| Author |
Message |
Harald Noack
Guest
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 Posted:
Sun Feb 13, 2005 6:10 am Post subject:
Q: Construction of a 8.5 digit multimeter |
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Hello!
Has someone of you experience how a high precision multimeter is constructed
?
e.g.: http://www.home.agilent.com/USeng/nav/-11250.536881781/pd.html
How is the voltage reference stabilized? (by using an oven?)
Are the input voltage divider temperature stabilized ? (temp. coeff.)
How is the self calibration done?
What type of ADC is used (maybe time to digital converter) ?
How is the integral and differential non-linearity measured and compensated
?
THANKS a lot for your help
Harald Noack
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Ken Smith
Guest
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| Posted:
Sun Feb 13, 2005 6:10 am Post subject:
Re: Q: Construction of a 8.5 digit multimeter |
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In article <377kiqF59opsfU1@individual.net>,
Harald Noack <noack@sbox.tugraz.at> wrote:
It could well be an ovened reference, but over a fairly narrow temperature
band, references can be made to be nearly flat. Without reading the
spec's carefully, you won't know.
[...]
| Quote: | What type of ADC is used (maybe time to digital converter) ?
|
These sort of things usually use a "multiple slope" converter. Many
simple ones use a dual slope converter. A capacitor is charged up by the
input for a fixed time and then discharged by a reference current. The
time taken for the capacitor to return to zero gives the reading. This
assumes that the capacitor is ideal.
When you get into higher end DVMs, things are trickier. The circuit
doesn't assume that the capacitor is ideal. Instead, it is run up and
down and extra time using reference currents. The soakage and resistance
effects of the capacitor are canceled.
| Quote: | How is the integral and differential non-linearity measured and compensated
?
|
Multi-slope converters generally have very good differential linearity.
With careful design capacitors can be made very linear. The design of the
integrator is very likely to be the magic of the design.
| Quote: |
THANKS a lot for your help
Harald Noack
|
--
--
kensmith@rahul.net forging knowledge |
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Joerg
Guest
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| Posted:
Sun Feb 13, 2005 6:11 am Post subject:
Re: Q: Construction of a 8.5 digit multimeter |
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Hello Ken,
| Quote: | Multi-slope converters generally have very good differential linearity.
With careful design capacitors can be made very linear. The design of the
integrator is very likely to be the magic of the design.
|
Sometimes companies go as far as having their own capacitors custom made.
Regards, Joerg
http://www.analogconsultants.com
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Winfield Hill
Guest
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| Posted:
Sun Feb 13, 2005 6:11 am Post subject:
Re: Q: Construction of a 8.5 digit multimeter |
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Harald Noack wrote...
| Quote: |
Has someone of you experience how a high precision multimeter is constructed?
e.g.: http://www.home.agilent.com/USeng/nav/-11250.536881781/pd.html
How is the voltage reference stabilized? (by using an oven?)
Are the input voltage divider temperature stabilized ? (temp. coeff.)
How is the self calibration done? What type of ADC is used
(maybe time to digital converter)? How is the integral and
differential non-linearity measured and compensated?
|
These are questions you have to answer and thoroughly understand to
evaluate a 5.5 or 6.5 digit voltmeter. You could start by studying
the service manual of Agilent's popular 34401A 6.5-digit multimeter.
For example, study its input-protection circuitry. A 7.5 or 8.5-
digit instrument raises these and many other issues, such as guards,
dc thermoelectric voltages, normal-mode ac-line-signal rejection,
etc., to dramatically higher levels.
--
Thanks,
- Win |
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John Larkin
Guest
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| Posted:
Sun Feb 13, 2005 6:11 am Post subject:
Re: Q: Construction of a 8.5 digit multimeter |
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|
On 12 Feb 2005 17:06:57 -0800, Winfield Hill
<hill_a@t_rowland-dotties-harvard-dot.s-edu> wrote:
| Quote: | Harald Noack wrote...
Has someone of you experience how a high precision multimeter is constructed?
e.g.: http://www.home.agilent.com/USeng/nav/-11250.536881781/pd.html
How is the voltage reference stabilized? (by using an oven?)
Are the input voltage divider temperature stabilized ? (temp. coeff.)
How is the self calibration done? What type of ADC is used
(maybe time to digital converter)? How is the integral and
differential non-linearity measured and compensated?
