Rectifier for biomedical analog front end

Discussion in 'The Projects Forum' started by alto125, Sep 14, 2013.

  1. alto125

    Thread Starter New Member

    Mar 15, 2013
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    Hi there!
    I'm working on a project that requires reading biomedical signals from the body, I'm making an analog front end with differential amplifier, bandpass filter, full wave rectifier, integrator and variable amplifier.
    I'm testing the system with a sine wave. Everything is fine until the output of the rectifier - it seems extremely unstable! The amplitude after the bandpass filter is a 1.5v p-p sine wave and after the rectifier it ranges from 2.15 to 3.08V p-p but the shape of the waveforms is not very uniform

    Any ideas?

    Thanks
     
    Last edited: Sep 14, 2013
  2. wayneh

    Expert

    Sep 9, 2010
    12,087
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    Can you attach your schematic in a picture format, such as .png? I can't use the .asc file.
     
    #12 likes this.
  3. alto125

    Thread Starter New Member

    Mar 15, 2013
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    Sorry, here is the rectifier
     
  4. #12

    Expert

    Nov 30, 2010
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    A copy of a googled precision rectifier only tells us that the rectifier is correct. How you built it must be wrong.
    Is that scope showing the first 3 pulses at start up or a steady state condition?
     
    Last edited: Sep 14, 2013
  5. alto125

    Thread Starter New Member

    Mar 15, 2013
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    That's in steady state
     
  6. t06afre

    AAC Fanatic!

    May 11, 2009
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    What kind of bio signal are you measuring. And also do you use a dual supply voltage
     
  7. #12

    Expert

    Nov 30, 2010
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    Which amplifier chip are you using? How much voltage did you give it to work with (at the supply terminals)? How much current does it have to provide to its load? Stuff like that.
     
  8. GopherT

    AAC Fanatic!

    Nov 23, 2012
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    Looks like you are trying to rectify the signal from your sensors. Normally, you would amplify the signal from the sensor before rectification.
     
  9. alto125

    Thread Starter New Member

    Mar 15, 2013
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    I'm measuring EMG signals, first of I'm amplifying with a ina129 differential amplifier (gain of 735) and then bandpass filtering it between (10-500Hz). It's a dual power supply of 9V supplied to the circuit

    Thanks!
     
  10. #12

    Expert

    Nov 30, 2010
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    and you still have not told which chip you used for the precision rectifier.

    We can not examine the precision rectifier unless you disclose what you used to make it.
     
  11. alto125

    Thread Starter New Member

    Mar 15, 2013
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    I apologise, the rectifier is built with a TL072
     
  12. #12

    Expert

    Nov 30, 2010
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    OK. That chip should do nicely. You have an available input range of +/- 5 volts and the available output into 10k ohms is +/- 7 volts. I shall now try to imagine which diodes you used and how the precision rectifier is connected to the previous and following circuits.

    No, actually, I will not. I can not examine a circuit which you refuse to reveal. I am done begging you to let me help you.
     
    Last edited: Sep 15, 2013
  13. alto125

    Thread Starter New Member

    Mar 15, 2013
    12
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    Take it handy!

    I've no problem putting up all of my circuit at all, and the diodes are clearly labelled as 1N4148 on the post I put up earlier so there is no need for mad imaginations skills....

    ;)
     
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