Hello Randy,

with a capacitive divider 2:1 or 3:1 based on a LTC1043 + LT1051
you can extend the 5V range to 10 or 15 V. The capacitive divider
is very stable over temperature and time.

With my 2:1 dividers I have less than 1 ppm drift over a
10-40 degrees (celsius) temperature range.

Absolute accuracy of the divider factor depends mainly on
charge injection of the switch and capacitor losses.
I measured values between 2-10ppm depending on different
pin configurations. Im not shure wether the examples in the
datasheet are using the correct pin numbers for minimum charge injection.

Another Question: how do you get a unipolar 0..5V range with a ADS1282?
The absolute input of the PGA-Amplifier is specified from 0.7 .. 3.75V.

With best regards

Andreas

----- Original Message -----
From: "Randy Evans" randallgrayevans@yahoo.com
To: "Михаил" timka2k@yandex.ru; "Discussion of precise voltage
measurement" volt-nuts@febo.com
Sent: Friday, November 11, 2011 5:01 AM
Subject: Re: [volt-nuts] Homebrew DVM

Hi,

I have not built one yet, but it seems to be one of a very few that goes beyond 24 bits.

MK

Randy Evans randallgrayevans@yahoo.com writes:

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

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@devereux.me.uk
To: volt-nuts@febo.com
Cc: Discussion of precise voltage measurement volt-nuts@febo.com
Sent: Sunday, November 13, 2011 11:42 PM
Subject: Re: [volt-nuts] Homebrew DVM

Randy Evans randallgrayevans@yahoo.com writes:

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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To unsubscribe, go to https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts
and follow the instructions there.

The Wikipedia page on the integrating ADC also has some more
description on the basic dual-slope ADC as well as some of the
enhancements that have been made to it over the years.  Some
of the material from the patents is mentioned as well.

http://en.wikipedia.org/wiki/Integrating_ADC

Many of the designs end up being quite simple, but the devil

is in the details, obviously.

What I find most interesting about the design is that, in many
cases, components with absolute precision are not required.
Stability and low TC are what is needed and sometimes the
ratio of certain critical values are important.  Everything
else can be calibrated out at the end (within reason, of course).

Randy.

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@yahoo.com het volgende geschreven:

In message 1321314126.66316.YahooMailNeo@web161704.mail.bf1.yahoo.com, Randy
Scott writes:

Well, that's the fundamental trick of the integrating A/D:  You
move the precision requirement to your timebase, and we are
much better at those, than at precision resistors/capacitors etc.

--
Poul-Henning Kamp      | UNIX since Zilog Zeus 3.20
phk@FreeBSD.ORG        | TCP/IP since RFC 956
FreeBSD committer      | BSD since 4.3-tahoe
Never attribute to malice what can adequately be explained by incompetence.

Hello Fred,

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@gmail.com
To: "Randy Evans" randallgrayevans@yahoo.com; "Discussion of
precisevoltage measurement" volt-nuts@febo.com
Sent: Tuesday, November 15, 2011 12:56 AM
Subject: Re: [volt-nuts] Homebrew DVM

Andreas Jahn wrote:

The HP3458A and the HP34401A ensure that the integrator output voltage
only represents a small fraction of the integral of the signal input.
This significantly reduces the effect of dielectric absorption in the
integrator feedback capacitor.
The integrator and control logic act somewhat like a sigma-delta
integrator during the runup phase.
The effect of dielectric absorption can be further reduced by suitable
choice of runup algorithm.
The on resistance modulation of analog switches in series with the
resistor connected between the signal and the integrator summing
junction can be a significant source of nonlinearity, however it is, in
principle, possible to correct for this.

Bruce

No, did not check all. Just hooked up a powersupply and looked what it did. It was at first absolute not lineair. Before it was finished my ardiuno went up in smoke because of the breadboard ( +5 V jumped out and I did not think and just pushed it back in the +15V rail) but It was not very lineair until I made a sort of autozero and used a correction factor. After that it was sort of lineair, I had made it so that it did a measurement of the test-voltage then It switched the input to ground and made a second measurement. That value I used to correct the other.

It was just for fun, to learn about the principle of ADCs. But it infected me with the volt nut virus.
It was not intended to become a real meter.

http://www.pa4tim.nl/?p=2482 there you see the schematic and some pictures ( the top one is from the circuit that, i was told, turned out to be a VF converter)

Fred PA4TIM

Op 15 nov. 2011 om 06:38 heeft "Andreas Jahn" Andreas_-_Jahn@t-online.de het volgende geschreven: