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LM35: Output readings for negative temperatures

Russ Tribe
Prodigy 10 points
Part Number: LM35
Other Parts Discussed in Thread: TMP235, , LM235

I wish to use this part to read temperatures between about -20C and +80C.  The data sheet, page 16, figure 18 shows the use of diodes to produce a negative reference.  I have read the two related threads on the forum.

It appears from the data sheet that (without the diode offset) 0C gives 0V output and 25C gives 250mV output.  Also, -55C will give -550mV output.

What I cannot understand is what temperature will produce 0V with the diode offset.  I wonder if this will be -55C?

Thanks.

  • 0
    TI__Mastermind 41985 points

    I recommend using TMP235 instead. It will produce 300mV at -20C without any additional circuitry. 

    Figure 18 shows a differential measurement. This measurement will give -550mV at -55C. If your measurement is not differential, it would have two times the forward voltage added to the LM35's output when referenced to ground. The forward voltage will vary and provide a very poor reference, so it's important that this circuit be measured differentially.

    thanks,

    ren

  • 0 in reply to Ren Schackmann
    Prodigy 10 points

    Thanks for your reply, Ren.  I will look at the LM235 as you suggest.

    I was considering using two ADC inputs to measure the outputs of the LM35 and subtracting one from the other.  This would eliminate errors caused by (temperature) variations in voltage across the diodes.

    If I did this, are you saying that 0C would give an output of 250mV?

    Thanks.

  • +1 in reply to Russ Tribe
    TI__Mastermind 41985 points

    LM235 is a different product that is not compatible. I recommended TMP235, which is a modern LM35 replacement with low voltage operation. 

    Yes, the difference of two single-ended measurements should suffice.

    If you measure the circuit in Figure 18 differentially as shown, it should output 0V at 0C. The circuit is needed to allow LM35 to output a voltage "below ground" in an application with a single supply. Please understand that LM35 is from the 80s, and it was common for systems to have positive and negative voltage rails. Please also note that LM35 is specified at Vs = 5V, so it may not meet specification if the supply voltage in Figure 18 is not at least 5V+2*Vf. The Line Regulation spec only covers operation down to Vs = 4V, for example.

    thanks,

    ren

  • +1 in reply to Ren Schackmann
    Prodigy 10 points

    Ren, sorry, my mistake.  I did mean TMP235!  Having looked at the data sheet, I don't think the package is as useful to me as the TO92 which houses the LM35.  I'm glad you commented on the problem that would arise from feeding 'fig.18' with a 5V supply.  I had seen this and decided that I could use my 12V supply instead.  Thanks again.