Multimeter specifications: what does "counts" mean?

[Cemoi]

Despite searching this forum for "DMM counts" I still don't understand what the number of "counts" means in the DMM specs. The Simple guide to using a DMM for measurements thread listed in "Thread of interest" doesn't address this topic.
So I'd be grateful if someone could explain what it means and how it translate in terms of measurement accuracy, or suggest some URLs worth reading.

For example, the specs for the Fluke 114 (a 6000 count multimeter) say:
Accuracy: ± ([% of reading] + [counts]): 0.5% + 2
What is the resulting "true" voltage range for e.g. a 4.000V reading?
How different would it be for a 2000 count / 50000 count DMM?

 

[VidPro]

http://www.reference.com/browse/circuit+analyzer
scroll down to the part on "resolution digital"

 

[Justin Case]

The counts spec tells you the absolute value of the full scale value that a DMM can display, ignoring the location of the decimal point.

Ignoring other issues such as ADC resolution, noise, etc, a 6000 count DMM can read out to 3 decimal places when measuring a 4V source. A 2000 count DMM can display only 2 decimal places. A 50000 count DMM can display 4 decimal places.

For a 6000 count DMM with VDC spec of +/-(0.5% + 2 digits), 4V has an accuracy range of

4 +/- (4*0.5% + 0.002) = 4 +/- 0.022 -> 3.978V-4.022V

2000 count DMM: 4 +/- (4*0.5% + 0.02) = 4 +/- 0.04 -> 3.96v-4.04V

50000 count DMM: 4 +/- (4*0.5% + 0.0002) = 4 +/- 0.0202 -> 3.9798v-4.0202V

For low count DMMs, the offset accuracy spec (the "digits") generally is a significant fraction of the total measurement accuracy range. So even if the % of range spec is low (e.g., 0.1%), the "digits" can still result in relatively large error.

 

[NextLight]

A 'count' in an accuracy spec is "1" in the least significant digit. So if we put ~ 1.950000 V into a 3.5 digit DMM (a "2000 count" meter) on the 2 Volt scale and the DMM shows 1.955V, that demonstrates a 5 count error.

In this example, the meter might be 0.1 % basic DC accuracy (mostly limited by the analog stages) +/- 4 counts (the counts error addresses limitations of the digitizer part's performance, and display resolution) For example here the meter's digitizer might have 4096 discrete steps, or +/- 2048 steps. The " counts" portion of the overall accuracy spec accounts for (a-counts for) the limits of errors in digital resolution in the analog-to-digital converter and the display. And, if the manufacturer is thorough, system noise and non-linearity.

 

[Cemoi]

Thanks a lot for your replies, very informative.

So if I understand well, a two-digit error on a 5000 count DMM is much worse than a four-digit error on a 50000 count one.