Dave, thank you for the tips !
Vasile

On Tue, 8 Jan 2002, Dave Dilatush wrote:

> Vasile Surducan wrote...
>
> >I wasn't follow this subject but if delta means diference I have some
> >doubts about the interchangeability of the sensors. Maybe a ratio.
>
> This "delta Vbe" technique is a well established and documented
> technique.  Linear Technology, Inc. application note AN45, by Jim
> Williams,  discusses this method and shows an example design on page 7,
> Figure 11.
>
> Circuits of this type produce a voltage output which is proportional to
> the log of the ratio of the two currents times the absolute temperature.
>
> The output shows very little dependency on transistor characteristics or
> device type; Williams obtained less than 0.4 degrees C spread over 25
> randomly selected 2N3904s and 2N2222s from various manufacturers.
>
> >There are five basic methodes of measuring temperature with a transistor:
> >1. Ib = constant, measuring Vbe, only be junction is used
> >2. Vbe = constant, measuring Ib, only be junction is used
> >3. Ib = constant, measuring Ic
> >4. Ib = constant, measuring Vce
> >5. Ib = constant, measuring both Vce and Vbe
>
> >But all methodes are too complicated if we think to the LM35 family.
>
> I think that last statement oversimplifies just a bit: there are many
> ways to measure temperature, and each method has its strengths,
> weaknesses, and a set of applications where its use is appropriate.
>
> The "delta Vbe" method of sensing temperature has two application areas
> where it is especially attractive:
>
> 1.  When the temperature sensing element MUST be a semiconductor
>     junction, such as measuring the die temperature of a CPU chip
>     for purposes of cooling control.  The delta-Vbe technique allows
>     highly accurate die temperature measurement without any need for
>     calibration.
>
> 2.  When there is a need for EXTREMELY low sensor cost (but NOT total
>     circuit cost), together with a requirement that sensors be
>     interchangeable without re-calibration.  For example, I can bond
>     a small, cheap transistor to an assembly to monitor its
>     temperature during testing or some other process (e.g., potting)
>     and then discard it when I'm done, at little expense.
>
> If you'd like to go back and follow the original thread to see how we
> got to this discussion, look in the archives for "[PIC]: current
> generator with a PIC" by Ashly Dearden on 27 December; that thread will
> provide some context.
>
> Dave Dilatush
>
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