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chelletengchelleteng Posts: 29
edited 2012-03-09 23:06 in General Discussion
How to bias the output at op-amp C to half of the Vcc input? The output idle voltage is saturated.
Thanks.
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Comments

  • Beau SchwabeBeau Schwabe Posts: 6,568
    edited 2012-03-09 22:50
    You would be better off purchasing an instrumentation amplifier, than rolling your own. The reason is because of the voltages involved in what you are trying to do. An instrumentation amplifier will reject any common mode signals much better than what you can put together using discrete Operational amplifiers.

    To answer your question however "How to bias the output at op-amp C to half of the Vcc input?" ... what you want to do is buffer the midpoint of R2, meaning that R2 becomes two 5k resistors instead of one 10k. This portion of the buffering does not need to be as precise as the instrumentation amplifier on the main inputs.

    See this link for a Reference:
    http://www.edn.com/article/520460-Use_Spice_to_analyze_DRL_in_an_ECG_front_end.php

    Instrumentation Amplifier reference with ECG example (see page 11):
    http://focus.ti.com/lit/ds/symlink/ina118.pdf
  • LoopyBytelooseLoopyByteloose Posts: 12,537
    edited 2012-03-09 23:06
    The TL084 is a good little op amp chip, but you may run into several problems.

    First is that it is one of four in the package - so if you use +5VDC for powering the first 3 op amps, the fourth has to do so as well. Generally the op amp will perform better and be simpler to wire if you use +15/-15 VDC.

    Try Elliot Sound Products for information about why. And look at the frequency response charts in the PDF.

    sound.westhost.com

    You can even buy a little board from them that will work the TL082 or maybe the TL084 to adapt to what you want. Everything he has ships as a letter via post, so costs are cheap.
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