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The circuit as you described it, with +550 mV on the (+) input of the
diffamp and the LM35 output connected to the (-) input of the diffamp,
would give 0V output at +55C, not -55C.  The diffamp output would go
positive for temperatures below 0C, and negative for temperatures above
0C.  Also, the output of the LM35 can't go below ground (at 0C) without
some help.

The LM35 data sheet (http://www.national.com/ds/LM/LM35.pdf) recommends
a couple of ways for dealing with this.  One way, shown in Figure 7 of
the data sheet (copy attached as Image2.gif), allows temperature sensing
below 0C by placing a couple of diodes in series with the ground lead of
the LM35, and lightly pulling down the LM35 output with a resistor to
ground to allow the LM35's output to go a bit negative with respect to
its ground pin.

This method requires that two of the PIC's analog inputs be used; one to
read the LM35's output pin, and the other to read the potential on the
LM35's ground pin.  After taking the two A/D readings, a simple
subtraction yields the temperature result.  The only components required
are two 1N4148 (or equivalent) diodes and an 18K resistor, which should
be somewhat less expensive than a differential amplifier.

Dave

Vasile Surducan wrote...

>A) Use a diferential amplifier ( I hope you are familiar with the
>configuration), Vout =3D vp - vn where vp is the potential on
>+ input and vn is the potential on - input.=20
>On Vp put a reference of +550mV, then for -55C the output will be 0V.
>For positive temperature the output will be +550mV +temperature =
potential.
>=20
>B) If you can go only to -45...-50 C then no external operational is =
need.
>Take a look to my site:
>http://www.geocities.com/vsurducan/electro/PIC/f877.html
>
>On Thu, 10 Jan 2002, Gabriel Caffese wrote:
>
>>    I=B4m trying to read negative temperature using LM35, and one =
16F877.
>>    It gives me 10mv/=BAC, so at -55=BA, I=B4ll have -550mv.
>>    But I want to have 0 Volt at -10=BAC, How do I change the zero =
position ??
>>=20
>>     It it for being able to read this temperature with the 16F877.
>>=20


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