A/D conversion techniques in multimeters - Page 1
Forum section: electronics / testgear

author: nightfire
author_rank: Frequent Contributor
timestamp: December 12, 2023, 10:54:58 pm
content: Situation: I want to try and learn a bit more about the inner workings of multimeters and their signal conditioning.So far I have just spend some time to look into old Fluke material for descriptions and circuit diagrams of the 8020A and 8600A (desk model, not typo).So it seems that I grasped the idea of how a dual-slope DMM works, including the description of how the cycle parts like Autozero, integration, read, and if applicable, overload are handled.- I guess thats what they say nowadays about a DMM having a "analog" Frontend?- Are there more modern techniques of reading a signal, especially when using TrueRMS and distorted AC measurements? I remember that Keysight (and maybe Hioki?) were mentioned as having a way more modern way of sampling AC, and I would be interested to read more about the techniques used in there

author: David Hess
author_rank: Super Contributor
timestamp: December 13, 2023, 01:34:12 am
content: Not all old meters used dual-slope integration. Charge balancing, run-up, and run-down were common before Intersil released their integrated dual-slope solution which had the virtue of being inexpensive and high performance.Newer designs for modern meters may use delta-sigma conversion because it integrates well with CMOS logic.High resolution successive approximation conversion can be done for a reasonable price now and simple digital processing will return all measurements simultaneously. Some older meters use successive approximation to achieve high measurement rates at moderate resolution.

author: nightfire
author_rank: Frequent Contributor
timestamp: December 14, 2023, 10:32:21 pm
content: Thanks for the pointer with delta-sigma conversion!I just want to understand in which regard to look for pointers how the insides of a certain DMM would work, as this does not get much coverage.From what I have read and understood so far, the current setup of Fluke DMM uses still the old-school way of doing things like pre-2000, whereas some other manufacturers like Keysight and Hioki have this method employed in their recent portfolio?

author: Kleinstein
author_rank: Super Contributor
timestamp: December 14, 2023, 11:14:11 pm
content: There are / were quite different ADCs used in DMMs. For a long time handheld meter were mainly using dual slope (the ICL7106 data-sheet may be worth a look to learn about the dual slope) - modern ones since about 2000 may be SD type integrated in the chip set.For the higher resolution bench tops it is mainly charge balancing or SD ADCs. In the old days they also had 5 and 6 digits from dual slope and Fluke had a odd succcessive approximation variant (recirculating reminder).Modern handheld DMMs usually have a speciallized chip that does most of the functions.For the AC part the classical way is some analog conversion from AC to DC, with a simple rectifier or a RMS-DC onverter chip like AD636 and similar.A relative modern alternative is sampling the AC signal with a relatively fast ADC and calculate the RMS value digitally. This is found in modern Keysight bench DMMs, AFAIK the Keithly DMM6500, but also some relatively cheap handhelp DMMs (e.g. Uni-T 133A) integrated in the DMM chip. With the handheld meters this is usually limited to a relatively low frequency (e.g. 1-5 kHz).

author: Phil_A
author_rank: Newbie
timestamp: December 14, 2023, 11:23:49 pm
content: One resource are the IC datasheets. For instance the ICL7206 datasheet has the schematics for inexpensive 3 1/2 digit multimeters along with references to app notes. Hycon Technology has detailed IC datasheets used in higher end multi meters (mixed signal microcontrollers, analog front ends and LCD controllers).