[volt-nuts] Transmille 8081
John Phillips
john.phillips0 at gmail.com
Wed Apr 22 14:15:48 EDT 2015
Looking at the video measuring 1 G ohm I think they must have some very
good (intelligent) filtering going on to get the display to be that stable.
I do realize they are using a high voltage and coaxial cables to make the
measurement but even a person breathing in the room would case more real
fluctuation in the measurement.
On Wed, Apr 22, 2015 at 7:36 AM, Ken Peek <ken.peek at diligentminds.com>
wrote:
> I'm a little bit suspicious of this one. Rumors have it that they use a
> V/F converter as opposed to the "normal" multi-slope technique. This
> may work just fine, and then again maybe not. I noticed that they write
> about "high linearity" in their brochure, but no figures for ADC
> linearity [either INL or DNL] are to be found anywhere on their
> data-sheet or in their manual. If Transmille is so proud of their DMM's
> linearity, then why not show it prominently in the data-sheet?
>
> The *average* HP/Agilent/Keysight 3458A has 0.02ppm [that's 20 parts per
> BILLION] INL--- better than *ANY* other 8.5-digit DMM out there. Why is
> this such a big deal? Well, for metrology [which is what you are likely
> doing with an 8.5-digit anything], a main technique for working with
> standards is to measure ratios. Every other function on an 8.5-digit
> DMM is marketing drivel-- and I have no expectation that their
> performance will be that good.
>
> The Datron 1281 and the Fluke 8508A are always used to show how good a
> DMM can be when measuring a resistor, and yet neither of them come even
> close to an old ESI 242D bridge or a direct current comparator [for
> lower resistances]. Not even *close*...
>
> People are always looking for a single tool to do everything, and just
> like with all-in-one stereo sets, you end up getting less than stellar
> results than you might if you purchased each component separately.
>
> If, [for some reason], you think you need an 8.5-digit DMM, the 3458A is
> *still* king of INL, after all of these years. The reason? It's analog
> ASIC. Matched JFET switches to ultra-high ratio stability highly
> interdigitated resistors. This, and balancing JFET switch transitions
> during an ADC cycle is the "secret sauce" that is at the heart of the
> 3458A's stellar INL spec. No one else has that. No one else ever will.
> *WAY* too expensive for even Keysight to redesign these days. Just is
> not going to happen...
>
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--
*John Phillips*
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