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When a DMM is set to measure DC voltage, does a read-out value correspond to an output of the ADC inside the DMM, or do multiple ADC outputs are averaged over a time interval and then converted to the DMM read-out value?

An oscilloscope always reads fluctuations in voltage, but a DMM tends to give stable readings. I wonder where the source of the difference is.

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    \$\begingroup\$ an oscilloscope will not show any fluctuations if you measure true DC voltage \$\endgroup\$ – jsotola May 27 '19 at 2:46
  • \$\begingroup\$ @jsotola For the same voltage input, OSC readings tend to fluctuate more. \$\endgroup\$ – Nownuri May 27 '19 at 3:16
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A typical DMM will only give 1-3 readings per second, whereas a DSO is expected to make potentially billions of readings per second.

The primary consequence of this is that fewer readings per second lets you integrate the reading over time, which is how a DMM can give true RMS readings, whereas a DSO can only give instantaneous readings.

If you slow a DSO down to just a few readings per second and tell it to average the results, you will get something closer to what a DMM gives you. But realize that it is not designed to do this, so a DMM will still do a better job at the tasks it is made for.

Another aspect of the difference is that even high-end DSOs rarely exceed 8 bits per sample, meaning only 256 values per range. Even a cheap DMM will generally give at least 2000 counts per range, and many go more than an order of magnitude higher than that.

These two aspects — speed and precision — are inversely related: it takes time to do a 16-bit or so measurement with high accuracy, so 50000 count DMMs either can only give a few readings per second or they're really expensive. Flipping it around, it's cheap to make a DSO that can do 500 megasamples per second over a 100 MHz measurement bandwidth if you only ask for 8-bit sampling.

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  • \$\begingroup\$ So, both OSC and DMM average input voltage over time, but they use different scale of intervals? \$\endgroup\$ – Nownuri May 27 '19 at 3:29
  • \$\begingroup\$ @Nownuri: It's better to think of a DSO as measuring each reading as fast as it possibly can at the expense of accuracy, relative to a DMM. The compensating advantage is that you get a lot of readings, so you can build up a pretty good picture of what the signal is doing down at the microsecond scale and below. \$\endgroup\$ – Warren Young May 27 '19 at 5:37
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Having a display which 'jumps all over the place' is bad for user confidence and also bad for meeting the typical use-case of "Tell me: ABOUT what voltage is present?" in many cases.

"Once upon a time", and quite likely still, DMMs use(d) "dual slope" analog to digital conversion. This has the advantage of being able to achieve a large number of bits of accuracy (and/or resolution) at relatively low cost.
It has the disadvantage of tending to be slower to much slower per conversion cycle.

DMM's once (and probably still) used to often make the conversion period a multiple of the mains cycle time (60 or 50 Hz mains) to minimise the impact of mains AC noise.
AND if you have a somewhat suspect conversion system, not only averaging the result but doing some sort of sliding window averaging when the input signal is 'a little noisy' and/or the conversion process is pushing its limits somewhat will add apparent stability.

All these (or some subset) add up to a high resolution, OK accuracy stable display.
If you look in the spec sheet (that small sheet of close typed thin paper that often comes with meters) you'll find that on a 1999 display (which thus has 0.05% resolution), the accuracy is usually around 1% (ie merits a 199 display.)

On a digital display of actual voltage, the last digit "should" hunt between two adjacent digits, with the time on each proportional to the undisplayed remainder. eg a voltage of 1.9876 V on a perfect accuracy "three and a half digit" 1999 display should alternate between 1.987 V and 1.988 V. On avergae it should display the former 40% of the time and the latter 60% of the time as the undisplayed 6 is 6/10 of the way between 1.987 and 1.988. Many meters do NOT have the least significant digit "hunting" in this manner.

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A DMM takes a certain amount of time to digitize each reading. There are trade offs to that: longer is less responsive in the case of “varying DC”, but averaged out AC noise better.

Precision instruments sometimes have this adjustable. Some call it “reading time”, for example. NI has a nice discussion in one of their manuals.

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  • \$\begingroup\$ Do you mean that each voltage reading corresponds to an ADC output, and the noise component is averaged out inside the ADC? \$\endgroup\$ – Nownuri May 27 '19 at 3:26
  • \$\begingroup\$ Different instruments work differently. Some average a bunch of ADC measurements, but there’s often some averaging in the analog front end. \$\endgroup\$ – Bob Jacobsen May 27 '19 at 3:29
  • \$\begingroup\$ Okay, so somewhere inside the DMM the reduction seems to happen. Does a DMM reading correspond to an ADC output? \$\endgroup\$ – Nownuri May 27 '19 at 3:31
  • \$\begingroup\$ Varies. Cheap ones, yes. Precision ones do quite a bit more. \$\endgroup\$ – Bob Jacobsen May 27 '19 at 3:32

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