multimeter resolution help - Page 1
Forum section: electronics / beginners

author: xtekkenman
author_rank: Newbie
timestamp: May 06, 2014, 12:11:09 am
content: so I have a multimeter that is reading 18.350 volts on the 0-20 volt range. the multimeter is an 8-bit resolution device. what is the resolution of my measurement? ..in other words.. my voltage should be 18.350 volts +/- "___" something.thanks for helping!d.

author: w2aew
author_rank: Super Contributor
timestamp: May 06, 2014, 12:22:40 am
content: 8 bit resolution doesn't tell you enough to give you what you're looking for. For example, the meter can average multiple readings to achieve better than 8- bit resolution. The best bet is to consult the manual or data sheet for the meter to check the accuracy spec.

author: xtekkenman
author_rank: Newbie
timestamp: May 06, 2014, 12:29:16 am
content: the manual says the multimeter's resolution is 8-bit..is the formula.. (delta.V) / (2^n) where n is the number of bit. which is 8 here. and delta.V being 20Volts.. 20-0 volts.. the range.

author: Wytnucls
author_rank: Super Contributor
timestamp: May 06, 2014, 12:32:54 am
content: You may find this article interesting:http://www.maximintegrated.com/app-notes/index.mvp/id/748

author: Rick Law
author_rank: Super Contributor
timestamp: May 06, 2014, 12:39:56 am
content: Quote from: xtekkenman on May 06, 2014, 12:11:09 amI am not one with much experience, but since no answer to your post yet I will give it a shot until some better answer come around. (And those with better answers, please do correct me if I am wrong)8bit resolution - I think it means an 8 bit ADC. You should check the menu/specs for sure. If it is an 8 bit ADC, this has to be the worst I've seen. Even the $1 Chinese DT830B use a 12bit ADC.8bit means it can go from 0 to 255. For every count, it is +- 1/255 of scale best case. That is about 0.4% of scale best case. Averaging will give a slightly better reading, but only fractionally better - just fraction of 1/255 better.At 20V range, 20/255 = 0.078 assumingeverything else being perfect. So,.the 18.35volt is 18.35+-0.0.78V best case. There is no chance of "everything else being perfect". The forth digit is garbage for sure, and the third digit is at best +- 1.Rick

author: Wytnucls
author_rank: Super Contributor
timestamp: May 06, 2014, 01:10:00 am
content: Resolution is determined by your DMM IC. If the meter reads 18.350 volts on the 0-20V scale, it has a resolution of 19,999 count.Accuracy is another matter and is usually quoted in the manual as a percentage of the reading + percentage of range +/- x digits.Accuracy depends on voltage reference, ADC resolution, oversampling and countless additional errors.8-bit ADC doesn't sound right for a 20,000 count meter. More likely 2,000 count and your reading was 18.35V.

author: Rick Law
author_rank: Super Contributor
timestamp: May 06, 2014, 04:48:33 am
content: Quote from: Wytnucls on May 06, 2014, 01:10:00 amI didn't think it sounded right either. That is why I suggest he checks for sure.If it is really an 8-bit resolution meter, I wonder if it is some kind of Scope meter or Scope display. 8-bit is awful low for a real DMM. The 18.350V could be merely a side effect of how it was calculated with the display code written merely in a software-convenience way.Question of the day - if 12bit ADC DT830B can be had for $1USD, how would they package and sell DMMs with 8bit ADC?Answer - bubble gum machine - the customer put in a quarter, twist and turn the knob and the DMM drop down to the slot.

author: ejeffrey
author_rank: Super Contributor
timestamp: May 06, 2014, 05:03:19 am
content: A 20,000 count DMM should have more like an 18-bit ADC. I have a hard time imagining anyone making an 8-bit DMM, since I think it would actually be more expensive than buying a DMM-on-a-chip with a built-in 12 bit ADC. Only way I can see an 8-bit ADC in a multimeter is if some l33t haX0r d00d made an 'arduino based' DMM using the AVR built-in ADC.

author: David Hess
author_rank: Super Contributor
timestamp: May 06, 2014, 03:29:29 pm
content: 3.5 digit meters read up to 1999 but with plus and minus, are actually 4000 counts or roughly 12 bits. It is often more useful to think in counts because integrating converters and converters that use BCD may not produce power of 2 outputs.A meter with actual 8 bit resolution from 0 to 20 volts would have a resolution of 78.125 millivolts making it barely better than 2.5 digits and that assumed it does not read out negative values. Some panel meters might have that low a resolution.