[volt-nuts] Best reference after LTZ1000

Charles P. Steinmetz charles_steinmetz at lavabit.com
Tue Aug 24 21:02:57 UTC 2010

Marv wrote:

>Overall though, the more important item is taken as a whole is this 
>box with the patented stabilizing circuit a better reference than 
>something else, assuming this is what your friend has?

I don't know -- he has not characterized it rigorously AFAIK, nor 
compared it to an SVR.  I have on several occasions compared it to my 
Fluke 732A, and would say that it seems to work fine at the tens of 
ppm level for drift and probably less than that for tempco, although 
that is anecdotal -- I made no attempt to separate the drift and 
tempco effects.

>I agree with your criticism of the circuit function.  See the 
>schematic on the patent.  There is a novel item, notice that the 
>supply voltage for the LM399 comes from the vref output of the AD587 
>op amp, rather than V+ as expected?

Any proper voltage regulator circuit (not to mention voltage 
reference!) does this, doesn't it?  Certainly, everything I've ever 
designed does.

>Not sure what this does to the ppm/oC over time

I have not observed any effect on tempco using this connection 
(assuming equivalent tempcos of the bias circuits for the two connections).

>Geller does offer the patented circuit on a stabler and more costly 
>version of SVR, but you have to ask him for it.  Its near $300 
>compared to the $35 SVR.

Interesting.  I wonder why he doesn't list it on his web site?

>But I think there are more conventional ways to achieve stability 
>without the uncertainty of this circuit's function.

As well as low noise.  There is nothing wrong with the concept -- at 
the end of the day, it's just an ovenized reference (LM399 or 
similar) with a scaling amplifier.  But the op amp and resistors in 
the AD587 are not the most optimal choices, so why pay for a 
reference you're not using, only to get an op amp and divider string 
that aren't the best-suited to the task?  I suppose if you require 
"pretty well optimized" and not "best I can do in an amateur lab," it 
makes things easier.

Best regards,


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