Homebrew DVM
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.
Thanks,
Randy Evans
In message 1320939951.98248.YahooMailNeo@web37902.mail.mud.yahoo.com, Randy E
vans writes:
My hope is that the DVM will be able to measure to 8.5 digits [...]
Do you mean 8.5 digits single-shot, or 8.5 digits with averaging ?
Considering that you can get off-the shelf ADC's where 24 bit is routine
and 26 bits available, I'm not sure I'd bother...
--
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.
8.5 digits with averaging.
I did consider that but the ADCs I am familiar with only have a 5V input range so range resistors would be required, not desirable for me. Also, they won't directly take the 7.2V ref input from a LTZ1000. Plus the AN-260 circuitry is not too complex. Otherwise, I agree with you.
Randy
From: Poul-Henning Kamp phk@phk.freebsd.dk
To: Randy Evans randallgrayevans@yahoo.com; Discussion of precise voltage measurement volt-nuts@febo.com
Sent: Thursday, November 10, 2011 7:51 AM
Subject: Re: [volt-nuts] Homebrew DVM
In message 1320939951.98248.YahooMailNeo@web37902.mail.mud.yahoo.com, Randy E
vans writes:
My hope is that the DVM will be able to measure to 8.5 digits [...]
Do you mean 8.5 digits single-shot, or 8.5 digits with averaging ?
Considering that you can get off-the shelf ADC's where 24 bit is routine
and 26 bits available, I'm not sure I'd bother...
--
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.
Op 10-11-2011 16:51, Poul-Henning Kamp schreef:
Do you mean 8.5 digits single-shot, or 8.5 digits with averaging ?
Considering that you can get off-the shelf ADC's where 24 bit is routine
and 26 bits available, I'm not sure I'd bother...
Oooh, gorgeous!
Where do I buy one and what does it cost?
- Rob.
Hi, Randy!
AN-260 is a very bad example of slow medium scale ADC.
I think that 130 db SNR ADS1282 is much better. Also, 8,5-digit Solartron 7081
have a simple and cheap delta-sigma ADC with 0.06 ppm noise and 51 sec.
integration time.
Mickle T.
Thursday, November 10, 2011, 8:20:58 PM, you wrote:
RE> 8.5 digits with averaging.
RE>
RE> I did consider that but the ADCs I am familiar with only have a 5V input
RE> range so range resistors would be required, not desirable for me. Also,
RE> they won't directly take the 7.2V ref input from a LTZ1000. Plus the AN-260
RE> circuitry is not too complex. Otherwise, I agree with you.
RE>
RE> Randy
Date: Thu, 10 Nov 2011 17:24:02 +0100
From: rob.klein@smalldesign.nl
To: volt-nuts@febo.com
Subject: Re: [volt-nuts] Homebrew DVM
Op 10-11-2011 16:51, Poul-Henning Kamp schreef:
Do you mean 8.5 digits single-shot, or 8.5 digits with averaging ?
Considering that you can get off-the shelf ADC's where 24 bit is routine
and 26 bits available, I'm not sure I'd bother...
Oooh, gorgeous!
Where do I buy one and what does it cost?
- Rob.
Hi Rob,
Try the LTC2441, 8.5 hz updates at about 26 bits with pretty good linearity.
Most of the "24 bit" DAC are only really good for about 20-22 bits due to noise and non-linearity.
MK
Op 10-11-2011 20:39, m k schreef:
Hi Rob, Try the LTC2441
Ehm, a search on LTC's website for "LTC2441" returns this:
Your search - ltc2441 - did not match any documents.
No pages were found containing "ltc2441".
No 8.5Hz, 26-bit converter shows up in the parametric search, either.
- Rob.
Hi Rob,
It was the LTC 2440 and 2442.
Date: Thu, 10 Nov 2011 22:36:10 +0100
From: rob.klein@smalldesign.nl
To: volt-nuts@febo.com
Subject: Re: [volt-nuts] Homebrew DVM
Op 10-11-2011 20:39, m k schreef:
Hi Rob, Try the LTC2441
Ehm, a search on LTC's website for "LTC2441" returns this:
Your search - ltc2441 - did not match any documents.
No pages were found containing "ltc2441".
No 8.5Hz, 26-bit converter shows up in the parametric search, either.
- Rob.
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and follow the instructions there.
Hello MK,
did you already build a real cirquit with the LTC2440?
It has a differential input 2.5+/-2.5V for bridge measurement cirquits like
weight scales.
So I think it will be difficult to get a unipolar 0..5V measurement out of
this device without resistive divider.
Which buffer cirquits do you use to feed the low-impedant (capacitive)
inputs?
With best regards
Andreas
----- Original Message -----
From: "m k" m1k3k1@hotmail.com
To: volt-nuts@febo.com
Sent: Thursday, November 10, 2011 10:50 PM
Subject: Re: [volt-nuts] Homebrew DVM
Hi Rob,
It was the LTC 2440 and 2442.
Date: Thu, 10 Nov 2011 22:36:10 +0100
From: rob.klein@smalldesign.nl
To: volt-nuts@febo.com
Subject: Re: [volt-nuts] Homebrew DVM
Op 10-11-2011 20:39, m k schreef:
Hi Rob, Try the LTC2441
Ehm, a search on LTC's website for "LTC2441" returns this:
Your search - ltc2441 - did not match any documents.
No pages were found containing "ltc2441".
No 8.5Hz, 26-bit converter shows up in the parametric search, either.
- Rob.
volt-nuts mailing list -- volt-nuts@febo.com
To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts
and follow the instructions there.
volt-nuts mailing list -- volt-nuts@febo.com
To unsubscribe, go to
https://www.febo.com/cgi-bin/mailman/listinfo/volt-nuts
and follow the instructions there.
The ADS1282 alswo only has a 5V unipolar input range and I want a 0 to 12V input range without dividers. I don't have any information on the Solartron 7081.
Randy
From: Михаил timka2k@yandex.ru
To: Randy Evans randallgrayevans@yahoo.com; Discussion of precise voltage measurement volt-nuts@febo.com
Sent: Thursday, November 10, 2011 9:17 AM
Subject: Re[2]: [volt-nuts] Homebrew DVM
Hi, Randy!
AN-260 is a very bad example of slow medium scale ADC.
I think that 130 db SNR ADS1282 is much better. Also, 8,5-digit Solartron 7081
have a simple and cheap delta-sigma ADC with 0.06 ppm noise and 51 sec.
integration time.
Mickle T.
Thursday, November 10, 2011, 8:20:58 PM, you wrote:
RE> 8.5 digits with averaging.
RE>
RE> I did consider that but the ADCs I am familiar with only have a 5V input
RE> range so range resistors would be required, not desirable for me. Also,
RE> they won't directly take the 7.2V ref input from a LTZ1000. Plus the AN-260
RE> circuitry is not too complex. Otherwise, I agree with you.
RE>
RE> Randy