[time-nuts] Low Cost Voltage Transfer References

[Joe Geller]

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