How does a modern digital multimeter "sense" electricity? - Page 1
Forum section: electronics / beginners

author: stitch
author_rank: Regular Contributor
timestamp: April 13, 2014, 10:04:42 pm
content: In the early days of electronics, a galvanometer was used to sense electric current. The galvanometer used magnets and a coil attached to a needle to do this. Can anyone explain to me what part of a modern digital multimeter replaces the coil and magnets of the galvanometer? Asked another way, what electrical phenomena are exploited by modern DMM's to sense current?

author: TimNJ
author_rank: Super Contributor
timestamp: April 13, 2014, 10:19:54 pm
content: I'm pretty sure it's as simple as ohm's law. I believe a DMM measures the voltage drop across a known resistance, also known as a current shunt.

author: Wytnucls
author_rank: Super Contributor
timestamp: April 13, 2014, 10:22:41 pm
content: The heart of the modern DMM is the integrating ADC:Read this article:http://en.wikipedia.org/wiki/Integrating_ADC

author: stitch
author_rank: Regular Contributor
timestamp: April 14, 2014, 12:10:51 am
content: Quote from: TimNJ on April 13, 2014, 10:19:54 pmYes, buthowdoes the DMM measure the voltage drop?

author: FrankenPC
author_rank: Unknown
timestamp: April 14, 2014, 01:10:14 am
content: Quote from: stitch on April 14, 2014, 12:10:51 amThat's a good question. I don't know how modern ADC's work. I THINK I know how simple ones work. They take in a voltage and pass it by a resistor ladder. As each divider is passed, it triggers a pulse that goes through a counter which ultimately puts out a byte (0-255) depending on how far the voltage passed through the ladder.TI has a white paper that describes doing something similar using capacitors:http://www.ti.com/lit/ds/symlink/tlv1548-q1.pdfThere's also successive approximation and sigma-delta which look pretty sophisticated.

author: IanB
author_rank: Super Contributor
timestamp: April 14, 2014, 01:28:51 am
content: Quote from: stitch on April 14, 2014, 12:10:51 amIf it's a digital voltmeter (DVM) it's done digitally.The fundamental element is a comparator, which is a circuit built out of transistors that compares two voltages A and B. If A is greater than B then the comparator produces an output, otherwise it doesn't. To make sure the circuit draws very little current (which would disturb the reading) the transistors are field effect transistors which have a very high input impedance.A single comparator can produce a single on/off state, which means that in binary terms it is a "1-bit" converter.However, various clever circuit arrangements can use many comparators to produce a more precise binary output with 8 or 12 bits of precision.These multi-bit samples can then be filtered and scaled and otherwise processed to produce a reading on the display.

author: FrankenPC
author_rank: Unknown
timestamp: April 14, 2014, 01:36:19 am
content: Quote from: IanB on April 14, 2014, 01:28:51 amSo, if a ADC has 8 bit resolution, you're saying there's 256 comparators in that chip? Or does it do something tricky like see if the voltage is half the reference, then half that, then half that, etc...?

author: IanB
author_rank: Super Contributor
timestamp: April 14, 2014, 01:43:45 am
content: Quote from: FrankenPC on April 14, 2014, 01:36:19 amThere is not necessarily a need for many comparators. You can trade off the number of comparators for operational speed.In an application like a multimeter you only need a slow sample rate. In this case, the integrating ADC referenced in the post above can in its simplest form use only one comparator. Instead of many comparators, it instead uses a timer to measure a length of time proportional to the input voltage. Of course the timer now has to be a digital timer or the measurement problem has just been transferred from one place to another.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 14, 2014, 01:51:26 am
content: By counting time and using the fundamental capacitor equation I = C * dV/dt, holding I, dV or dt constant in various ways (see the earlier link about slope conversion).Tim

author: jeremy
author_rank: Super Contributor
timestamp: April 14, 2014, 06:50:48 am
content: Quote from: FrankenPC on April 14, 2014, 01:36:19 amThe 256 comparator version is what is known as aflash ADC(not to be confused with flash memory). They are used nowadays in really really fast ADCs (like digital oscilloscope fast) but are very expensive due to their size and usually lower resolutions.The most common one I see is asuccessive approximation register ADC. These represent a good tradeoff between speed, cost and resolution. Most microcontrollers use a SAR ADC.The best resolution ADCs are usuallydelta-sigma ADCs. These are usually super accurate and >=16 bits but slow and expensive.Of course there are other special versions as well as hybrids of these three, but these are the most common ones I have seen.Most measurement equipment like multimeters use either SAR or delta-sigma ADCs. My beloved agilent 34405A uses a kind of delta-sigma method I believe.

