Need helping figuring out something vague in a datasheet for an LM35

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1

I'm looking to use the Texas Instruments LM35 temperature sensor to (yes, you guessed it) monitor ambient temperature in a refrigerator.

The overall goal is to eliminate the existing Temperature Control Knob and replace it with an Arduino controlled/triggered Solid State Relay. This will also allow me to have an LCD readout of the temperature and be able to "dial in" a Set/Desired Temperature via programming or some other source such as a potentiometer.

I'm looking to operate the LM35 in Full-Range (-55°C to 150°C). Figure 2 on page 1 of the datasheet (found here: http://www.ti.com/lit/ds/symlink/lm35.pdf) shows that a resistor must be added to the output of the LM35 in order for it to operate in Full-Range.

I'm confused as to what the other side of the resistor (labeled -Vs) should be connected to and how to figure out what size resistor I'll need. Also, what is the purpose of listing the different Vout ratings in this figure? Are they simply stating that the output in mV is directly proportional to °C? Seems like an odd place to put this if that's what they are.

Here is Figure 2 from the datasheet:

EDIT: I just noticed Figure 18 on page 11 of the datasheet. Now I'm even more confused as to whether I should wire it up as shown in Figure 2 or Figure 18??? What's the difference between the two?

Figure 18:

Any help would be greatly appreciated.

2

Either circuit will work. In Fig. 18, the two diodes have a voltage drop of 1.2 to 1.4 V, which is, in effect, the same as providing a negative voltage supply as in Fig. 2. The difference is they don't tell you what the output of the Fig. 18 circuit will be as a function of temperature. So, you would have to calibrate the final circuit. (I would do that in any case).

Fig. 2 is a bit vague, because it implies that "-Vs" should be the same voltage as "Vs" but with the sign reversed. I don't see why the two voltages should have the same absolute value, but in any case, there is a formula for the correct resistor value, depending on your choice of "-Vs".

You should always verify circuit function over as much of the temperature range as possible, before trusting the results.

3

use the diodes.. Read both voltages .
The difference is your base to calculate temp