These are questions you have to answer and thoroughly understand to
evaluate a 5.5 or 6.5 digit voltmeter. You could start by studying
the service manual of Agilent's popular 34401A 6.5-digit multimeter.
For example, study its input-protection circuitry.
|
I have one of these. It's really a 4.5 digit box, except on AC where
it's more like 3.5, or 2.5 on some ranges. Internally, it's a variant
on dual-slope with HC4051's as the switches. Its biggest problem is
the huge amount of noise the VF display kicks into the front end. An
old Fluke 8842 has less digits but is a far better instrument.
John |
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Mike Harrison
Guest
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| Posted:
Sun Feb 13, 2005 5:49 pm Post subject:
Re: Q: Construction of a 8.5 digit multimeter |
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On Sun, 13 Feb 2005 01:15:47 +0100, "Harald Noack" <noack@sbox.tugraz.at> wrote:
| Quote: | Hello!
Has someone of you experience how a high precision multimeter is constructed
?
e.g.: http://www.home.agilent.com/USeng/nav/-11250.536881781/pd.html
How is the voltage reference stabilized? (by using an oven?)
Are the input voltage divider temperature stabilized ? (temp. coeff.)
How is the self calibration done?
What type of ADC is used (maybe time to digital converter) ?
How is the integral and differential non-linearity measured and compensated
?
Remember that an 8.5 digit DMM will typically not have anything like 8.5 digits absolute accuracy. |
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Winfield Hill
Guest
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| Posted:
Sun Feb 13, 2005 7:47 pm Post subject:
Re: Q: Construction of a 8.5 digit multimeter |
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|
John Larkin wrote...
| Quote: |
Win wrote...
... You could start by studying the service manual of Agilent's
popular 34401A 6.5-digit multimeter. For example, study its
input-protection circuitry.
I have one of these. It's really a 4.5 digit box, except on AC where
it's more like 3.5, or 2.5 on some ranges. Internally, it's a variant
on dual-slope with HC4051's as the switches. Its biggest problem is
the huge amount of noise the VF display kicks into the front end. An
old Fluke 8842 has less digits but is a far better instrument.
|
Sheesh, there must be something wrong with your meter! We have six
of them, of various vintages (including some from eBay) and haven't
experienced the degradation you describe. Furthermore, used in the
SLOW high-resolution 6.5-digit mode (integrates over 100 power-line
cycles), they show an amazing capability for a $1k instrument.
John, have you checked out how to get the 34401A in this mode (use
the MEASurement menu), and tried it? BTW, this instrument clearly
wasn't meant to be a micro-volt meter. Note, the Fluke 8842 has a
20mV scale, compared to the 34401A's 200mV lowest scale. Perhaps
that's what you like about it. We use Keithley meters for most of
our low-voltage measurements.
We have several high-performance digital voltmeters in my lab, but
they're large 19" relay-rack beasts and real space hogs on the bench.
--
Thanks,
- Win |
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John Larkin
Guest
|
| Posted:
Mon Feb 14, 2005 1:38 am Post subject:
Re: Q: Construction of a 8.5 digit multimeter |
|
|
On 13 Feb 2005 05:47:02 -0800, Winfield Hill
<hill_a@t_rowland-dotties-harvard-dot.s-edu> wrote:
| Quote: | John Larkin wrote...
Win wrote...
... You could start by studying the service manual of Agilent's
popular 34401A 6.5-digit multimeter. For example, study its
input-protection circuitry.
I have one of these. It's really a 4.5 digit box, except on AC where
it's more like 3.5, or 2.5 on some ranges. Internally, it's a variant
on dual-slope with HC4051's as the switches. Its biggest problem is
the huge amount of noise the VF display kicks into the front end. An
old Fluke 8842 has less digits but is a far better instrument.
Sheesh, there must be something wrong with your meter! We have six
of them, of various vintages (including some from eBay) and haven't
experienced the degradation you describe. Furthermore, used in the
SLOW high-resolution 6.5-digit mode (integrates over 100 power-line
cycles), they show an amazing capability for a $1k instrument.
|
Well, mine kicks out so much spikes it disturbs active circuits. Check
yours; mine's an early unit, so maybe they fixed it without changing
the "A" in the model number.
