At 07:14 AM 6/27/2011, you wrote:

>I doubt if many spec sheets cite non a non-ideality factor, alas.

I doubt if even one does.

Diode-connected transistors don't have that problem (n=3D1).

The next factors in line limiting accuracy are the base spreading resistanc=
e
(rbb), which is a (typically unspecified) transistor parameter (use a part
such as MMBT4401 or 3904) and wire resistance. A third current can be used
to eliminate those, with some increase in complexity and additional
sensitivity to component values.

The sensitivity (microvolts per degree) of this approach is not that much
higher than that of Pt100 RTD or thermocouple, and the complexity is not th=
at
low. Since the output is proportional to absolute temperature, a 0.1% error
represents several degrees C, but a single-point adjustment could be used.

In a way, this idea is similar to the internal circuit in your garden-varie=
ty
band-gap voltage reference, except rather than using two closely matched
(fabricated on the same monolithic substrate) BJTs operating at different
currents, a single device is used with a switched current.

It's a fine technique for certain high-volume niche applications (such as
measuring CPU die temperature, but I'm not sure I see much in the way of
general application areas for it given the limitations it has.

Best regards,

Spehro Pefhany --"it's the network..."            "The Journey is the rewar=
d"
speff@interlog.com             Info for manufacturers: http://www.trexon.co=
m
Embedded software/hardware/analog  Info for designers:  http://www.speff.co=
m



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