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The 3456a is probably the best deal you can find. I haven't ever had a
problem with mine and it's 6.5 digits. It's not really hard to find
them under $100 in working condition (I paid $75 for mine and have seen
references for as low as $20). The only complaint is it does take up a
little space.

As for serial data, here's a thread on eevblog about building a
serial adapter:


Not the world's smarted gpib adapter, but trivial to implement and it
gives you the serial data through a USB connection. A proper gpib
connector will cost you more than the processor, but the picture in the
post shows a cringe worthy budget approach.

Calibration of any 6.5 digit meter on a hobbiest budget is a challenge,
but if you have access to a recently calibrated 6.5 digit or better
meter you can at least compare it and get an idea of how close it is.

This is starting to get into volt-nuttery and I'll leave calibration
advice to those who know more than me. Suffice it to say I have an
ongoing project restoring a Keithley 2001 7.5 digit meter to tell me if
my other meters are close to spec (note this one cost me $500 for a
broken, maybe repairable example, jury's still out on that).


Paul

On Thu, Apr 09, 2020 at 05:28:53PM +0000, Daniel Koller via groups.io wrote:

Hi All,
? I am wondering what sort of DVM the collective wisdom here might
suggest for precision work.
? I am working on a project for work that involves designing a
precision voltage controlled current source.? But due to the
current pandemic, I am working from home and assembling and testing
stuff in my own well equipped basement shop.? ?The current source
is more or less a design out of Horowitz and Hill's "The Art of
Electronics".? ?I am working to keep the full scale and linearity
of this current source to within 0.1% or better.? ?
? My current best DVM is a Keithley model 173 DMM "4 and 1/2"
digit, 4-wire meter (the leading digit reads to 3).? It's accurate
to ~0.009%+1 digit over 24 hours on the V scales and ~0.035%+1
digit on the Ohms scales.? ?I think it's good enough for what I
need, but it has a few limitations, and I am wondering if it is not
time to upgrade to a real bench instrument.
? ?First of all, my DVM probably hasn't been calibrated in
decades.? ?How do I know it's correct without some externally
calibrated transfer standard?? ?It *may* be a possibility for me to
use a meter at work (sent regularly to calibration services) and
calibrate a portable voltage reference and bring it home.? ?But I
don't have a resistance standard.? ?
? The other problem is that even with 4 wires on the lowest Ohms
setting, I don't have enough resolution to measure a 0.100 Ohm
sense resistor to within 1%.? ?
? ?So, I think I could use a better meter.? ?The HP34401A DVM seems
like a nice meter to have.? It's even used as a circuit design
example in H&H's text.? But they seem to go for several hundreds of
$$ on e-bay.? ?Remember, this is coming out of my hobby budget.
? So, I looked at the? HP3455A / 3456A on e-bay.? These seem to be
significantly cheaper, and I love vintage instruments.? BUT, do I want to take up a new project?? How reliable are these old beasts??
Are they easy to repair and calibrate?? ?What tends to go wrong
with them?
? ?Even though it's more expensive, one major strong point of the
HP34401A is in addition to GPIB, it's got a SERIAL interface on the
back!? That is a whole different ballgame in terms of interfacing to a PC or laptop (and a whole lot easier for me, for sure).? But I
do think I want to have a meter I can use, rather than one I will
have to repair.? ?
? Are there any other models I am missing?? ?Are there any that
come standard with an RS232 interface?? Please let me know and
thank you in advance for your suggestions.
? Dan

??





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Paul Amaranth, GCIH | Manchester MI, USA
Aurora Group of Michigan, LLC | Security, Systems & Software
paul@... | Unix & Windows