Re: OT Advice on good choice of Voltage references and Precision resistors

Fabio Trevisan
 

Hello all,
First of all, thanks for the input given so far.
I got really interested on those things made by
http://www.voltagestandard.com <http://www.voltagestandard.com>
It's a pity to realize they're not making the "DMMCheck_Plus" anymore...
That one would be a real life-saver.
Still, any of the other references they still make are very interesting.
At some tens of dollars ballpark, they're more expensive than buying the
reference ICs alone (but not by much), and they come pre-built and
calibrated on the other hand, which is great.
As I write this, I`m already trying to make up my mind on which one to pick
now. Luckily I have relatives in Canada... because them not shipping
elsewhere is surely a game stopper.
Now, regarding precision resistors, thanks David, for the tip on the higher
value precision resistors... (that they're more expensive and it worth more
to make associations).
As a matter of fact, in line with your rational, it makes sense to make the
choice of values based on possible associations which tends to minimize the
error by averaging (statistically, at least).
In that line, I can get a relatively larger number of mid-valued resistors,
and get the higher values by series association, and the lower values by
parallel.

Thanks for now and I keep watching whatever other suggestions come up.

Rgrds,

Fabio



2017-11-22 5:56 GMT-02:00 Adrian Nicol fenland787a@... [TekScopes] <
TekScopes@...>:



Hi Tom,I used Absolute Calibration Ltd (www.absolutecal.co.uk) for this,
they are based in Portchester, Hants, and no, they didn't charge extra for
the tweak! I'm near Cambridge and used their collect and return (a week
later) service which reduced shipping worries as you hand equipment over
directly to one of their employees in one of their vans. It didn't add much
to the cost either IIRC. Great service and really cheerful, helpful folks.
- no connection, just a happy customer - in fact, as a backup, I also got
them to do my 3457A.
Adrian


On Wednesday, November 22, 2017 1:10 AM, "David @DWH
[TekScopes]" <TekScopes@...> wrote:


On Tue, 21 Nov 2017 17:28:13 -0200, you wrote:

Hello,
In the quest for keeping my old stuff calibrated (or reasonably so), and
knowing many of us can't afford calibration services, or they simply
aren't
available anymore to old equipment, I wish to setup some basic calibration
standards that can be counted-on without needing for sending them to
calibration services, so I ask your advice.
I might recommend something like this but they stopped making them:

<http://www.voltagestandard.com/DMMCheck_Plus.html>

From what's currently available in the market, and some of you may know
well:
1. What is a voltage reference IC (if any) that can be trusted
out-of-the-box, for calibrating, say, voltmeters in the 0.05% precision
range... or if not feasible, at least in the 0.1% range.
I would look through the Mouser or Digikey selection guide for the
various options sorted by initial accuracy. 0.02% is feasible with a
better temperature coefficient than required for a reasonable price:

MAX6350CPA 5V 0.02% 0.5PPM/C $10.55

2. What is a good choice of resistor family (there are so many of them at
Digikey or Mouser that I simply can't choose) that can be purchased in
"hobby" quantities, in the precision range of 0.05% or at least 0.1%.
What's your advice regarding values and temp. coeff.? I was thinking of
values close to, but smaller than 2, and its multiples, like *say* 196,
1k96, 19k6, 196k, 1,96M and 19.6M. With 3 of each of them, it could be
enough to check / adjust voltmeters of 2000 counts, 4000 counts, 6000
counts and 10000 counts (20000 counts too, but it would require other
precision class for that).
Temperature coefficient generally tracks accuracy so parts may be
selected based just on accuracy. 0.05% parts up to 1 megohm are just
a couple dollars and limited values at 0.02% and 0.01% are available
as Vishay/Dale metal film resistors.

Above 1M, tolerances of 0.1% or better are very expensive. It might
be better to place several lower value precision resistors in series
up to 20M.

And I know that it's usually not possible to adjust every range
separately,
so it may sound overkill to have resistors to check almost all possible
ranges... but that's just my first "hunch".

Thanks for any tip and regards,

Fabio
The different ranges should be checked whether they can be calibrated
independently or not to ensure proper operation.
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