[volt-nuts] Homebrew DVM
Fred Schneider
pa4tim at gmail.com
Mon Nov 14 23:56:29 UTC 2011
I did some dual slope ADC experiments. Just on a breadboard and using an Arduino for the math and display. No fancy stuff. The VRef was just a resistive divider from the 5 V. It was just an experiment. Used a 4066, LF356 and a LM339 but the thing worked wonderfull. Got it stable up to 1 mV in a 1 V range. Not shocking but with a nice Vref, better switches, a real good capacitor it can be made better. While figuring out how that worked I misunderstood how it should work and by accident re-invented a ( self nulling) VF converter, using my counter as display. That was fun too.
Now I have a 26 bit ( 24 bit usable) ADC waiting to be used interfaced to the arduino, I am making/experimenting with some Vrefs and have parts for a compound chopper based on a LT1052. But the realisation will be up in the future, first gathering more knowledge about percision and micro volt techniques.
Fred PA4TIM
Op 15 nov. 2011 om 00:10 heeft Randy Evans <randallgrayevans at yahoo.com> het volgende geschreven:
> 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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