Q: Construction of a 8.5 digit multimeter

[Harald Noack]

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

 

[Winfield Hill]

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. 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

 

[Joerg]

Hello Ken,

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

 

[Mike Harrison]

On Sun, 13 Feb 2005 01:15:47 +0100, "Harald Noack" <noack@sbox.tugraz.at> wrote:

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.

 

[Chris Carlen]

John Larkin wrote:

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,

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.

 

[Treeline]

"John Larkin" <jjSNIPlarkin@highTHISlandPLEASEtechnology.XXX> wrote in message
newsb9k115d4pnvfud4crhs7ib655jatp09tn@4ax.com...

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.