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LM34: OP amp needed

Alvin P. Schmitt
Intellectual 940 points
Part Number: LM34
Other Parts Discussed in Thread: TMP235, LMT84, LMT87

I have used the LM34 with a microprocessor with 10 bit a to d in a room without varying temperature. The readings always jump wildly about . Does this device require a Op Amp between the device output and the input to the a to d? 

  • 0
    TI__Mastermind 40545 points

    Hi Alvin,

    Could you share a schematic? Do you have R or RC (see datasheet figures 12 and 13) on the output of LM34? What is the input impedance of the ADC? Some ADCs require input capacitance; have you reviewed the requirements for your ADC?

    If your 10 bit ADC has a 3.3V reference (the LM34 can go to 3V output full range) then you have 3mV LSBs. Noise of +/-1 LSB would cause +/-0.3F variation. It's fairly common for an ADC to have at least a few LSB of noise. 

    The LM34 doesn't have stellar output current drive. The spec table guarantees all parameters under a nominal load of 50uA. TMP235, for example, is a modern analog output temperature sensor with a 500uA output spec. The LMT84 through LMT87 devices are available in the through-hole TO-92 package and also offer a 50uA output spec. However, the LMT8x devices also specify that they can tolerate 1100pF on their output, where LM34 description (page 11) states the LM34 can only handle 50pF without resistor isolation.

    thanks,

    ren

  • 0 in reply to Ren Schackmann
    Intellectual 940 points

    The LM34 plus connected to +5 volts. Minus is connected to ground and Vout is connected to a microchip pic AtoD pin unless something is wrong using a Pic?

  • 0 in reply to Alvin P. Schmitt
    TI__Mastermind 40545 points

    Hi Alvin,

    Could you review some of my previous questions and consider answering them? I'm not familiar with PIC, and not able to advise you on it's ADC. If you could be more specific about your schematic and ADC's specifications I could make further suggestions.

    thanks,

    ren

  • 0 in reply to Ren Schackmann
    Intellectual 940 points

    As far as the schematic goes I can't be more specific as it is just a simple connection to power, ground and vout. Power for both the LM34 and micro (PIC12F1840) is +5 volts, gnd id the same and vout goes to ADC pin 10 bit ADC pin of which there are 5. I will include the part of the manual on the ADC. I use CCS C which takes care of the particulars to access the ADC. The ADC may be being used wrong and maybe I should be going to Microchip and CCS with this question. I will attach the manual pages. I do not see how to attach a file. I probably need your email address to attach a file. I will attach the file to the email I got from the system.

  • 0
    Intellectual 940 points
    Attached manual page

    Alvin P. Schmitt
    APS PRODUCTS LLC
    P.O. Box 201
    Newport VA 24128-0201

    1-540-951-3961

     



  • 0 in reply to Alvin P. Schmitt
    TI__Mastermind 40545 points

    Hi Alvin, 

    I didn't receive the attachment. Please give a link or simply the model number should be sufficient.

    thanks,

    ren

  • 0 in reply to Ren Schackmann
    Intellectual 940 points

    PIC12F1840  the ADC section pages 127->140

  • 0 in reply to Alvin P. Schmitt
    TI__Mastermind 40545 points

    I wasn't able to find an internal reference, so I have to assume the ADC uses the 5V supply as reference. That would mean that it can only resolve 0.5F changes. This ADC may not be suitable. You could try buffering with an opamp, but this low-quality ADC is likely to always have at least +/-0.5F noise. There isn't a spec for the ADC's noise or input impedance, so I can't make any further comments on its suitability for LM34. 

    You could try an LMT87, which has a higher gain of -13.6mV/C, but it's not a straight comparison to the LM34's 10mV/F. There isn't another fahrenheit sensor in our portfolio.

    thanks,

    ren

  • 0 in reply to Ren Schackmann
    Intellectual 940 points

    I told CCS about it (they make the C compiler) and they said they did it without any problems with no op amp used. They suggested using more time and waiting longer between ADC conversions. So I will try it and work with them.