[volt-nuts] Homebrew DVM
Randy Evans
randallgrayevans at yahoo.com
Mon Nov 14 23:10:28 UTC 2011
John,
Thanks for the info. I will read them tonight and assess wether I want to proceed with my original plan or revisit it. I have no particular preference, just what is easiest and cheapest but high resolution and accurate.
Randy
________________________________
From: John Devereux <john at devereux.me.uk>
To: volt-nuts at febo.com
Cc: Discussion of precise voltage measurement <volt-nuts at febo.com>
Sent: Sunday, November 13, 2011 11:42 PM
Subject: Re: [volt-nuts] Homebrew DVM
Randy Evans <randallgrayevans at yahoo.com> writes:
> I am in the process of designing a homebrew DVM that initially will
> only cover 0 to 12VDC measurement range. Perhaps in the future it
> could be expended for a wider range but the issue is getting very
> accurate and stable (stable is the key operative issue) range divider
> resistors. The design is based on the National Semiconductor app note
> 260, but modified with more modern components such as low charge
> injection analog switches, faster comparators, LTZ1000 reference,
> extremely linear integrator, low cost uprocessor, etc. My hope is
> that the DVM will be able to measure to 8.5 digits over this limited
> range. We shall see. It should be a fun project, regardless.
> The big issue is the lack of noise rejection characteristics. A
> substantial amount of averaging may be required so fast 8.5 digit
> measurements might be out of the question. However, it should be a
> low cost measurement technique for high accuracy measurements of
> stable voltages. Obviously, the measurement accuracy would be largely
> determined by the accuracy of the LTZ1000 reference.
> Since the integrator is the key circuit for this design, I would
> really like to see the integrator design in the HP-3458A, if anyone
> could provide a copy of the circuit. Comments on the approach would
> be apprecitated.
Hi Randy,
I have an ambition to do this too. But I think there are better circuit
arrangements that are not really any more complicated. Or at least it
seems that way right now, before I have tried to make them work!
Firstly you have to make a pretty good circuit before you can beat a
cheap single-chip sigma-delta ADC, as others have pointed out. That
application note predates these I think.
You could look at some of the patents on multislope converters (as used
in the HP3458A, and all the top meters as far as I know). They are
surprisingly readable. Also many of them have expired by now, although
that does not matter for educational use.
The "basic" HP patent is 4357600:
<http://www.google.com/patents?id=vCUxAAAAEBAJ>
I think this is the HP3458A.
There is a later HP one 4951053 which refines it with a greatly improved
switching technique <http://www.google.com/patents?id=INImAAAAEBAJ>. But
the 3458A does not use it AFAIK.
Then fluke get in the game with 5321403 and 5565869, describing a
switching sequence with imnproved elimination of charge injection
errors. (These have not expired).
There are lots of others if you search on "multislope" but I found these
the most interesting.
--
John Devereux
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