Andy Errington wrote:
>
> 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.


LM355. Hmmm, Maybe you mean LM335? I think so. But maybe your should consider
the LM35, since it is calibrated for Centigrade. (0v = 0 degrees C)

> 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!)


The LM335 has a transfer function of

Vot(T) = 2.732V + (10mV/C) * T ...............(1)

Where, T is the temperature in degrees Centigrade.

However, you desire a different transfer function:

Vd(T) = K * T ................................(2)

Where, T is again the temperature in degrees C and
K, the proportionality constant is around 10mV/C.

Consider the op-amp circuit shown below:

                                  R4
                         +------/\/\/\----+
               R3        |   |\           |
    Vref ----/\/\/\------+---|-\          |
                             |  \         |
                             |   \        |
                             |U1 /--------+--- Vd(T)
               R1            |  /
   Vot(T) ---/\/\/\------+---|+/
                         |   |/
                         \
                         /  R2
                         \
                         /
                         |
                        ----
                       / / /

The transfer function for this circuit is

Vd(T) = K1 * Vot(T) - K2 * Vref .............(3)

K1 = R2 * (R3 + R4) / [ R3 * (R1 + R2)]
K2 = R4 / R3

Substitute (1) into (3)
Vd(T) = K1 * (2.732 + 10mV/C *T) - K2 * Vref

and equate to (2)
K * T = K1 * (2.732+ 10mV/C *T) - K2 * Vref


This gives you two equations, one for the coefficient of T:
  K * T = K1 * 10mv/C * T
      K = K1 * 10mv/C

and the other associated with the constant term:
         0 = K1 * 2.732  - K2 * Vref
K1 * 2.732 = K2 * Vref

Now, it's a grind. You need to choose a Voltage reference, Vref. If you can live
with some inaccuracy, then low pass filter Vcc (i.e. 5V) and use it. Otherwise,
an LM385 (1.2V reference) is a very reasonable reference.

Next, you need to pick resistor values for R1 - R4. But, I can't provide them
 here
unless you tell me your K, i.e. your temperature to voltage proportionality
 constant.

Finally, you need a 5V, single-supply op-amp. Most single-supply op-amps accept
 common
mode input voltages all the way to the negative rail (0 Volts). However, they
 only go
to within 1.5 to 2 volts of the postive rail. The LM324 is good example. If you
 are
willing to tolerate low bandwidth, and I mean really low bandwidth, then
 consider Maxim's
rail-to-rail opamps like the MAX406A. The gain bandwidth is 40kHz(!), yet for a
 5V
supply it is guranteed to swing within 30mv of either rail. This will probably
 work,
because temperature is almost always a slowly varying signal.

Hope this helps,
Scott