DC offset of signal

Discussion in 'The Projects Forum' started by nazcov, Mar 20, 2013.

  1. nazcov

    Thread Starter New Member

    Jan 20, 2013
    8
    0
    Hi

    I'm making a circuit for measuring, amplifying, and filtering an EMG signal.

    In one of my last parts i have to offset the signal so it will become positive only. This is because i have an MSP430 (eZ430-RF2500), which works between 0-3.3V.

    My problem is that with an normal op-amp offset trim circuit, i can't seem to make it work properly. And the datasheet also says it only works with +/- 4mV.

    So can any of you help me with another method, or maybe just the method for doing it correctly? I'd rather not wan't an advanced method, but just as simple as possible.

    Hope you can help

    Thank you in advanced
    Sonny
     
  2. JMac3108

    Active Member

    Aug 16, 2010
    349
    66
    I think you are mixing up two different things.

    The 4mV you mention is the "input offset" of the op-amp. This is the output of the op-amp with 0V input. This "offset trim" circuit is probably referring to input offset trim.

    From your description, I bellieve what you're actually trying to do is to add a DC offset to your signal so that it is always positive going into the MSP430 (probably into the ADC). Right?

    Try posting your schematic with a better description of what you're trying to accomplish and I'm sure someone will help.
     
  3. nazcov

    Thread Starter New Member

    Jan 20, 2013
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    You are completely right JMac, it is of course DC offset i wan't, I just figured i could use the input offset for an easy way out.

    I have attached the schematics as it is now. My signal is 400µV PP (Amplitude 200 µV) and it will be gained by 200 in my in-amp, to 80mV PP, after that i will AA filter with cutoff at 500Hz, and gaining a second time until 1.65V which will be a gain around 20. This will leave me with ~+/-1.65V which I need to offset to +3.3V.

    After that i will place a unity gain buffer (single supply), for safety measure regarding my MSP so I wont risk it being burned by +/- 9V

    I hope someone can help.
    Thank you so much
     
  4. nazcov

    Thread Starter New Member

    Jan 20, 2013
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    0
    Oh sorry, I had one of the op-amps turn upside down without changing the supplies, i'll upload it here.
     
  5. t06afre

    AAC Fanatic!

    May 11, 2009
    5,939
    1,222
    Can you post a schematic in PDF format. Not everyone can read asc files
     
  6. nazcov

    Thread Starter New Member

    Jan 20, 2013
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    Of course, it is attached.

    A correction: 800 µV PP (400 µV amplitude).
     
    Last edited: Mar 21, 2013
  7. ramancini8

    Member

    Jul 18, 2012
    442
    118
    Read chapter 4 of Op Amps for Everybody because it explains exactly how to achieve your goal. Start with section 4.3. This book is available free from the TI website.
     
  8. nazcov

    Thread Starter New Member

    Jan 20, 2013
    8
    0
    Hi all

    Thank you for your answers. Since we weren't able to get it to work we ended up with an rectifier which works now.

    Thank you again for your help!
     
  9. Brownout

    Well-Known Member

    Jan 10, 2012
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    I always wanted to ask you. Did you author that book?
     
  10. nazcov

    Thread Starter New Member

    Jan 20, 2013
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    His profile states that he did ;)
     
  11. Brownout

    Well-Known Member

    Jan 10, 2012
    2,375
    998
    Guess I should'a looked. In that case, it's my extreem honor. I read your column for years. I remember when you wrote that your boss said you dress like you're leaving a burning building. HA! That was priceless.

    BTW, I know one of your fellow EDN authors personally. Clive "Max" Maxwell. Heckuva guy.
     
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