What is 6-digit multimeter?
It's multimeter that allows you to measure voltage, resistance and current with resolution up to 6 digits, that is resolution of on millionth parth of full scale. For example, when voltage range of this multimeter is 1V, smallest change it can differentiate is 1uV.
This multimeter is AC powered and is can be controlled via serial port, useful for connection to computer for logging or more advanced test setups, for example.
Why did you build this multimeter?
For years I was using cheapest multimeters with great success and had no need to explore realms of more advanced test instruments. Friend of mine had old Czechoslovakian Metra M1T242. This one probably isn't well known to folks from the better part of ex-iron curtain and in fact, it isn't something stellar after all. Four digit multimeter with nixie tubes (manufactured up to 80's!, later modified to LED displays and sold as M1T242A) but there is still quite a few of those instruments among hobbyists. I was able to cheaply acquire this meter too, and after quick repair it worked as expected. During the repair I noticed how much the meter suffers from having to use early 70's technology and thought of reworking the instrument to modern ICs, hoping to decrease loss heat (causing heating up the components - not something you want in precise test gear) and improve stability of the meter. Doing too deep cuts into existing meters (having some historical value, too) seemed somehow brutal, so I decided to build a replica with modern components. I started reverse engineering and redrawing of the meter with aforementioned friend, but after a while the initial enthusiasm evaporated, leaving me halfway in the project. I realized investing too much time into 4-digit meter has little to no practical value, on the other hand the knowledge I acquired really called for building something related. I built a 1000 count integrating AD converter few years ago, made of a few opamps, handful of 74xx chips and beefy 5V power supply, but realm of 5+ digit AD converters was very hazy to me. So, scope creep happened, I tossed the old 4-digit meter plans and decided to better understand how the integrating converters do work and i started with 6-digit voltmeter, being at the borderline between practical tool and low-end of high-end laboratory gadget. The voltmeter was working as I intended, but I was aiming higher, but converting voltmeter to ohmmeter and ampermeter should be easy enough, right? (spoiler alert: not exactly rocket science, but not easy)
Why do I need 6-digit multimeter?
Well, in most cases you probably don't. For majority of typical DIY hobby work, classic 3,5 digit multimeters like venerable DT-830B are totally adequate to determine whether the 5V regulator is blown or not, whether that thermistor reacts to hand heat or if the current through LED is reasonable.
On the other hand, you might use it, if it was obtainable, see below. Hobbyists usually aren't doing projects that require precision test gear, because, well, they don't have precision test gear to start with.
You explained I likely don't need this instrument, why should we bother?
In recent years, the term citizen scientist is getting traction. Not only at hackaday, but since we are at hackaday, perhaps this is good entry point. There is a lot of projects, falling outside the usual DIY tinkerer stereotype and reaching more into field of applied research, biology experiments and material science, where measurement is keystone of research. Take a look at garage of Sam Zeloff or Ben Krasnow - I believe their research would be more difficult without proper tools. Fortunately for all of us, they can work and share their tremendous knowledge also thanks to precision test instruments and tools.
Commercial entities doing this kind of research have access to instruments with high price tags; buying such as instrument for DIY work may be simply too expensive. There is possibility of acquiring older instrument via usual online sources, but second hand instrument will have unknown use history, perhaps even hidden problems and repair of such as instrument could be sometimes impractical or impossible without access to unobtainium-made parts. I believe in knowledge sharing and create or support open-source projects, so I decided to start this project to share knowledge I acquired during my personal research, bringing result that can lower the barrier to high performance measurement tools for DIY-ers, as well as open the world of test gear to mere mortals.
Apart from multimeters, there are other test gear projects, that can bring cheaper test gear closer to anyone interested, like this #8 GHz Sampling Oscilloscope from Ted Yapo or #Scalar Network Analyser
Are there any other open-source high-resolution multimeter projects?
AFAIK not yet.
While we can find a few multimeters projects built around arduino or perhaps simple AD converters, most of them aren't built with high precision in mind. Especially, DIY high resolution integrating AD converters are extremely rare and as far I know, there are no other finished and open implementations around the internets.
Can I replicate this projects?
Yes, you can. I entered this project as a part of Tech at home challenge, so I'm building this project entirely from parts I had in my junkbox. Many of those components are obsolete for years or decades, but whenever possible I'll try to point out modern equivalent of such as part.
All schematics and all source codes will be released to public during this project.