[time-nuts] Low Cost Voltage Transfer References
Mike Monett
XDE-L2G3 at myamail.com
Sun Dec 14 13:07:16 UTC 2008
> I thought I might make a couple of comments about the Geller SVR
> board (I am a long time Time-Nuts reader, although sometimes only
> an occasional reader of the archives).
> The SVR board is nothing more than a high quality platform for a
> monolithic reference chip. The goal of this project was to provide
> a relatively low cost way to transfer voltage calibration
> (primarily with hobbyists and amateur scientists in mind). The SVR
> board is not intended to be a voltage standard. The absolute
> calibration of our set point is directly traceable to our Fluke
> 732B, which is in the Fluke calibration program. We rate the
> transfer accuracy at +/- 5 ppm which I believe to be conservative.
> With some care, generally a 2 to 3 ppm absolute transfer accuracy
> is realizable. In recent years, most SVR boards are burned-in for
> 200 hours before final calibration.
> Although there has been some industry support for the project,
> including some support from ADI, this project has largely been
> accomplished at a financial loss. Our costs include the hp3458A,
> the Fluke 732B, (including overnight shipping, calibration cost is
> over $500 / year) not to mention component parts, shipping boxes,
> and eBay and PayPal fees.
> Since the relatively low cost AD587LN grade is now discontinued, I
> have been debating whether to continue testing and design efforts
> and to continue the low cost transfer project, or to just abandon
> the effort altogether. There have been several promising new
> reference chips in recent years, yet none so far that match the
> performance of the AD587LN considering stability with time,
> stability with temperature, output noise spectrum (both below 10
> Hz and higher frequency noise), stability with power supply,
> stability with load (output drive), and ease of assembly and use.
> I agree that the LTZ1000 appears to be the best ovenized reference
> available, especially with burn-in temperature cycling techniques
> and/or part selection. However, at >$35 per part in quantity, and
> the need for an amplifier, and ultra low tempco scaling resistors,
> it does not appear to be practical for use in a low cost product.
> Regards,
> Joe Geller
Hi Joe,
I want to congratulate you for your excellent work. You have a
well-deserved reputation for quality and fair dealing.
It is unfortunate the AD587LN is no longer available, but would a
part like the AD780BN be suitable? It lists for only $6.39, which is
considerably considerably better than the LTZ1000:
http://www.analog.com/en/other/militaryaerospace/ad780/products/product.html
The AD780BN spec is 3ppm/C part vs the 5ppm/C of the AD587, so it is
a bit more stable with temperature. Both references have the same
noise spec of 100nV/sqrt(Hz). The initial accuracy is a bit better
at 1mV, but I don't see anything on long-term stability for either
part.
If it is suitable, it seems your customers may not mind a small
increase in price, especially considering the improvement in
performance over the AD587LN. Also, I'm sure if you put the above
explanation on your web site, your customers would certainly find it
understandable if you increased your price to cover the costs of
maintaining the product. In fact, many products do better when the
price is raised to match the customer's perception of the value. So
keeping a low price may give the wrong impression of the actual
caliber of the service.
It would be a shame to see such a valuable concept disappear. I'd go
for it.
Regards,
Mike Monett
P.S. I liked your page on the HP 53310A. I have the file you are
missing, so if you would like, I can email it to you along with some
others that might be useful.
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