author: Hideki
author_rank: Frequent Contributor
timestamp: April 14, 2014, 11:28:10 am
content: That's actually not the case for most multimeters. Look at what Wytnucls posted about the Integrating ADC -- dual-slope or something more fancy.

author: jeremy
author_rank: Super Contributor
timestamp: April 14, 2014, 11:43:46 am
content: Ah interesting. I'm pretty sure that most of my multimeters are delta-sigma, but perhaps that is just luck!

author: stitch
author_rank: Regular Contributor
timestamp: April 14, 2014, 09:44:40 pm
content: Thanks for all of your responses to my topic question. From your responses, I think I have been able to find an answer to my extreeeeeeemly fundamental question: "Can anyone explain to me what part of a modern digital multimeter replaces the coil and magnets of the galvanometer?"I'm pretty sure now that the answer is … a resistor and a capacitor. Who knew?Apparently a modern DMM (DVM) makes use of the RC Time Constant. By counting and comparing the time it takes to charge a capacitor, the DMM (DVM) calculates voltage. But here's what's really interesting …The theory behind the coil and magnets of the galvanometer (electromagnetic induction theory) was developed by Joseph Henry and Michael Faraday around the year 1830. The invention of the capacitor (Leyden Jar) was made around the year 1745 by Ewald Georg von Kleist and Pieter van Musschenbroek. The modern DMM (DVM) uses an invention that is almost 100 years older than the antique galvanometer!

author: Wytnucls
author_rank: Super Contributor
timestamp: April 14, 2014, 10:00:15 pm
content: It won't work without a voltage reference (Clark Cell 1873)1.434 voltshttp://en.wikipedia.org/wiki/Clark_cell

author: pickle9000
author_rank: Super Contributor
timestamp: April 14, 2014, 10:18:23 pm
content: A good question judging by the answers.

author: Richard Crowley
author_rank: Super Contributor
timestamp: April 14, 2014, 10:56:01 pm
content: As Wytnucls said, the meter uses some internal reference to COMPARE the voltage under measurement with the internal reference. In the old-school d'Arsonval, the "reference" was a very small, internal "hair-spring". In modern solid-state, digital meters, a very crude process known as "approximation" is done.From the reference voltage they use resistors to create all the intermediate voltages: 0.1, 0.2, 0.3 etc....Then they ask, over and over again, is the measured voltage = 0.1?, is it = 0.2, is it = 0.3, is it = 0.4, is it = 0.5, etc. etc. Then, when they find a match, it is displayed on the readout. NOTE: This is an extremely truncated and over-simplified explanation, but I think it is what stitch is asking.As Jeremy explained, there is a different kind of analog to digital converter ("flash converter") where they compare the input voltage SIMULTANEOUSLY from 0.0V all the way up to to 2.56V That method is much faster, but it takes more power and has lower resolution. Now in some applications (very fast signals like video, etc.) that trade-off is worth the extra power and lower resolution. But typically it is not suitable for a proper test meter.That is how modern digital multi-meters measure VOLTAGE. They measure CURRENT simply by using Ohm's Law and measuring the voltage drop across a resistor (actually a very low-resistance "shunt"). And they measure RESISTANCE, again by Ohm's Law, where they apply a current across the resistor under test, and they measure the voltage across the unknown resistor. So you see that no matter what you are measuring (volts, ohms, amps, etc.) it comes down to some voltage that is converted from analog to digital (A to D) and shown on the display.

author: stitch
author_rank: Regular Contributor
timestamp: April 15, 2014, 03:59:23 am
content: Actually my question was not about how tomeasureelectricity, but rather how to firstlysenseit so that it can subsequently be measured. In my topic question, I used the example of the galvanometer which employed "magnets and a coil attached to a needle". I probably should not have mentioned the needle (the measuring part), because the sensing is done with the coil and magnets (the coil magically moves when current is passed through it). Yes, there was a needle attached to the coil to measure the movement of the coil, but it was the magical movement of the coil which revealed to human observers that something was happening - regardless of the magnitude of what was happening. That's the sensing part that I was talking about … you know, the kernel, the phenomenon, the very first step.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 15, 2014, 05:14:52 am
content: Well, when an electron and a proton love each other very much....Neglecting the underlying physics though, it's just Ohm's law through a bunch of switched resistors, of the order 1-10M for the outward most ones.

