> Hi to any analogue gurus,
>
> I want to use a National Semiconductor LM355 absolute temperature sensor to
> measure ambient temperature.  This 3pin device behaves as a zener diode
> whose voltage is linearly proportional to absolute temperature.  At 25degC
> you get 2.982 volts (=298.2Kelvin), and the voltage changes by 10mV per
> degree.
>
> I want everything to operate on a single rail 5V supply, so I need to put
> the output of the sensor into a circuit which has a gain of about 4, and
> which subtracts about 2.6V from the sensor output first.  This will give me
> a sensible output voltage range which can be read using an ADC (connected to
> a PIC of course!)
I think you can use a op amp as follow:
                             +---+
                             v   |
signal  ------10K--+--------50k--+-----+
                   |                   |
                   |                   |
         5V        |  |\               |
          |     +--|--|-\              |
          |     |  |  |  \             |
         10K<+  |  |  |  /-------------+-------- out
          |  |  |  +--|+/
          |--+  |     |/
          |     |
          +-----+
          |
         10k
          |
          |
         GND

Well ASCII art is not what I do best but I will try to explain.
This is a subtracting citcuit, the signal is subtracted from the
signal at invert input and amplified.
Adjust 10K trimpot to read 2.982V at invert input and adjust
50K trimpot to 40k to gain 4.
I didn't test this circuit but I think it works

Cheers

Octavio
nogueira@mandic.com.br