ECG amplifier

Discussion in 'The Projects Forum' started by andrew24, Mar 2, 2010.

  1. andrew24

    Thread Starter Active Member

    Aug 20, 2008
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    Hello, i'm designing an ECG amplifier(to amplify the electric heat signal). My ADC reference voltage is 3.3V, so assuming that ecg signal is ~3mV i need a gain of 1000. The heat of this project is instrumential amplifier AD8221. (datasheet - http://www.analog.com/static/imported-files/Data_Sheets/AD8221.pdf)

    ECG signal bandwidth is 0.05Hz - 150Hz, diferential amplitude 1-5mV with a DC component od 0.3V which limits first stage gain.

    First stage has a gain of 100.The feedback integrator(U3,R4,C4) that drives ad8221 reference pin should cancell out DC component.R4 and C4 were selected for the integrator to output voltage only if there is DC component (0.05Hz threshhold). The last stage is active filter with a cutoff frequency of 150Hz and a gain of 100.
    Tell me what do you think about my design!
     
    Last edited: Mar 2, 2010
  2. t_n_k

    AAC Fanatic!

    Mar 6, 2009
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    The 0.3V DC offset on the input side will translate to a 30V shift at the 1st stage output with x100 gain. So the 1st stage output will saturate, and you won't be able to compensate for this DC magnitude with the integrator feedback to pin 6.

    Are you sure you will expect 0.3V DC offset?
     
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  3. t06afre

    AAC Fanatic!

    May 11, 2009
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    Hi Andrew
    You have done some good work but perhaps your circuit is not optimal. I would have used 3 electrodes as a start. I think you will find some useful information in this document
    http://focus.ti.com/pdfs/vf/medeq/informationformedicalapplications.pdf
    I have not studied Andrews circuit in detail. But the technique he has used to apply AC coupling to an instrumentation amplifier is quite common and is described here http://focus.ti.com/lit/an/sboa003/sboa003.pdf
    Also remember Andrew most of the methods described in the papers above will apply very well to other types of instrumentation amplifiers. You do not have use the exact same circuits that are used in papers. Use the design principals, and modify if needed
     
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  4. russ_hensel

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    Jan 11, 2009
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  5. andrew24

    Thread Starter Active Member

    Aug 20, 2008
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    Thank you all for your replys and recomended documentation!

    BTW, Recently it came to me that i've designed amplifier with bipolar voltage, but my ADC reference can be only positive..How could i shift my signal upwards that it would be only positive?

    Also i have a question.. Yes, i've used that popular DC canceling technique with a feedback integrator, but now i can't figure out any advantage of it versus the simple RC high pass filter right after instrumentation amplifier.. Here's a scheme to get my idea:

    Electrodes --> InAmp(low gain 5-10) --> RC high pass filter 0.05Hz --> high gain --> out

    what DC canceling method do you think is better? with integrator or with RC filter(in this case we connect REF to ground)?
     
    Last edited: Mar 3, 2010
  6. t06afre

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    This will depend on your ADC. It is quite common that a ADC only do have a positive reference but convert in +/- Vref range. Which ADC do you use?
     
  7. andrew24

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    Aug 20, 2008
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  8. t06afre

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    In that case you can use the design I pointed you to here http://focus.ti.com/pdfs/vf/medeq/in...plications.pdf as a reference design. Read the paper, simulate, and make your modifications, then simulate again. As I said use the principals. You do not need to have all the fancy components used in the paper. You may also use this thread to ask questions. But as I understand so is this a school project, so you must put in some effort your self (D'oh) at least if you want some more help from me
     
  9. andrew24

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    Aug 20, 2008
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    offcourse i don't want others to do the job instead of me :) I will use that reference design, thanks. can you tell me what are so special InAmps, stated to be single supply ? are they just Rail - to - rail and thus better than others, or they have other usefull characteristics that bipolar?
     
  10. t06afre

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    As a thumb rule most opamps need a split supply (+/- supply voltage). But some opamps do not need this, in order to work, they only need positive supply.The - supply voltage is then connected to GND. And that is single supply type opamp. Normally a opamp will not have output range that cover the supply voltage. It is usually around a few volt below. But some opamps can do this. And that is rail to rail opamp. The term come from the expression power supply rails. http://en.wikipedia.org/wiki/Power_supply_rail
     
  11. andrew24

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    Aug 20, 2008
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    But i think we can connect any opamp for example to GND and 5V, and make virtual ground in the middle (2.5V ) and this will work. or it won't unles the opamp is rated single supply?
     
  12. t06afre

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    You can make a virtual ground, and use a opamp that require +/- 2.5 from a 5 volt supply. But that will complicate your design. It is a awkward and impractical design strategy. I have never used it in any design. It is then often more practical to use single supply opamps and lift the signal up to half the supply voltage. What are you going to use in your project?
     
  13. andrew24

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    Aug 20, 2008
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    I think i will use INA326 and OPA335 as proposed in the application note and power them from a 3V battery
     
  14. t06afre

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    Just to so I do not mislead any way. It is perfectly OK to combine batteries in such way that you get split power supplies. Example combine 4 1.5 volt cells to give +/- 3 volt. Or two 9 volt cells to give +/- 9 volt and not 18 volt. That will not be a virtual ground solution.
     
  15. andrew24

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    Aug 20, 2008
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    i undestand that, but since ADC can take only 0-3V i think its inpractical to use split power supply
     
  16. t06afre

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  17. andrew24

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    Aug 20, 2008
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