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My DMMs ($50-$100 range) refresh the displayed value every ~250mecs. Are there technical reasons (limitations) for this, or is this just a UI thing?

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Your question makes little sense. We can't even tell whether you think this limit is on the low end or the high end. Which way would you rather have it, faster or slower? –  Olin Lathrop Jun 28 at 11:32
    
I accidentally dropped the qualifier; mistake corrected. I'd like to see higher refresh rates. For example, when measuring the voltage across a charging capacitor. –  Duoran Jun 28 at 12:07

3 Answers 3

up vote 6 down vote accepted

Most DMMs are integrating type (of which the dual-slope was one of the first methods developed suitable for high resolution and accuracy).

An advantage of integrating is that the integration period can be designed to be a multiple of 50Hz and 60Hz line frequencies. For example, 300 msec is 18 cycles of 60Hz and 15 cycles of 50Hz. This has the effect of a natural notch filter at mains frequency so that hum caused by mains noise is cancelled out and the reading does not jump around as much.

Integrating converters can also be built with high resolution and pretty good (< 0.1%) linearity with cheap parts (all the errors cancel out to a first order except for the reference voltage)- 0.1% linearity and accuracy, even with a 5% film capacitor, 5% resistors, a crude RC clock, and with adequate refresh rate for visual purposes (a few Hz)

For UI reasons you really don't want to update the display too too fast, 2-5Hz is about right- if the display rate is too fast, it could jump back and forth between (say) 201 and 101 and it would look almost like 301. If it is too slow, you don't get to see the reading stabilize and how much apparent noise there is in it.

More modern high-resolution converters are often made using sigma-delta techniques, which eliminates at least one second-order effect (capacitor dielectric absorption) from the integrating converter error budget (improving linearity*). They can be used in a DMM by low-pass filtering and decimating the result to an appropriate display rate (or just averaging over a suitable time). You'll also see low-end voltmeters and ammeters that use the built-in 10 or 12-bit successive-approximation converter built into a micro and add some averaging to get a sort-of acceptable reading with a crummy (low) input impedance.

*Although most users can't see nonlinearity without a better instrument to compare the DMM against, they can simply flip the leads on a stable DC source and see that (say) a reading of +10.00V reads -9.98V when the leads are reversed. Of course these days such an effect could just be fiddled out with a microcontroller.

Modern higher-end DMMs like my Agilent have options for reading speed, here is an example table showing capabilities: enter image description here

NPLC refers to the number of power-line cycles that the reading is integrated over.

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ADC converters tend to get more expensive with increasing resolution and sample rates, so for a regular DMM there's probably not much point to trying to make them faster at an increased cost. Afterall who can read an LCD display updating more than four times a second?

I have a multimeter that can take 1000 readings per second, but that only makes sense because it can also record readings to a USB stick and transfer over ethernet etc for later analysis. Also for some of the in-built math functions it can make sense to have a higher sample rate, but that's not the sort of thing hand-held meters are normally made for at the lower end of the market.

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I believe this is the correct answer. On my Fluke 87 DMM, If I increase the precision from 3½ to 4½ digits, the refresh rate drops noticeably. This suggests that for a given ADC there is a trade-off between precision and sample rate. –  RedGrittyBrick Jun 28 at 12:16
    
That really is strange though because it still does not take that long to perform an ADC conversion. I do not recall exactly but I want to say it can be done for 8 bit resolution in 8 clock cycles. –  sherrellbc Jun 28 at 12:26
    
@sherrellbc, I wouldn't want a DMM with a 8-bit ADC ;-) But a lot of high-resolution ADCs do really only take under ten samples per second. –  PeterJ Jun 28 at 12:31
    
I was just taking 8 bit as an example because with it I am familiar. I was looking to point out that answers like RedGrittyBrick above do not make a great deal of sense considering increasing the sample rate as he mentioned should not have a noticeable impact on the refresh rate of the DMM. There has to be something else going on there - perhaps it's more accurate now and changes fewer times over a comparable amount of time? At lower resolution maybe there is more rounding contributing to erroneous readings that tend to jump around a bit. –  sherrellbc Jun 28 at 12:40
    
There is no one correct answer - wholly ignoring Sphero's dual rmap/ integrating response is 'risky'. –  Russell McMahon Jun 28 at 16:22

The electronics could refresh the display both faster and slower than 250 ms. The tradeoff is between updating it so fast that it is hard to read, versus so slow that the user has to wait too long between readings. 1/4 to 1/2 second is usually a good refresh period.

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So, it's a UI thing. Hm, it would be nice if this could be made configurable, eg with a button that steps through 1/2/4/10/100Hz or some such. –  Duoran Jun 28 at 12:13
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@Duoran: That wouldn't make any sense. You can't see individual readings at much faster than 4 Hz. Even that can be annoying. 10 Hz or more is useless. You'll see a blur for some of the segments of the digits, which leaves you with little idea what even the range of numbers are. You can't smoothly interpolate the output of a 7-segment display. –  Olin Lathrop Jun 28 at 15:34
    
Indeed, I wouldn't be able to see individual readings, but I might be able to deduce a trend. Say I would like to observe the voltage across a charging capacitor (Platt's MAKE: Electronics contains such an experiment), doing so with a high refresh rate would be more helpful and informative to me. (Comment edited because my tablet screwed up again, sorry). –  Duoran Jun 28 at 15:46
    
Perhaps I should have said: observe a trend, instead of deduce. –  Duoran Jun 28 at 15:51
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@Duoran: What you are asking for is inappropriate for a digital meter. The low digits would be a blur which you'd get no useful information from. You'd be able to see the trend from the higher digits that are updating slowly enough, but then you have lower resolution and a slower update rate. You're back where you started except with less resolution. If you want to see that kind of trend, use a analog meter, or a data logger and plot the numbers afterwards. –  Olin Lathrop Jun 28 at 16:20

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