On AC, as you reduce the input voltage, at some point it just drops
off to zero indication, clearly a software kluge to hide the spikes. A
careful reading of the spec shows that they fudged the specs and the
firmware, rather than cleaning up the noise problem. Tacky.
| Quote: | John, have you checked out how to get the 34401A in this mode (use
the MEASurement menu), and tried it?
|
Yes, but don't get me started on the nightmare menu structure!
| Quote: | BTW, this instrument clearly
wasn't meant to be a micro-volt meter.
|
Agreed!
| Quote: | Note, the Fluke 8842 has a
20mV scale, compared to the 34401A's 200mV lowest scale. Perhaps
that's what you like about it.
|
Yes; at low level, it's much better than the Agilent, even though
superficially it has less resolution.
| Quote: | We use Keithley meters for most of
our low-voltage measurements.
|
After mixed experience, we only buy Fluke handhelds and Keithley
benchtops now. Too bad Fluke gave up the precision end of the
business.
John |
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Chris Carlen
Guest
|
| Posted:
Mon Feb 14, 2005 6:11 am Post subject:
Re: Q: Construction of a 8.5 digit multimeter |
|
|
John Larkin wrote:
| Quote: | On 13 Feb 2005 05:47:02 -0800, Winfield Hill
hill_a@t_rowland-dotties-harvard-dot.s-edu> wrote:
John Larkin wrote...
Win wrote...
... You could start by studying the service manual of Agilent's
popular 34401A 6.5-digit multimeter. For example, study its
input-protection circuitry.
I have one of these. It's really a 4.5 digit box, except on AC where
it's more like 3.5, or 2.5 on some ranges. Internally, it's a variant
on dual-slope with HC4051's as the switches. Its biggest problem is
the huge amount of noise the VF display kicks into the front end. An
old Fluke 8842 has less digits but is a far better instrument.
Sheesh, there must be something wrong with your meter! We have six
of them, of various vintages (including some from eBay) and haven't
experienced the degradation you describe. Furthermore, used in the
SLOW high-resolution 6.5-digit mode (integrates over 100 power-line
cycles), they show an amazing capability for a $1k instrument.
Well, mine kicks out so much spikes it disturbs active circuits. Check
yours; mine's an early unit, so maybe they fixed it without changing
the "A" in the model number.
On AC, as you reduce the input voltage, at some point it just drops
off to zero indication, clearly a software kluge to hide the spikes. A
careful reading of the spec shows that they fudged the specs and the
firmware, rather than cleaning up the noise problem. Tacky.
John, have you checked out how to get the 34401A in this mode (use
the MEASurement menu), and tried it?
Yes, but don't get me started on the nightmare menu structure!
BTW, this instrument clearly
wasn't meant to be a micro-volt meter.
Agreed!
Note, the Fluke 8842 has a
20mV scale, compared to the 34401A's 200mV lowest scale. Perhaps
that's what you like about it.
Yes; at low level, it's much better than the Agilent, even though
superficially it has less resolution.
We use Keithley meters for most of
our low-voltage measurements.
After mixed experience, we only buy Fluke handhelds and Keithley
benchtops now. Too bad Fluke gave up the precision end of the
business.
|
Well Fluke still has the 8050A 8.5 digit multimeter. How about that one?
I bought a Fluke 45 for home instead of the Agilent. I liked the dual
display, and a little bit cheaper. 0.025% DC is pretty good for testing
batteries ;-)
--
_____________________
Christopher R. Carlen
crobc@sbcglobal.net
SuSE 9.1 Linux 2.6.5 |
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John Larkin
Guest
|
| Posted:
Mon Feb 14, 2005 7:40 am Post subject:
Re: Q: Construction of a 8.5 digit multimeter |
|
|
On Sun, 13 Feb 2005 16:53:04 -0800, Chris Carlen
<crobc@BOGUSFIELD.sbcglobal.net> wrote:
| Quote: | John Larkin wrote:
On 13 Feb 2005 05:47:02 -0800, Winfield Hill
hill_a@t_rowland-dotties-harvard-dot.s-edu> wrote:
John Larkin wrote...
Win wrote...
... You could start by studying the service manual of Agilent's
popular 34401A 6.5-digit multimeter. For example, study its
input-protection circuitry.