author: Richard Crowley
author_rank: Super Contributor
timestamp: April 15, 2014, 05:24:31 am
content: Quote from: stitch on April 15, 2014, 03:59:23 amThat seems like an odd question. Clearly, the old d'Arsonville/Weston meter movement uses simple electro-magnetic effect to "measure" the current through the coil.But if you are asking about how that works in solid-state equipment, you are asking about current flow and holes and electrons, and solid-state physics. On one level a simple light switch is trivial to explain, but on another level, how current flows through a conductor has yet to be explained completely. It is not clear at what level of abstraction you are asking your question? Are you willing to take "for granted" what is inside the integrated circuits? Or are you asking about how the integrated circuits operate?

author: ejeffrey
author_rank: Super Contributor
timestamp: April 15, 2014, 06:53:35 am
content: Quote from: stitch on April 15, 2014, 03:59:23 amThen I think the answer to your question is "a transistor, usually a field effect transistor'. A voltage applied to the 'gate' of a FET creates an electric field inside the channel, which either allows or prevents carriers to flow from the source to the drain, and it does it while drawing negligible current at the gate electrode. If the gate voltage is above some threshold a large current flows and you have a one, otherwise a zero or small current flows which is zero. You can either have a fixed threshold (as in ordinary logic gates used to implement digital logic) or a comparator that gives you a 1 if voltage A is greater than voltage B.

author: wiss
author_rank: Frequent Contributor
timestamp: April 15, 2014, 07:11:52 am
content: May I try?The input voltage is feed via a buffer to a resistor, the current through that resistor charges a capacitor for a fixed amount of time (1 power-line cycle for example), the input voltage is replaced with a reference-voltage and the same capacitor is discharged through the same resistor and the number of clock-cycles it takes to get back to zero is counted.For higher voltages an input-divider is used, for lower voltages the input buffer usually can apply 10 times gain. Sometimes the reference voltage can be switched 2 and 20 volts, sometimes not.

author: Unknown
author_rank: Unknown
timestamp: April 15, 2014, 07:51:32 am
content: Quote from: stitch on April 13, 2014, 10:04:42 pmMy explanation is simply the ADC replaces the "sense" coil of the galvanometer; and there are many types of ADCs to choose from.http://en.wikipedia.org/wiki/Analog-to-digital_converter

author: Dave
author_rank: Super Contributor
timestamp: April 15, 2014, 01:54:13 pm
content: Quote from: wiss on April 15, 2014, 07:11:52 amYou have described a dual-slope integrating ADC.There are, of course, many other types of ADCs, the most accurate being the multi-slope ADC, a successor of the dual-slope ADC. They are used in high-performance multimeters like the spectacular 3458A.

author: wiss
author_rank: Frequent Contributor
timestamp: April 15, 2014, 03:03:24 pm
content: Quote from: Dave on April 15, 2014, 01:54:13 pmOf course!One point being that the same resistor/capacitor is used for both slopes. And you'd like to measure zero some way, in a fluke 8050 the zero-offset is stored in a capacitor during the integration-phase, in a keithley 192 the "zero" is counted and subtracted from the result for example. And it is possible to mix the integration/deintegration phases in various ways. I strongly recommend to read the fluke 8050, fluke 8800 and keithley 192 manuals (available via google).

author: stitch
author_rank: Regular Contributor
timestamp: April 15, 2014, 06:27:32 pm
content: Quote from: wiss on April 15, 2014, 07:11:52 amThat's a good answer. Very much on target to what I was inquiring about.

author: stitch
author_rank: Regular Contributor
timestamp: April 15, 2014, 06:30:18 pm
content: Quote from: T3sl4co1l on April 14, 2014, 01:51:26 amThat's a good answer. Also, right on target.

author: stitch
author_rank: Regular Contributor
timestamp: April 15, 2014, 06:32:51 pm
content: Quote from: ejeffrey on April 15, 2014, 06:53:35 amThat's a good answer also. That's what I was looking for - the fundamental part where the sensing begins.

