3.5 vs 4.5 digit bench DMM (NOT volt nuts thread) - Page 1
Forum section: electronics / testgear

author: 001
author_rank: Super Contributor
timestamp: September 04, 2018, 06:30:41 am
content: Hi!I see a lot of vintage 3.5 and 4.5 digit manual range bench meters (hamegs flukes hp etc) at yours workshop photos.Is it for nostalgy? Do they still usable? Is it practical? Any REAL everyday difference 3.5 vs 4.5 digits?

author: Fungus
author_rank: Super Contributor
timestamp: September 04, 2018, 07:45:05 am
content: 4.5 digits won't make much difference for measuring a typical 5% resistor, no, but you can use the 4.5 digit meter to calibrate your 3.5 digit meters.(How do you know your 4.5 digit meter is accurate? Simple: You use your 5.5 digit meter... )

author: dzseki
author_rank: Frequent Contributor
timestamp: September 04, 2018, 08:04:57 am
content: I also have a 4.5 digit bench DMM for daily use, and I'd say it is the best bang for the buck, you can get such instrument for less than 50USD, yet they display say mV resolution in 20V range, which is more than enough for general use IMHO. Sure you can get more digits, but if you do some basic repair it is usualy not the 6th digit (and not even the third) what makes the difference when something is wrong... but since they are so cheap why not? Also, old gears were produced with the spirit of state-of-the-art, it is always nice to have some gear that was made with "love" and effort, even if the specifications may seem modest these days.

author: Berni
author_rank: Super Contributor
timestamp: September 04, 2018, 08:31:53 am
content: My most often used DMM is a handheld Agilent U1273A with its 4.5 digits of resolution.It gives you enough digits to cover most uses. For example checking supply rails you can use the 30V range and will give you for example 12.001 V on the display. It lets you see that its the correct voltage and 1mV resolution lets you quickly see if the rail is jumping or drifting about.

author: Gyro
author_rank: Super Contributor
timestamp: September 04, 2018, 08:38:30 am
content: I use my old Datron 1041M 41/2 digit bench dmm daily too - it gives me high input impedance and 1uV resolution. 10 reading/sec (and autoranging) makes a big difference when waiting for readings to settle too.

author: ogden
author_rank: Super Contributor
timestamp: September 04, 2018, 08:49:39 am
content: Hi! Please don't spawn/spamsimilar threads. You can ask about DIY DMM in this thread as well. No need to waste internet ink. Now that thread is locked by moderator and my time I spent posting, is essentially wasted. Shame.

author: 001
author_rank: Super Contributor
timestamp: September 04, 2018, 09:03:41 am
content: Quote from: ogden on September 04, 2018, 08:49:39 amWhy so angry? It is "Products" forum here and I`m looking for oldtimer user opinions.

author: ogden
author_rank: Super Contributor
timestamp: September 04, 2018, 09:07:48 am
content: Quote from: 001 on September 04, 2018, 09:03:41 amDont'you see? - I feel that my effort investing time to answer you, is wasted.QuoteWell, Simon (moderator) do not agree.

author: David Hess
author_rank: Super Contributor
timestamp: September 04, 2018, 10:00:08 am
content: There is more subtlety to the design of a voltmeter than is readily apparent so novices may start designing one and get stuck with a seemingly intractable problem.Sometimes the difference between a 3.5 and 4.5 digit design is simply that the later can get by with one less range while achieving the same resolution.Only specialized digital meters do *not* use an integrating ADC in one form or another because selecting the proper integration time will reject 50/60 Hz interference which would otherwise compromise performance.

author: Kleinstein
author_rank: Super Contributor
timestamp: September 04, 2018, 12:10:33 pm
content: Quote from: David Hess on September 04, 2018, 10:00:08 amThere are a few older Fluke and Phillips meters that use an odd type of re-circulating ADC with a sample & hold input stage. Though not special meters they are an exception.Quite a lot of the lower end meters have a lowest range of around 200 mV (maybe 300/400 mV with an ADC for 3000 / 4000 counts). This is because this is the input range for there ADC. An important point for looking at 4.5 digit resolution can be to resolve lower voltages. Though lower ranges might do the same trick, they are no common. With the modern ADC's this even makes perfect sense as the higher resolution ADC might be cheaper than an extra range (gain setting).There seem to be not much market for 3.5 digit bench top DMMs, especially now that the low end ADC's are not that much cheaper. The higher price for the better meters is to a large part due to the better accuracy (reference and dividers, not the ADC itself).

author: 001
author_rank: Super Contributor
timestamp: September 04, 2018, 12:12:24 pm
content: Thanx!What ADC (16bit in 4.5 region?) is right for You?Is 20ppm reference will be ok for 4.5 digit meter?

author: nctnico
author_rank: Super Contributor
timestamp: September 04, 2018, 12:17:20 pm
content: I always want 4.5 digits at least because then you can still see mA current changes on a 10A or 20A measurement range.

author: iMo
author_rank: Super Contributor
timestamp: September 04, 2018, 02:47:01 pm
content: QuoteYou do not need 7.5 DMM to watch ~microvolts drifts of your voltage sources.I own a METEX 3850D (3 3/4) for more than 20y and it still works fine. It is actually a 40000 ADC inside (MAX134), OP07s, etc..For differential measurements I've built myself a 100x diff amplifier (from junk box parts) and I can measure at 1uV resolution (+/- 3999uV). With a stable external reference you may get nice results..

