Electronics > Metrology

7.5digit diy voltmeter?

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Bootalito:
I was thinking of embarking on a voyage: constructing an open source DIY 7.5digit volt meter.  Now I know doing this is more than just buying highly speced parts and slapping them together, but it doesn't seem out of the realm of possibility to do the following:
1. Construct an xdev's LTZ1000 reference, and burn it in for 1000hours
2. divide the reference in half(or something) using a precision resistor network and feed this into a 24ENOB ADC like AD71772 (has built-in rail to rail op amps).  Its not like a can design my own custom multi-slope integrator....
4. Feed test voltage in through another precision resistor network (so only a single range volt meter), or not...make it like a 5V max volt meter or something...
5. Add some input protection: PTC, MOV, maybe a Zener in case someone feeds in to much voltage.
6. Use a linear power supply with a properly isolated digital ground and analog ground
7. Use some sort of uC, a decent size LCD, a couple of rotary encoders, a couple of pushbuttons
8. Bob's your uncle!

Of course I would expect a long process with experimentation, and many board revs, but with the right engineering a 7.5digit volt meter, or at the very worst a 6.5digit noiseless volt meter.  I couldn't find anywhere where someone has taken this on, or perhaps I'm wrong?

The Soulman:
I think you meant AD7172?

http://www.analog.com/media/en/technical-documentation/data-sheets/AD7172-2.pdf


--- Quote ---24 noise free bits at 5 SPS
--- End quote ---

In theory it can reliable produce 16777216 counts or 7,5 digits, accuracy and linearity at that level are a entirely different thing.

Edit: Scullcom hobby's on youtube did a similar project, was going to link it here but youtube ain't co-operating.  :rant:

james_s:
It sounds like an interesting project if your primary goal is to stretch your engineering abilities and pull off a unique challenge. On the other hand if you're trying to reduce the cost of 7.5 digit meters I don't think that's going to happen, frankly I'd be surprised if you can get all the parts for less than what a good used commercial meter goes for.

Bootalito:
@Soulman
I meant the AD7177-2 with the 24.7ENOB @ 5SPS http://www.analog.com/media/en/technical-documentation/data-sheets/AD7177-2.pdf
I've watched all of Scully's videos, and this project would kind of be an extension of that. It doesn't look like he ever attained the precision, accuracy, he was shooting for, nor did he complete any long term stability testing.  Greg Christienson improved upon the design here: https://www.barbouri.com/2016/05/26/millivolt-meter/#comment-523, however upon close inspection of his PCB layout I think he may have made a few errors like not including the vREF and ADC in AGND with a star ground at the ADC, not providing a direct current path back to ground for the VREF and ADC (which would probably be fixed if they were put in AGND), placing DGND under the VREF and ADC, using a common LM7805 instead of a low noise linear reg, placing the linear reg close to the ADC and VREF (heat) , placing a ground plane right next to power traces (capacitance), and placing some bypass caps down an "alley way" trace away from pins.

@james_s

Yes this project would be solely as a new challenge for myself.  It would force me to upgrade my equipment so I can take the appropriate measurements (my wife is going to kill me...), I'd have to develop all kinds of testings procedures to collect accurate repeatable data, etc.  I'm not even an electrical engineer (Chemical Eng by degree, Control Systems Eng / programmer by trade) or have any formal electronics training.  Even if I can only pull of a decent 5.5digit meter the experience would be invaluable.

ez24:

--- Quote from: Bootalito on January 04, 2018, 03:05:15 am ---
--- End quote ---

Thanks for your info and links.  I am trying to determine if there was alien interference into the electronics field in the 1990's.  I had given up until I found a link to a link in your post.  So I am working with what I can find now.   :-+  There were power supplies that were very advance and in 20 years they have completely disappeared (the people).  There was a photo in the links that said it was them but it was a government building that has rocket shape designs on it.  I hope they find out about this message and invite me to their planet to stop me from exposing them because I need a vacation.


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