[volt-nuts] Homebrew DVM

Andreas Jahn Andreas_-_Jahn at t-online.de
Tue Nov 15 05:38:25 UTC 2011


Hello Fred,

> Got it stable up to 1 mV in a 1 V range.

Did you check only noise and temperature or also linearity?

My experience is that with a simple dual slope integrator the capacitor 
soaking creates big linearity errors.
I built my first dual slope with a 2,49V LM336 reference a 4051 and a TLC271 
and a simple 1uF mylar capacitor.
Due to single supply I used the 2,5V of the reference as vitual ground. So 
the input range of 0..5V was
divided into two ranges from 0..2.5 (negative with respect to virtual 
ground) and from 2.5 to 5V. (positive).
With a simple mylar capacitor the non-linear gap between negative and 
positive range was around 1 mV.

So linearity is only around 12 bits with this design. To reach this 
linearity I had to do some software
trick to deal with the soakage. After each switching of the input 
multiplexer to another channel I did
some waste A/D conversions with the new channel to let the capacitor 
stabilize.

Years later I read the article of Bob Pease about capacitor soaking and 
exchanged the mylar against
polypropylene. The non-linear gap is now around 100uV wich is around 15 bits 
linearity.

At the moment I'm experimenting with the 24 bit LTC2400.
He has a unipolar input range of 0..5V (+ some mV overrange) with a 5V 
reference.
The best results I get with a LT1027CCN8-5 reference. From the many 
references
I tried up to now the LT1027 and MAX6250A in DIL-housing seem to be the best
when regarding hysteresis.

The effort to get around 10 ppm stability is enormous:
Galvanic isolated battery powered design.
Temperature of reference is measured and compensated by a NTC (3rd order 
polynominal compensation)
Linearity of A/D-Converter is compensated. (see AN86 of LT)

The main issue is noise of the converter of around 20uVpp.
So to get less than 1 uV I have to integrate measurements over 10 minutes.
Reference drift is around 10 ppm over 1 year.

I am looking now for a converter with less noise.
So what ADC are You using?

With best regards

Andreas



----- Original Message ----- 
From: "Fred Schneider" <pa4tim at gmail.com>
To: "Randy Evans" <randallgrayevans at yahoo.com>; "Discussion of 
precisevoltage measurement" <volt-nuts at febo.com>
Sent: Tuesday, November 15, 2011 12:56 AM
Subject: Re: [volt-nuts] Homebrew DVM


>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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