[volt-nuts] Homebrew DVM

[John Devereux]

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