Multimeter display counts - Page 1
Forum section: electronics / general

author: Unknown
author_rank: Unknown
timestamp: November 23, 2009, 03:50:50 pm
content: I was wondering why do multimeters have 7000 or 50000 counts or the like.Why can't they just give the first digit a full 0-9 range if they are anyway using seven segment digits?I understand that in cheaper multimeters they only have a 1 as the first digit to save space but what about the others.Does it have something to do with the ADC or is it pure price targeting on behalf of the companies?Edit: does it purely depend on the a/d or is it a marketing thing also?

author: EEVblog
author_rank: Unknown
timestamp: November 24, 2009, 05:23:59 am
content: It all comes back to the *accuracy* of the ADC and other stuff.There is no point having a 50,000 count resolution meter that has a 0.5% basic accuracy for example.So the cheaper (less accurate) meters limit the resolution to a usable percentage of full scale based on the basic DC accuracy.Dave.

author: Zero999
author_rank: Super Contributor
timestamp: November 24, 2009, 11:21:10 am
content: I can see his point, there's no point in a high sample rate for a DVM unless it has more fancy functions such as data logging or a 'scope.Some DVMs don't use a full seven segment display for the first digit, it's just -1.

author: Unknown
author_rank: Unknown
timestamp: November 24, 2009, 07:29:20 pm
content: I think I was a bit unclear. I only meant the first digit. Let us say we have a 7000 count DMM with 0.1% accuracy which is an uncertainty of 6.999 at a full scale. Why then can't it have a 10000 count readout using the first digit to its full potential. Of course the uncertainty will then be 9.999 which will make the last digit unreliable but it was already quite unreliable with the 7000 counts. The accuracy is usually even worse on the other ranges like current so why not just have the first digit go up to nine? Or is it just me with my metric system way of thinking.Another thing is do the companies use better ADCs in the more expensive multimeters in a single product range or just use the same board and ICs but omit some components?Hero999: where does sample rate come in?

author: Neilm
author_rank: Super Contributor
timestamp: November 25, 2009, 01:48:05 pm
content: The reason you will find the same board in several products is easy - economies of scale. If I am producing several flavours of widget I can get the cost of each one down by using as many common parts as possible - cases, boards, chips and so on. As the cost of each bit will fall the more I buy it will dramatically lower the cost to build the products. Same selling price - less cost = more profit.It goes a bit further than just not fitting some things. There are some products out there where the only difference between the lower spec product and the higher spec product is a switch in software - so if you knew the "secret password" you could change your bottom of the range widget into a top of the range widget. Of course not fitting bits makes the widget cheaper still.

author: Unknown
author_rank: Unknown
timestamp: November 25, 2009, 08:33:13 pm
content: It would be fun to see if big multimeter companies use the same hardware and downgrade the meter with crappier software in the cheaper models.Dave, perhaps you could check that if they send you two or more multimeters from the same product series? (that is if they allow you to open them)Although they might not be too happy if you rattle the cage and get too nosey.

author: qno
author_rank: Frequent Contributor
timestamp: November 27, 2009, 06:40:51 pm
content: It also has to do with the ADC.This is usually a power of 2. A 10 bit ADC can have 1024 different states. This can be divided into a range of -500 - 0 - +500 leaving some counts to indicate overflow.A popular display with a range of -1999 to 1999 has a 12 bit range ( 4096 counts).A 16 bit counter will have a range of 65535 counts allowing a display range of -32000 to +32000.You can see on the display range what kind of ADC is used.Then there is another thing, accuracy. If your ADC has an accuracy of 0.5% it makes no sense to have a display range of 16 bit.Note that the accuracy of the meter is always specified as the percentage of the full range.If you have a multimeter with a display range of 32000 and the accuracy of the meter is 0.25% (which is pretty good, ask Dave) at 10.000 Volts the meter can indicate anything between 10.040 and 9.9960. Hardly any better than a simple 1999 count meter with the same spec of 0.25%.To design a good multimeter you have to have a good stable reference over a large temperature range and keep out the noise. This is more important than adding a few digits.

author: Unknown
author_rank: Unknown
timestamp: November 29, 2009, 07:22:20 pm
content: Thanks! That clears up a lot of things.I thought they had some way of dealing with polarity before the voltage gets to the ADC so that they can use the full ADC range.