributor
timestamp: December 23, 2023, 04:03:08 am
content: In many particle physics sensors individual separate events (photons, particles, collisions) with a known minimum interval in between, tend to be easy to handle. Many such sensors have a dead time after each individual event. It is when you want to measure both event counts and individual event properties (wavelength or energy, typically), and want/need to detectconcurrentevents, that things become hairy.For this reason, particle colliders don't even try to measure the collision result particles directly, and instead use many layers of detectors measuring the dissipation of energy, often with a heavy electromagnetic field so that the actual trajectory reveals further information on charged particles. Indeed, each collision event is more like a tree, with each branching point being a subsequent collision or decay event; with the entire tree of events and (intermediate) particle properties forming a system offuzzyequations –– fuzzy in the QM sense! –– that is solved as a system, to identify the original collision result particles (even if they themselves left no directly measurable traces in the detectors, for example due to being too short-lived, with the closest layer of detectors being too far from the collision site).When measuring e.g. radiation spectrum, using a filter that only allows a specific energy (wavelength) at a time, makes things easier. Then, the energy of each photon is known, and a relatively cheap CCD element can count individual events with very short dead times. Adjusting the sensor distance from the source affects the solid angle the sensor covers with respect to the sample, so the event intensity can be adjusted by adjusting the sensor distance.Calculating the actual activity of radioactive samples using such equipment is where I personally first used elliptic integrals in anger. (Calculating the exact effective solid angle of the detector with respect to an uniform-activity sample volume yields elliptic integrals.) I quickly found out that using a suitable program (with a sufficiently random pseudo-random number generator) to simulate a radioactive sample emitting individual photons is not only faster (in terms of amount of human work needed;notin raw computation time needed), but also easily visualized and verified. Even a cheap laptop can simulate a few billion photons with planar, conical, cylindrical, and spherical surfaces and filters, within a few seconds to a couple of minutes, so getting a precise enough result with tight enough error boundaries, with sufficient margins, does not take long at all. It is particularly useful when the detector distance from the sample and detector geometry is such that not all points on the sensor surface are directly "visible" from all points within the radioactive sample.I did end up calculating it both ways, with the results (effective solid angle of the sensor) agreeing with each other, of course.Quote from: T3sl4co1l on December 22, 2023, 11:35:29 amI prefer digital:analog ≃ discrete:continuous, because it is more useful when building specific domain knowledge on top. In particular, "to convert to digital" ≃ "discretize".In the example of using a capacitor discharge to measure a current, or an RC lowpass filter to measure resistance, by converting the current or resistance to a time interval (still in the analog domain), the conversion to digital occurs when the measurement isdiscretized.Now, in quantum mechanics and related physics fields, most things arequantized. Thing is, that does not mean they are also necessarilydiscretized, even though the observables related to the quantized propertiesoftenhave discrete values.Very often the quantized properties can be described using complex numbers. Then, the magnitude, or absolute value, is an integer multiple of some real positive value ("one quantum"), but the direction in the complex plane may vary. This is also why summing two or more such quantized values is rarely equal to the sum of their magnitudes.In thedouble-slit experiment, where emitting individual photons or electrons (all with the same energy) through a double slit (with suitable size and slit separation) yields an interference pattern, the particle location and trajectory is described using a quantum wave function. The double slit acts like a filter, so that the resulting wavefunction is a sum of two wavefunctions. Because these wavefunctions can be described in complex number form, the above note about summing applies, and the resulting wavefunction has the same shape if we had two particles at the same time, one particle through each slit, interfering with each other; just with lower amplitude. Because of how the amplitude of wave functions is defined, the probability distribution of where the particle will hit the display surface is thesquare(and not just magnitude) of the wave function at the display surface.In a real sense, the double slitquantizesthe location of the particle. It does not, however, mean that the particle will pass through one or the other slit; it does notdiscretizethe path or trajectory. In fact, if you do something that causes each particle to pass through just one of the slits, for example measuring which one it passed through, you lose the interference pattern, and just get a sum of two gaussian distributions. This is because then the resulting wave function is no longer a sum of two wave functions; it is always one of two possible wave functions instead.If you grok that, you grok the core idea in quantum mechanics, I believe. Thus, I don't think considering "quantum" a superset of "analog" is useful at all.

author: Richard_S
author_rank: Contributor
timestamp: December 23, 2023, 05:14:34 am
content: Quote from: joeqsmith on December 22, 2023, 05:02:31 pmAh ha, the original!Not electrical but then say a modern electronic vernier which uses magnetic pulse counting would be exactly that. I once had the privilege of a visit and tour around a factory where they made such devices.

author: tggzzz
author_rank: Super Contributor
timestamp: December 23, 2023, 08:51:55 am
content: Quote from: Nominal Animal on December 23, 2023, 04:03:08 amYes, particularly since new scopes do that in two dimensions, some older ones in one dimension, and the original scopes in neither dimension. The middle variant seems hard for youngsters to believe.

author: TUMEMBER
author_rank: Regular Contributor
timestamp: December 23, 2023, 12:38:17 pm
content: Quote from: Nominal Animal on December 23, 2023, 04:03:08 amquantum theory of everythingeach of these particles is the same in terms of shape, but is variable "in terms of content" and only "randomly" assembled gives the correct image (result). How much work can be done by an ordinary brain.

author: fourfathom
author_rank: Super Contributor
timestamp: December 23, 2023, 07:28:57 pm
content: Analog to timing joystick: Back in the mid-1970's I made a "Pong" game equivalent, using an oscilloscope in X/Y mode (rather than using a TV modulator). Each player-side used the joystick voltage (single axis, it just moved the paddles up and down) into a comparator, with a ramp on the other input. This, and one-shots (another analog-controlled digital device) created the on-screen graphics. I don't remember *any* of the details, but doubt if I did anything for retrace-blanking, etc. My buddies and I did this when we were supposed to be testing and troubleshooting stuff.