instrumentation amplifier in 16F877A

Discussion in 'Embedded Systems and Microcontrollers' started by mgfr, Nov 28, 2006.

  1. mgfr

    Thread Starter New Member

    Nov 28, 2006
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    hello,

    I used LM348 opamp as instrumentation amplifier,I can read fine the output voltage in digital voltmeter.
    When I connect it dirrectly to ADC of PIC16F877A, I can not read the value because very fluctuative.
    My ADC is working fine for other voltage value except from instrumentation amplifier.
    Is it the noise of instrumentation amplifier? What should I do to solve it?
    Thanks,

    mgfr
     
  2. Papabravo

    Expert

    Feb 24, 2006
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    There is probably nothing wrong. Most multimeters average the voltage readings over a comparitively long period. When you use an ADC you get on instantaneous reading. If the next one occurs 100 or 200 microseconds later the it may be different by several LSBs. The only way to be sure is to look at the opamp output with an oscilliscope.

    The LM348 or quad 741 has such a low GBW and crappy response that it is unlike to be susceptible to significant amounts of high frequency noise.

    Two questions:
    Did you bypass the supply pins?
    Did you tie off the inputs to the forth amplifier in the package?
     
  3. beenthere

    Retired Moderator

    Apr 20, 2004
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    Hi,

    Another source of noise is in the proper circuit grounding. The PIC does not let you separate analog and digital ground lines, so not only do you have to have a good connection to a single point on the board, but, as Papabravo said, you need to use capacitors to bypass your power supply lines for the PIC and the op amp. A better op amp couldn't hurt.
     
  4. mgfr

    Thread Starter New Member

    Nov 28, 2006
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    0
    hello,

    Thanks for your reply.
    I build instrumentation amplifier (IA) in dual polarity voltage as supply and bypass the supply pins for IA and PIC.
    It needs 3 op amps and tie off for the 4th. I measure 3 kind of voltage outputs,one of them is from IA when I used them together sometime all of them become very fluctuative, but if I disconnect output from IA they are working fine.
    If I connect the ADC channel for IA with output from regulated power supply, it's working fine.
    Is it wrong connect dirrectly from IA to PIC ? I can not finish my project because of this problem.
    Please help me to solve this problem.
    Thanks,

    mgfr
     
  5. Papabravo

    Expert

    Feb 24, 2006
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    1st, fluctuative is not a word. It is hard to know what you mean when you make up words which have no meaning.

    2nd You're going to have to produce a schematic so we can spot any obvious problems which your descriptions do not adequately convey.
     
  6. beenthere

    Retired Moderator

    Apr 20, 2004
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    Hi,

    I think the word is fluctuating, if I've spelled it correctly.

    If the problem appears whenever you have the IA connected, then the problem is associated with it. Perhaps a ground loop. Do the grounds of the PIC Vcc supply and the power supply ground for the +/- voltages to the IA have a common point? If the IA supply ground is not referenced to the PIC's ground, then the IA signal will float, and appear very noisy.
     
  7. Papabravo

    Expert

    Feb 24, 2006
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    I was hoping that he could characterize the fluctuations in amplitude and frequency so we are not left to wonder what he's talking about.

    BTW changing "fluctuative" to "fluctuating" is equally useless in the original context.

    ...all of them become very fluctuating,

    Do you have a sense of what he is observing? I sure don't.
     
  8. beenthere

    Retired Moderator

    Apr 20, 2004
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    Hi Papabravo,

    I get the idea that the IA "destabilizes" the whole system when it's signal is brought in. That speaks to me of ground loops or lack of a common reference between the digital and analog power supplies.
     
  9. Papabravo

    Expert

    Feb 24, 2006
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    If the IA is running off of a bipolar supply what is to prevent the output from drawing excessive current through the A/D inputs as they go below the negative analog reference(Vss(?)) on the 16F877? If that's happening it would be a big Bozo No No.
     
  10. beenthere

    Retired Moderator

    Apr 20, 2004
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    Hi,

    Yeah, bipolar signals and single-ended inputs don't play well together. Need a schematic and some waveforms to tell what's happening.
     
  11. mgfr

    Thread Starter New Member

    Nov 28, 2006
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    Sorry for my english.Here is the schematics.I tried 2 kind of ps but not good in result.
    If bipolar can not work well in single ended, how to make nice IA for it?
     
  12. Papabravo

    Expert

    Feb 24, 2006
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    You have not included any details on the PIC portion of the schematic.

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  13. mgfr

    Thread Starter New Member

    Nov 28, 2006
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    Please don't attack other members personally.

    Dave
     
  14. Dave

    Retired Moderator

    Nov 17, 2003
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    Ok guys, enough of this. May I bring your (both Papabravo and mgfr's) attention to the rules:

    This tone is neither healthy nor appropriate. If neither of you have anything positive or constructive to say to each other, don't say it at all. mgfr has asked a valid query relating to an instrumentation amplifier and deserves to be replied to in an appropriate manner by all members. At the same time should you feel a reply is inappropriate this is not a valid reason to personally attack another member. If you have an issue with the tone or what a member has posted here at AAC, please alert a Mod to it and we will deal with.

    Consider this a warning to both of you.

    Dave
     
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