author: stitch
author_rank: Regular Contributor
timestamp: April 15, 2014, 06:48:26 pm
content: Quote from: Richard Crowley on April 15, 2014, 05:24:31 amThanks for your thoughtful answers, Richard. You are quite right about my not wanting to "take for granted" what is inside the integrated circuit. But I'm also not really asking about how the integrated circuits operate. I think I am asking about what fundamental sensing comes first ... before the integrated circuit.So let's say a doctor wants to assess his patient's heart rate. One way he might do this is to take the patient's pulse. To do so the doctor place his fingers on the patient's wrist. At that point, the doctor "senses" through his fingertips the swelling movement of the patients arteries. That's the part that I was inquiring about. Now of course the doctor can continue and measure each pulse against his wrist watch to ultimately get the heart rate, but that is not the part that I was curious about. I was curious about just the fingertips part ... the sensing part.

author: Wytnucls
author_rank: Super Contributor
timestamp: April 15, 2014, 06:58:21 pm
content: The sensing part would have to be the charging capacitor. Suspend a small object with a negative charge between the plates and watch it deflect towards the positive plate as the electric field increases. Theoretically, you should be able to measure the unknown voltage, with just a stopwatch.

author: Richard Crowley
author_rank: Super Contributor
timestamp: April 15, 2014, 07:17:29 pm
content: If you look at actual schematic diagrams of commercial DMMs, you will discover that the signal being measured goes directly into the IC (after appropriate "signal conditioning"*) The internal Analog to Digital converter (ADC) takes the signal directly and compares it with the scaled reference to develop the numbers to display.That is why I asked if you really wanted to get into semiconductor physics and analyze how an integrated ADC works? I already described how a typical ADC works in a DMM (successive-approximation). Rather like a small child guessing a number: "Is it 1?, Is it 2? Is it 3? Is it 4?....." etc. ad.nauseum Your quest for some similarity to how an analog meter (or a physician taking a pulse rate) rather breaks down, because the world of digital circuits really is much different than the Real World. Things in the Digital World are frequently much more crude than in the Analog World. But since digital circuits are so many orders of magnitude faster than anything analog, you can get away with crude methods because they are very fast and don't affect the final performance of the gadget.* By "signal conditioning" I mean scaling the voltage down to the safe operating range of the chip (or amplifying it where it is a very low voltage). Or it means converting AC to DC using a rectifier (or a true RMS converter in higher-end meters). Or it means using a resistance to voltage converter (using a constant-current source) for measuring resistance.

author: stitch
author_rank: Regular Contributor
timestamp: April 15, 2014, 09:10:15 pm
content: Quote from: Wytnucls on April 15, 2014, 06:58:21 pmYes. The capacitor is emerging to be the central player in what I was inquiring about - it senses a building charge over time. For the same reason, the FET mentioned earlier by ejeffrey sounds good also.

author: IanB
author_rank: Super Contributor
timestamp: April 15, 2014, 09:35:43 pm
content: Quote from: stitch on April 15, 2014, 09:10:15 pmIt really doesn't. When the capacitor charges up it acquires a voltage, but a voltage is what we started with at the measurement terminals. So we have just moved the problem of measuring a voltage from one place to another. Something still has to measure the voltage on the capacitor!In physics a sensor, otherwise known as a transducer, converts a signal from one physical form to another more useful form.So in the case of our galvanometer, a voltage across the terminals is translated into a current through the coil, which is translated into movement of the coil against a spring, which moves a needle on the display. So electric current has been transduced (converted) into a physical displacement. Electricity has been converted into movement.In the case of a DVM the situation is slightly different. The transducer here is not converting a voltage to some other analog signal, but instead is converting it to a digital signal. A voltage is being converted into bits of information.The sensor, or transducer, that is essential to this conversion process is, as mentioned above, the voltage comparator. This sensor is able to produce an on/off signal when one sensed input voltage is greater than another. The analog voltage signal is thus transformed into a digital information signal that answers the question, "is this voltage greater than that voltage?"The key element of the voltage comparator is the transistor, but the transistor itself is not actually the sensor. The transistor is more like the magnet in the galvanometer: it plays an essential part, but by itself it is not enough. It needs more parts connected up the right way to make a complete measuring system.

author: Richard Crowley
author_rank: Super Contributor
timestamp: April 15, 2014, 09:49:57 pm
content: I only looked at half a dozen schematics for commerical DMMs, but i did not see ANY of them with an input capacitor. As IanB says, it isn't as simple as that.

author: stitch
author_rank: Regular Contributor
timestamp: April 15, 2014, 10:09:29 pm
content: Quote from: IanB on April 15, 2014, 09:35:43 pmBut doesn't the rate at which the capacitor charges up tell us something about the voltage in question (RC Time Constant)? Can't we "compare" that rate to a know rate? If so isn't the capacitor still fundamental?

