[volt-nuts] Transmille 8081

Ken Peek ken.peek at diligentminds.com
Wed Apr 22 10:36:26 EDT 2015


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...



More information about the volt-nuts mailing list