I have one of these. It's really a 4.5 digit box, except on AC where
it's more like 3.5, or 2.5 on some ranges. Internally, it's a variant
on dual-slope with HC4051's as the switches. Its biggest problem is
the huge amount of noise the VF display kicks into the front end. An
old Fluke 8842 has less digits but is a far better instrument.
Sheesh, there must be something wrong with your meter! We have six
of them, of various vintages (including some from eBay) and haven't
experienced the degradation you describe. Furthermore, used in the
SLOW high-resolution 6.5-digit mode (integrates over 100 power-line
cycles), they show an amazing capability for a $1k instrument.
Well, mine kicks out so much spikes it disturbs active circuits. Check
yours; mine's an early unit, so maybe they fixed it without changing
the "A" in the model number.
On AC, as you reduce the input voltage, at some point it just drops
off to zero indication, clearly a software kluge to hide the spikes. A
careful reading of the spec shows that they fudged the specs and the
firmware, rather than cleaning up the noise problem. Tacky.
John, have you checked out how to get the 34401A in this mode (use
the MEASurement menu), and tried it?
Yes, but don't get me started on the nightmare menu structure!
BTW, this instrument clearly
wasn't meant to be a micro-volt meter.
Agreed!
Note, the Fluke 8842 has a
20mV scale, compared to the 34401A's 200mV lowest scale. Perhaps
that's what you like about it.
Yes; at low level, it's much better than the Agilent, even though
superficially it has less resolution.
We use Keithley meters for most of
our low-voltage measurements.
After mixed experience, we only buy Fluke handhelds and Keithley
benchtops now. Too bad Fluke gave up the precision end of the
business.
Well Fluke still has the 8050A 8.5 digit multimeter. How about that one?
|
Can't find that one. 8508A maybe? Looks expensive... price is $call,
not a good sign. Specs look pretty good.
John |
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Chris Carlen
Guest
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| Posted:
Mon Feb 14, 2005 9:45 pm Post subject:
Re: Q: Construction of a 8.5 digit multimeter |
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|
John Larkin wrote:
| Quote: | On Sun, 13 Feb 2005 16:53:04 -0800, Chris Carlen
crobc@BOGUSFIELD.sbcglobal.net> wrote:
Well Fluke still has the 8050A 8.5 digit multimeter. How about that one?
Can't find that one. 8508A maybe? Looks expensive...
|
Oops, sorry, that's the one I meant.
price is $call,
| Quote: | not a good sign. Specs look pretty good.
John
|
Well I'd think anyone considering to buy an 8.5 digit meter would pass
the old test of "if you have to ask the price..."
Good day!
--
_______________________________________________________________________
Christopher R. Carlen
Principal Laser/Optical Technologist
Sandia National Laboratories CA USA
crcarle@sandia.gov -- NOTE: Remove "BOGUS" from email address to reply. |
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Klaus Vestergaard Kragelu
Guest
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| Posted:
Mon Feb 21, 2005 10:46 pm Post subject:
Re: Q: Construction of a 8.5 digit multimeter |
|
|
John Larkin wrote:
| Quote: | On 12 Feb 2005 17:06:57 -0800, Winfield Hill
hill_a@t_rowland-dotties-harvard-dot.s-edu> wrote:
Harald Noack wrote...
Has someone of you experience how a high precision multimeter is constructed?
e.g.: http://www.home.agilent.com/USeng/nav/-11250.536881781/pd.html
How is the voltage reference stabilized? (by using an oven?)
Are the input voltage divider temperature stabilized ? (temp. coeff.)
How is the self calibration done? What type of ADC is used
(maybe time to digital converter)? How is the integral and
differential non-linearity measured and compensated?
These are questions you have to answer and thoroughly understand to
evaluate a 5.5 or 6.5 digit voltmeter. You could start by studying
the service manual of Agilent's popular 34401A 6.5-digit multimeter.
For example, study its input-protection circuitry.
