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Must I connect all 3 LMT84DCKT's Gnd Pin to ground?

Koyo Kegasa
Expert 1260 points
Other Parts Discussed in Thread: LMT84, LMT87, LMT86, LMT85

I tested the LMT84DCKT sample today.  

There are 5 pins. 

There are 3 gnd pins. 

I thought, connecting pin-1 gnd to the ground is enough.  

I left pin-2 gnd and pin-5 gnd open.  

Temperature is about 26 degrees C, so I expected 892mV at pin-3 output. 

But, when I applied 3.3V to pin-4 Vcc,  I got 1.43V at pin-3 output. 

So, the temperature is less than -50 degrees C?

Must I connect the all 3 gnd pins to the ground?

Hope to get the answer soon!

  • 0
    TI__Expert 7220 points
    Hi Koyo,

    Pin 2 is actually the true ground of the LMT84. The other pins, Pin 1 and 5 are digital input pins that sets the gain of the output. So if you connect all 3 pins to ground you should see the correct output voltage corresponding to 26C.

    -Michael Wong
  • 0 in reply to Michael Wong
    Expert 1260 points

    Oh!  I didn't know that!

    Dose that mean LMT84, LMT85, LMT86, and LMT87 are using the same silicone chip inside?

    It's acturally is variable gain temperature sensor with two bit digital input?

  • 0 in reply to Michael Wong
    Expert 1260 points
    I connected pin 1 and pin 2 to the ground. (I used solder bridge to connect them.)
    Now I can measure the 876mV which match the current room temperature of 29 degrees C all right.
    So, Can I leave the pin 5 open?
    Is it internally connected to ground by pull down resistor?
  • 0 in reply to Koyo Kegasa
    TI__Expert 7220 points
    Hi Koyo,

    You are correct that it is a variable gain temperature sensor and yes it is the same die as the LMT84-7 sensors.

    I recommend you connect pin 5 to ground, leaving a pin open may yield unpredictable results not guaranteed in the datasheet.

    -Michael Wong
  • 0 in reply to Michael Wong
    Expert 1260 points
    Thank you very much!
    I connected all the 3 pins to the ground.

    I made Visual C++ Form1.h program to turn the voltage[uV] into the temperature [degrees C] using the equation (2) on the page 10 in lmt84.pdf.

    private: double lmt84microVoltToDegreesC( double microVolt ) // LMT84 uV to Temperature
    {
    return ((5.506-Math::Sqrt((-5.506)*(-5.506)+4.0*0.00176*(870.6-(microVolt/1000.0))))/(2.0*(-0.00176))+30.0);
    }
    I got the temperature around 29 degrees C all right.
    ( It's hot in my room today. )

    Thank you very much again for you help.