author: stitch
author_rank: Regular Contributor
timestamp: April 15, 2014, 10:10:42 pm
content: Quote from: Richard Crowley on April 15, 2014, 09:49:57 pmI was just about to ask you about that.

author: IanB
author_rank: Super Contributor
timestamp: April 15, 2014, 10:14:16 pm
content: Quote from: stitch on April 15, 2014, 10:09:29 pmImagine you have a capacitor and series resistor on your bench, and an unknown voltage. You can see how fast the capacitor charges up when you apply the voltage to it. But how will you detect the voltage on the capacitor to find the rate of increase in voltage? Logically, you will measure the voltage on the capacitor with a voltmeter as you time the rate of increase to compare with your known rate. Except...do you see the problem?

author: stitch
author_rank: Regular Contributor
timestamp: April 15, 2014, 10:15:39 pm
content: If you haven't already guessed, I'm leaning towards the capacitor. Besides, it rhymes: Old school used electromagnetic phenomena to sense electricity, new school uses electrostatic phenomena to sense electricity.

author: stitch
author_rank: Regular Contributor
timestamp: April 15, 2014, 10:17:43 pm
content: Quote from: IanB on April 15, 2014, 10:14:16 pmYes, I see the problem. Let me think for a minute.

author: Richard Crowley
author_rank: Super Contributor
timestamp: April 15, 2014, 10:23:07 pm
content: Quote from: stitch on April 15, 2014, 10:10:42 pmWell, the answer is that1) A capacitor is not a "sensor". At best, it is a "bucket".2) An ADC that uses a R/C time constant is only ONE type of ADC, and not the kind typically found in meters (which is what you are asking about).You can "lean towards the capacitor" if it makes you feel better. But it doesn't really answer your question.

author: stitch
author_rank: Regular Contributor
timestamp: April 15, 2014, 10:47:01 pm
content: Quote from: IanB on April 15, 2014, 10:14:16 pmCan't the ADC convert the ramp of the charging capacitor to a digital output? I'm not opposed to the ADC or any other part of a DVM's digital circuitry. My question has always been what happens before that. After all, before an ADC can convert, it must have somethingtoconvert.

author: Len
author_rank: Frequent Contributor
timestamp: April 15, 2014, 11:20:06 pm
content: Quote from: stitch on April 15, 2014, 10:47:01 pmThat capacitor ispart ofan integrating ADC. The "problem" Ian was referring to is, if you use an ADC to make your ADC, where do you get the other ADC from?The basic answer to your original question of "what part in a DMM replaces the galvanometer?" is "the ADC", one type of which is an integrating ADC (with a capacitor and a timed voltage ramp). An ADC converts an analog voltage to a digital number. Everything else comes down to that.To measure a voltage, apply it to the ADC and read the result. To measure anything else, convert it to a voltage and measure the voltage. Current is measured by passing the current through a known sense resistor and measuring the voltage drop. Resistance is measured by passing a known current through the resistance and measuring the voltage drop. Etc.

author: IanB
author_rank: Super Contributor
timestamp: April 15, 2014, 11:34:59 pm
content: Quote from: stitch on April 15, 2014, 10:47:01 pmItdoeshave something to convert. It has the original voltage on the input terminals of the DVM. It can just convert that...

author: stitch
author_rank: Regular Contributor
timestamp: April 16, 2014, 12:06:37 am
content: Quote from: Len on April 15, 2014, 11:20:06 pmYes. I accept that.

