Q: Construction of a 8.5 digit multimeter

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

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.

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.

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.

Hello!

Has someone of you experience how a high precision multimeter is constructed ?

e.g.:

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

Hello Ken,

Sometimes companies go as far as having their own capacitors custom made.

Regards, Joerg

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.

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.

Yes, but don't get me started on the nightmare menu structure!

Agreed!

Yes; at low level, it's much better than the Agilent, even though superficially it has less resolution.

After mixed experience, we only buy Fluke handhelds and Keithley benchtops now. Too bad Fluke gave up the precision end of the business.

John

Remember that an 8.5 digit DMM will typically not have anything like 8.5 digits absolute accuracy.

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 ;-)

Can't find that one. 8508A maybe? Looks expensive... price is $call, not a good sign. Specs look pretty good.

John

Oops, sorry, that's the one I meant.

price is $call,

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!

Have you checked it for the kickout spikes and low-level AC dropout? It would be interesting to compare.

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

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

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.