author: Kleinstein
author_rank: Super Contributor
timestamp: September 04, 2018, 03:30:40 pm
content: For the reference quality, resolution and TC are in principle separate quality criteria. If the temperature is rather stable (e.g. in a lab) 20 ppm/K may still be still acceptable for a 4.5 digit meter, though not good. For outdoor field use I would consider in a problem / weak point.Comparing ADC's is not that easy, as it is not only the resolution / noise that matters. There are other properties that can be more important, like gain drift, linearity, input aperture, offset drift,... .The ICL7135 is a typical display oriented one: so 1 ADC step usually corresponds to 1 display step and adjustment / calibration is usually analog with trimmers, possibly in combination with very accurate dividers instead of individual adjustment.Many modern DMMs use an ADC with higher resolution than the display resolution and than use numerical adjustment by a multiplication in software. SD-ADC chips usually integrate essentially all the time, while the dual slope ones usually only measure during about 1/3 the time - this can give the SD ones an slight advantage when it comes to noise.The 4.5 digit ICL7135 and similar is a bit more then 15 bits of resolution (+- 19999 counts). With numerical adjustment it should be more than 16 Bits of resolution, more like 18 Bits at least, though some of this might be from oversampling.Modern low end DMMs are usually build around a special DMM chip set that includes also auto ranging and the input amplifier/buffer and AC functions. Today chances are high to find an SD ADC with numerical adjustment.

author: David Hess
author_rank: Super Contributor
timestamp: September 04, 2018, 05:19:58 pm
content: Quote from: Kleinstein on September 04, 2018, 12:10:33 pmThis became more common as ADCs and input amplifiers got better. In the past, 2 (or 4) volts would have been the standard ADC input range and 200mV (or 400mV) would have been a special x10 mode with separate offset and gain adjustments.I never noticed it at the time but I did later when I started studying digital multimeter design in detail and ran across several examples where the 3.5 digit meters had a 200mV input range and the 4.5 digit meters of the same basic design had a 2V input range.Then there were the weird meters which were 4.5 digits but keep the lower input voltage ranges or even had a 20mV range but I think they were intended for working with strain gauge transducers so not really relevant for most users unless microvolt resolution of a very small voltage range is desired. If you know what to look for though, these can be great deals.But I think better for precision analog design are meters which have a greater than gigohm input resistance up to +/-20 volts DC or more. I just wish modern meters like this had external binding posts so that a 10 megohm shunt resistor can be added if needed. You can add a dual banana plug but then the meter's original guarded probes won't fit ...

author: floobydust
author_rank: Super Contributor
timestamp: September 04, 2018, 05:49:23 pm
content: I have an old Wavetek BDM-40 4-1/2 digit bench DMM that uses ICL7135 and 1N829.Sometimes I use it because it has LED's which are much easier to readThe problem with older DMM's is they have terrible input protection circuits.I just killed the A/D with a few kV zap and after looking at the schematic- it has nothing at all for overvoltage protectionSomelow cost chinese 5-digit panel metersare usingMicrochip MCP342118-bit A/D with built-in voltage reference, PGA, SOT23-6.I'd like to build an old school LED or OLED DMM, using modern DMM IC like HY3131. I haven't found the IC for sale though.

author: 001
author_rank: Super Contributor
timestamp: September 04, 2018, 06:10:44 pm
content: Hi! China 5 digit panel meners are linear in VERY small range. It is ok for battery check but unusable as DMM

author: Kleinstein
author_rank: Super Contributor
timestamp: September 04, 2018, 06:25:33 pm
content: Those modern DMM chip sets are not generally available. Likely there are just no suitable customers: there is little use to buid a $20 DMM in a small series, and those companies who build them in the millions get there chips directly from the factory.Those small little SD-ADCs like MCP3421 are supposed to be not that bad. So they might actually be suitable for a simple 4.5 digit meter. So the trouble in some of the panel meters could be from the input and protection. Self heating of the shunt can be a big problem for a very cheap solution.The ADC is only a small part of a DMM - down to sot23-6, no more bulky DIP40 like the ICL7106. The expensive and tricky parts are more like shunts, dividers, switches and the case and testing (EMI, Safety).

author: David Hess
author_rank: Super Contributor
timestamp: September 04, 2018, 06:54:05 pm
content: Quote from: floobydust on September 04, 2018, 05:49:23 pmI have never run across one which lacks good input protection. But early CMOS integrating ADCs were both unreliable and very sensitive to ESD.QuoteThe HY3131 is one of those modern all-in-one multimeter ASICs so more suitable for a handheld meter.For a bench unit, I would hack something together with an 8-bit PIC or similar, high resolution delta-sigma instrumentation ADC, and external reference. If I wanted RMS measurement capability, then I would use a high precision sampling ADC and do the RMS calculation in the microcontroller instead of using a separate RMS converter IC.The tricky part is switching the input divider. The HY3131 is intended to use a ground switched input divider which is great for simple automatic ranging but it makes the input resistance vary with range which I hate.