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LM34: LM34 output at temps below 0F

Mike Wahler
Prodigy 175 points
Part Number: LM34
Other Parts Discussed in Thread: LMT87, LM94022

Hello all:

Searching around on the internet (including TI forums), I can't seem to find a definitive answer to my question:

I'm using an LM34DM (SO-8) in "minimal" configuration: connections to +5V, GND, and Vout to a PIC18 ADC pin.

Datasheet tells me Vout is 10mV/degF, which is what I'm getting at "normal" temps (above 0F). If temperature falls below 0F, will LM34DM Vout fall below GND? (I'm not equipped to test temps this low).

I'm using the LM34DM to have the PIC take action at a threshhold temperature (200F). Works great.  But I'm anticipating deployment of my circuit in Midwest U.S. where temps can fall far below 0F in winter. I have two concerns:

1. ADC input much more than a small amount (0.3V) below GND may damage the PIC.

2. Vout below GND might cause the PIC ADC register to 'wrap' around to a spurious high value above my threshhold.

The answer I "want to hear" :-) :  Vout never falls below GND.

Thank you for any assistance.

-Mike

  • 0
    TI__Expert 4905 points
    Hi Mike,

    Below 0F the LM34 output will want to go negative so you need a pull-down resistor to a negative supply to see the output (see the datasheet for examples - page 1 full-range Fahrenheit temperature sensor).

    If you do not add the pull down the output cannot go below ground as the LM34 does not have an internal negative voltage generator. Anyway at -32F the output would be at -320mV so with the pull-down.

    IF you are still concerned or would like to resolve temperatures below zero you could use a device like the LMT87 - it has a negative temperature coefficient but with the pic processor you can easily calculate the actual temperature. It has an offset such that temperatures below zero give a positive output voltage. Also these parts are in a much smaller package (still leaded). If you require better accuracy take a look at the LM94022 (it has a programmable gain you can tune to your requirements).

    Take care,
  • 0 in reply to Emmy Denton
    Prodigy 175 points
    Emmy:

    Thanks for answering.

    So let's please clarify: you wrote:
    "If you do not add the pull down the output cannot go below ground"
    Is that guaranteed? If so, then it's exactly what I need. Subzero temperatures giving an ADC reading of zero would work just fine for this application. Just wanted to make sure my ADC pin won't go below ground.


    Thanks,
    -Mike
  • 0 in reply to Mike Wahler
    TI__Expert 4905 points
    Hi Mike,

    The output of the LM34 cannot go negative unless something in your system allows it. There is no negative voltage generator inside the LM34. You need to make the assessment if that can happen in your system. BTW, this is true for all TI analog temperature sensors.

    Thank you for designing with TI temperature sensors and your patience.

    Take care,
  • 0 in reply to Emmy Denton
    Prodigy 175 points
    Emmy:

    That's what I needed to know (sorry if I seemed a bit dense; I'm just paranoid by nature :-). Now I know my system will work as desired in subzero temperatures.

    Thanks,
    -Mike
  • 0 in reply to Mike Wahler
    TI__Expert 4905 points
    Hi Mike,

    No problem - glad we were able to help you!

    Take care,