data logger

Discussion in 'The Projects Forum' started by nbc, Nov 21, 2008.

  1. nbc

    Thread Starter New Member

    Nov 21, 2008
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    I am constructing a temperature data logger.
    The output voltage of the 741 amplifier is connected to PORTA(analouge input pins) of PIC16F876 microcontroller. The output voltage reduce after connected to microcontroller. Can anyone tell me what's the problem and how to solve this problem??

    By the way, the 741 operational amplifier need +5V and -5V voltage supply. Can a voltage inverter used to generate -5V supply??
     
  2. Wendy

    Moderator

    Mar 24, 2008
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    What kind of voltage ranges are you using? If it is a thermocouple device the 741 won't hack it.
     
  3. nbc

    Thread Starter New Member

    Nov 21, 2008
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    Thanks for ur reply. After a voltage inverter was used the problem had solved. But the output voltage of the operational amplifier is slightly higher than expected value. Should I connect some component to get a correct value.

    A PIC16F876 microcontroller was used in the project. The microcontroller has built-in A/D converter. Can negative voltage be converted if the Vref- is set to negative?

    thanks....
     
  4. hgmjr

    Moderator

    Jan 28, 2005
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    Can you provide a schematic of your circuit to assist us in helping you out?

    hgmjr
     
  5. nbc

    Thread Starter New Member

    Nov 21, 2008
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    The schematic diagram was attached. But the diagram is not completed. I have forgotten to draw the input voltage of the op amp. The pin 3 of 741 op amp(non-inverting input) was connected to a thermocouple. The gain of the op amp is 101. The input voltage of the op amp was 1.1mV but the output voltage was 132mV. Can anyone tell me what's the problem?
     
  6. hgmjr

    Moderator

    Jan 28, 2005
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    What is the range of the input signal to your opamp?

    hgmjr
     
  7. nbc

    Thread Starter New Member

    Nov 21, 2008
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    -6mV to 50mV. Any problem?
     
  8. beenthere

    Retired Moderator

    Apr 20, 2004
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    Yes, that's way more signal swing than a 741 can handle. With a 5 volt supply, you need an op amp specifically built to go rail to rail. The 741, beside having very poor input impedance, will hardly go past about 3.5 volts.

    Look at the TI OPA4830 -

    • Quad, Low-Power, Single-Supply, Wideband Operational Amplifier (Rev. A) (opa4830.pdf, 953 KB)
      22 May 2008 Download.
    It is much closer to what will work in your application. Oops, now I see it's a quad. Look for single supply rail to rail in a single package (sorry).
     
  9. nbc

    Thread Starter New Member

    Nov 21, 2008
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    Sorry, I m under graduate student. I not quite understand what's the meaning.
     
  10. Wendy

    Moderator

    Mar 24, 2008
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    The 741 is a training chip. It was barely state of the art 30+ years ago, and is still good for beginners to burn up learning op amps. You need a modern part, such as was suggested. That was why I posted what I did in the beginning. I'm one of the folks who like the chippie, you will hear some comments that aren't so nice about 741's.

    Thermocouples are pretty high tech, low voltages, sometimes noisey. They need an op amp that is as stable as a rock and no offset, which does not describe a 741.
     
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