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LM35: Vout range and circuit for Negative temperature sensing

Sarmad
Intellectual 340 points
Part Number: LM35
Other Parts Discussed in Thread: ADS7852

I would like to confirm two things:

1) For converting Vout of LM35 read via ADC to temperature, almost all resources conveniently assume that Vout would be 0 @ 0°C. Is this true? because I couldn't find it in the datasheet or must have missed it. If it is then why LM35C's Vout range is not limited to -0.40V to 1.1V instead of -1 to 6V?

2) To sense negative temperatures, can I use the following circuit, instead of the one given in Figure 18. I have simulated it for the absolute range of Vout .i.e. -1V to 6V and it scales the output from 500mV to 4.3V.

  • 0
    TI__Expert 4875 points

    Hello,

    The LM35 has a linear scale factor of 10 mV/C. The output is proportional to -0.4 V because (-40 C)*(10 mV/C) = -0.4 V. This is the lower limit for the LM35C, as the temperature range is -40 to 110 C. As you can see, the upper limit will be (110)*(10 mV/C) = 1.1 V.

    If you are using the LM35 with the output range from -55 to 150 C, this will translate to a voltage output of -0.55 to 1.5 V.

    Regards,

    Audrey

  • 0 in reply to Audrey Dearien7
    Intellectual 340 points
    Thank Audrey,

    Since I do not have means to test this sensor, can you please let me know if above circuit can be used to monitor full range from -40°C to 110°C. My limitations are single supply and single-ended ADC (ADS7852) with analog input range of 0 to 5V.
  • 0 in reply to Sarmad
    TI__Expert 4875 points
    Hi Sarmad,

    I also simulated your circuit and found the transfer function, which is: Temp_out1(V) = VCC_5V+(((VOUT-VCC_5)/(R3+R4))*(R3)). You can adjust this based on your R3 and R4 values. According to this configuration, you can monitor the full range of -40 C to 110 C, which will correspond to 0.95 V to 2.075 V. This is within the range of your supply for your single-ended ADC in the range of 0 to 5 V. In doing this, you have reduced the output gain from the LM35's original 10 mV/C, so you should consider the resolution of your ADC to get your desired accuracy at the final output.

    Regards,

    Audrey