author: T3sl4co1l
author_rank: Super Contributor
timestamp: April 16, 2014, 12:38:12 am
content: Yes, certainly the capacitor is only as important as the surrounding circuitry allows it to be!As an example, suppose you had an ideal constant current coming down a wire. (So, you normally keep it grounded so it doesn't waste power, and you handle it very carefully so it doesn't start a spark -- because it will spark and arc as far as you care to open the circuit! Ideal constant currents are fun.) Suppose you have naught but your hands and some means of keeping time (a pendulum, a metronome, a stopwatch... or just counting mississippis out loud).If you connect the current source to the capacitor, start the timer, and disconnect it after a given time, you have delivered a known amount of charge to the capacitor. You can now measure the charge, say by connecting an electrometer. The foil electrodes deflect in front of a scale, allowing you to read a measurement (in effect, by calling out divisions of a scale, you're performing an A-D conversion manually, which can then be written down as a finite series of digits).You could also keep the electrometer connected, while charging the capacitor, and stop the timer when it reaches a certain charge. Your main source of reading error is now determining how many pendulum swings occurred, or how many mississippis were spoken, or whatever.In both cases, the electrometer scale must be calibrated, and you have two sources of errors: the ability to count time, and the ability to read a scale consistently (and its calibration).You can combine these methods. Perhaps you have a known current sink, so after charging the capacitor for some time, you discharge it until it reads zero again. Compare the ratio of times, and you know the ratio of currents. Now you no longer need to know what the delivered charge was; the electrometer scale doesn't need to have any calibration, or even any linearity, so long as it remains a one-to-one function of voltage, so you can tell when it passes the point it was initially at.You now need a calibrated current source instead of the electrometer scale, but the idea is to swap out sources of error until you have something easily calibrated and stable. An electrical current is easy to measure, set and calibrate, and has other advantages inside an integrated circuit. So this is good progress. The time measurement is still a matter, but it is actually one of the easiest and most accurate measurements that can be performed electronically.Perhaps, when you stop the discharge (with the known current sink), you accidentally went too far: you counted one full mississippi extra. Your time is well known (you counted a whole number, no rounding or guessing at fractions), but now the discharge is too far. You could measure the residual with the electrometer again, but then its scale must be calibrated; and in finer divisions than before!Suppose you connect another known accurate current source, of magnitude 1/N times the discharging one. Then you count until the charge comes back to (or just past) zero: now you have a triple slope integrating ADC, and the counts of that last step are exactly 1/N the weight of the discharge counts. So if you count 0-9 steps, and N=10, you go from, say, a measurement of X, to a measurement of X.Y, fully 10 times more precise. If the current ratio N can be generated geometrically, it won't need additional calibration, another big win.In the electronic implementation, ultimately, you have transistors (usually MOSFETs, since we're talking battery operated, most likely CMOS circuitry here), which are required to sense that the voltage has reached some value, or that it's crossed through zero, or whatever. Whether you want to put the emphasis on one thing or another (a transistor, a capacitor, a resistor..) is up to your own interpretation. Obviously, an instrument cannot exist unless there is both a sensing element (a transducer), and something to observe that activity, a sensor or amplifier or observer or whatever. Concentrating on any one component is like saying a human is nothing but a brain, or a heart, or a stomach; none of which can exist on its own without the help of many supporting parts!Tim

author: David Hess
author_rank: Super Contributor
timestamp: April 16, 2014, 01:41:47 am
content: Quote from: stitch on April 15, 2014, 10:09:29 pmIt does but by charging the capacitor with a current proportional to the voltage to be measured, an integrator is used, and then discharging it with the reference current, the capacitance cancels out as well as the capacitance drift over time and temperature. Further, if the capacitor charging time is fixed at some multiple of the cycle time of the power line frequency, then power line interference will cancel out which is handy for making accurate measurements in the presence of power-line noise.Alternatives to dual or multi-slope integration include delta-sigma conversion and voltage to frequency conversion. All three can achieve 100,000 counts of resolution and linearity or better depending on implementation.

author: Wytnucls
author_rank: Super Contributor
timestamp: April 16, 2014, 09:36:35 am
content: Quote from: IanB on April 15, 2014, 09:35:43 pmA dual slope integrator doesn't measure voltage directly. The input voltage is derived from the reference voltage and the charging/discharging time of the capacitor.They are plenty of multimeters using a dual slope ADC. I have two, at least, on my bench. And yes, they have an external integrating capacitor just for that purpose, next to the IC.

author: electr_peter
author_rank: Unknown
timestamp: April 17, 2014, 10:59:22 pm
content: Fundamentaly, voltage measurement procedure can be understood as cooperation of two devices - ADC and voltage reference.ADC's job is to compare input signal to voltage reference and output proportion of input vs reference. That is, ADC on itself can only output relative reading (10%, 43% or 100%) which doesn't say anything about absolute input voltage.ADC's relative reading is then converted to absolute reading by multiplication with voltage reference. If it is known that voltage reference for ADC is 10V and ADC outputs 76%, then input voltage is 7.6V.ADC without some reference or reference without ADC are both useless for multimeter function.In practice, ADC may be implemented in different ways. Voltage reference may be changed to time/capacitance/current reference - principle is still the same.