I have one of these. It's really a 4.5 digit box, except on AC where
it's more like 3.5, or 2.5 on some ranges. Internally, it's a variant
on dual-slope with HC4051's as the switches. Its biggest problem is
the huge amount of noise the VF display kicks into the front end. An
old Fluke 8842 has less digits but is a far better instrument.
|
I have used many of theese at work and liked it so much I bought one for
my home lab. I have never seen what you describe. On the contrary - our
Q-dept has increased the calibration intervals to two years because they
never find out-of-spec 34401A's
Cheers
Klaus |
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John Larkin
Guest
|
| Posted:
Tue Feb 22, 2005 12:21 am Post subject:
Re: Q: Construction of a 8.5 digit multimeter |
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|
On Mon, 21 Feb 2005 17:46:52 +0100, Klaus Vestergaard Kragelund
<klausvk@oncable.dk> wrote:
| Quote: | John Larkin wrote:
On 12 Feb 2005 17:06:57 -0800, Winfield Hill
hill_a@t_rowland-dotties-harvard-dot.s-edu> wrote:
Harald Noack wrote...
Has someone of you experience how a high precision multimeter is constructed?
e.g.: http://www.home.agilent.com/USeng/nav/-11250.536881781/pd.html
How is the voltage reference stabilized? (by using an oven?)
Are the input voltage divider temperature stabilized ? (temp. coeff.)
How is the self calibration done? What type of ADC is used
(maybe time to digital converter)? How is the integral and
differential non-linearity measured and compensated?
These are questions you have to answer and thoroughly understand to
evaluate a 5.5 or 6.5 digit voltmeter. You could start by studying
the service manual of Agilent's popular 34401A 6.5-digit multimeter.
For example, study its input-protection circuitry.
I have one of these. It's really a 4.5 digit box, except on AC where
it's more like 3.5, or 2.5 on some ranges. Internally, it's a variant
on dual-slope with HC4051's as the switches. Its biggest problem is
the huge amount of noise the VF display kicks into the front end. An
old Fluke 8842 has less digits but is a far better instrument.
I have used many of theese at work and liked it so much I bought one for
my home lab. I have never seen what you describe.
|
Have you checked it for the kickout spikes and low-level AC dropout?
It would be interesting to compare.
| Quote: | On the contrary - our
Q-dept has increased the calibration intervals to two years because they
never find out-of-spec 34401A's
|
I never said that mine was out of spec. I said that the VF display
made a lot of noise, and that the specs and firmware were fudged to
compensate. I measure a lot of low-level stuff, AC and DC, and the
Agilent is seriously inferior to a Kiethley 2000 or an old Fluke 8842.
John |
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Treeline
Guest
|
| Posted:
Tue Feb 22, 2005 5:57 am Post subject:
Re: Q: Construction of a 8.5 digit multimeter |
|
|
"John Larkin" <jjSNIPlarkin@highTHISlandPLEASEtechnology.XXX> wrote in message
news:pb9k115d4pnvfud4crhs7ib655jatp09tn@4ax.com...
| Quote: | I have one of these. It's really a 4.5 digit box, except on AC where
it's more like 3.5, or 2.5 on some ranges. Internally, it's a variant
on dual-slope with HC4051's as the switches. Its biggest problem is
the huge amount of noise the VF display kicks into the front end. An
old Fluke 8842 has less digits but is a far better instrument.
I never said that mine was out of spec. I said that the VF display
made a lot of noise, and that the specs and firmware were fudged to
compensate. I measure a lot of low-level stuff, AC and DC, and the
Agilent is seriously inferior to a Kiethley 2000 or an old Fluke 8842.
John
|
Thanks for the clarifications. I once looked into those when I guess
Agilent was still Hewlett-Packard. The digit numbers were impressive
and its ability to hold a calibration was impressive; but, when I studied
the specs closely and compared them to the Fluke 8842 - that's when
I began to suspect that you get what you pay for. I was hoping for
a great bargain, more numbers at lower cost. Neither device
could do what I needed so I forgot about them.
Hewlett Packard HP 34401A Digital Multimeter, 6.5 Digits, sells for
$1136.00 or so?
A dozen years later, and it's still selling for the same price?
You would think in a dozen years, for the same price, better
specifications for the money, or not? I guess here someone will chime
in with the value of the dollar but still...
Now I remember. I wanted to match resistors to a far higher degree
than I could order easily, even from Vishnay?. The reason was to reduce
the CMRR in the front-end of a circuit dealing with lots of noise
at millionths of a volt and picoamperes.
I was also trying to match capacitors, but that was even more of a problem.
Pease, a great analog engineer and writer, suggested an elementary op-amp
circuit instead to achieve specs on the compared parts. |
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