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Date: Sun, 25 Oct 2015 00:54:57 +0200
From: "Gabriel Paubert (paubert AT iram DOT es) [via geda-user AT delorie DOT com]" <geda-user AT delorie DOT com>
To: "Evan Foss (evanfoss AT gmail DOT com) [via geda-user AT delorie DOT com]" <geda-user AT delorie DOT com>
Subject: Re: [geda-user] Stable and low noise references (was: The
 Chippocolypse is coming!)
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    Hi Evan,

On Sat, Oct 24, 2015 at 06:15:22AM +0000, Evan Foss (evanfoss AT gmail DOT com) [via geda-user AT delorie DOT com] wrote:
> On Fri, Oct 23, 2015 at 9:28 PM, Gabriel Paubert (paubert AT iram DOT es)
> [via geda-user AT delorie DOT com] <geda-user AT delorie DOT com> wrote:
> > On Fri, Oct 23, 2015 at 09:25:52AM -0400, Evan Foss (evanfoss AT gmail DOT com) [via geda-user AT delorie DOT com] wrote:
> >> On Fri, Oct 23, 2015 at 9:16 AM, Evan Foss <evanfoss AT gmail DOT com> wrote:
> >> > On Fri, Oct 23, 2015 at 3:02 AM, Gabriel Paubert (paubert AT iram DOT es)
> >> > [via geda-user AT delorie DOT com] <geda-user AT delorie DOT com> wrote:
> >> >> For example, and as far as I can tell, Linear Technology has eliminated
> >> >> all TO-99 versions except the LTZ1000A, where the metal can is superior
> >> >> for reasons related to thermal insulation and mechanical stress on the die.
> >>
> >> Sorry I miss read this.
> >>
> >
> > Thanks for the second reply, I couldn't understand the first one.
> >
> >> > That is bizzar. I get that people hate paying for TO-99 stuff but a
> >> > reference really should be in that package. I have to wonder if they
> >> > are just switching people over to something newer that is available in
> >> > a TO-99. LT makes a few different references.
> >
> > This is becoming offtopic, but the LTZ1000A is, to my knowledge, still
> > the best reference available in terms of noise and long term stability,
> > and until a few years ago, I only used buried Zener references when I
> > needed high stability.
> 
> It is. If you look at any of the tear downs people do of meters above
> 6 digits they are all using that reference or one made by LT custom
> for Fluke.

I was not aware of that, but this means that this device has cornered a
market from people accepting to pay for the performance and might not be
EOLed too quickly.

> 
> > But then, Linear started to produce a line of low noise bandgap
> > references that are fairly close in performance to buried Zeners,
> > especially the LTC6655, which is better than most buried Zeners.
> 
> I will read up on those. That sounds cool.

It is an excellent reference, and it is much easier to use than the
LTZ1000A, which needs to be surrounded by a significant amount of 
carefully layed out analog circuitry.

> 
> > This said, I use more LT6654 than LTC6655, it's clearly worse in the low
> > frequency noise range, but sufficient for most of my needs and much more
> > robust on the input (38V max vs 13.2).
> >
> > Both the LTC6655 and LT6654 are available in SMD plastic and ceramic
> > package, but I've only used the plastic ones. The ceramic package
> > has stability advantages over long term and temperature cycling
> > which are of no interest in my applications.
> 

The main reason I've not systematically used the LTC6655 is that it
would require an additional linear regulator because I don't have
a power supply rail in the suitable range.

While it's limited to Linear products, http://www.linear.com/docs/29441
clearly shows which reference to choose when 0.1 to 10Hz noise is
important. With 0.25 ppm, the LTC6655 is about 1.5 times the value of 
0.17ppm given for the LTZ1000A, all the others are at least 0.6ppm, 
3.5 times higher than the LTZ100A.

Before the 6654/6655, the best compromise for my applications was often
the LT1021, and I only sweared by buried Zeners, thinking that bandgaps
would never give the required noise. Well, I was wrong, but it took
about 20 years.


> Nice
> 
> >     Gabriel
> 
> Thanks for this it actually is useful in something I am playing with now.
> 

Glad to help, but we've steered way off-topic. To be honest, the
original thread was not really geda specific either.


    Gabriel