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What opto-coupler to use to isolate the input from the rest of the circuit for ElectroCardiography application ?

Posted on 2011-10-24
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Last Modified: 2012-05-12
Hello Guys,

I'm designing a circuit to capture the heart signal and should output it to an oscilloscope (ElectroCardiography). I need to isolate the circuit from the human, I'm using an opto-coupler for this purpose. I'm designing the circuit in MultiSim (by National Instruments). I've attached a screenshot of the circuit and a screenshot of the input to the opto-coupler and the output from the opto-coupler.

The opto-coupler is causing my signal to be reduced in amplitude and also to a have a negative DC offset.

My question is what opto-coupler should I use that would be suitable for ElectroCardiography ? I want the output to be almost identical to the input because the only purpose of the opto-coupler is isolation, so how can I get rid of any side-effects of the opto coupler ?

Please advise, thanks

ecg-circuit-opto-coupler  ecg-circuit-opto-coupler-output
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Question by:adkry
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LVL 93

Assisted Solution

by:nobus
nobus earned 400 total points
ID: 37016433
you should be able to amplify it with another chip, and that would allow you to remove the offset as well
look at this youtube :  http://www.youtube.com/watch?v=b6fqmZ9suL4
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by:Dave Baldwin
Dave Baldwin earned 1200 total points
ID: 37016445
Digikey carries many of the standard opto couplers like http://search.digikey.com/us/en/products/4N35M/4N35MFS-ND/400350  Not very expensive either.
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Assisted Solution

by:d-glitch
d-glitch earned 400 total points
ID: 37017234
There are opto couplers optimized for linear operation:  http://www.avagotech.com/pages/en/optocouplers_plastic/plastic_high_linearity_analog_optocoupler/hcnr200/

There are also isolated amplifiers: http://www.analog.com/en/specialty-amplifiers/isolation-amplifiers/ad202/products/product.html

You could also convert your analog voltage signal to frequency and use a simple digital optocoupler.  A suitable V/F converter would be:

      http://www.analog.com/en/analog-to-digital-converters/voltage-to-frequency-converters/advfc32/products/product.html
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Author Comment

by:adkry
ID: 37026900
@nobus

The video wasn't in english therefore couldn't understand it. Could you clarify what the video was saying ?

@DaveBaldwin

Thanks for your link for opto-couplers, will be very useful if I want to get them, but I'm now thinking of different alternatives, mentioned below.

@d-glitch

Thanks for your link on opto-couplers with linear operation.

Isolated amplifiers is a really good option, so now I've got 3 options:
Opto-couplers
Isolated amplifiers

Which one is most suitable for ElectroCardiography circuit ?

The requirement is to isolate the patient from the circuit (my circuit operates using +5V voltage source, don't know the current, but could find out).

The patient will be connected using 2 electrodes to an instrumentation amplifier, this op-amp will output a signal which is around 5 V (peak to peak), I believe the isolator must be put after the instrumentation amplifier (not before it, i.e. not between the patient and the op-amp, because the op-amp is very sensitive to the 5mV input it takes from the human heart, an isolator will mess things up I think). So the question is which isolator will not disrupt the signal outputted from instrumentation op-amp, and also will protect the patient, and will not need more than a +5V, -5V supply ???

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Accepted Solution

by:
Dave Baldwin earned 1200 total points
ID: 37027295
Opto isolators show a great deal of variation in sensitivity.  You have to be able to adjust the gain and calibrate them no matter which one you select.

This page http://www.analog.com/library/analogDialogue/archives/37-11/ecg.html looks to me like a good discussion of what is needed.  It uses a more expensive ISO amplifier which will have circuits to linearize the output.
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Expert Comment

by:nobus
ID: 37029464
to me it was in English
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Author Comment

by:adkry
ID: 37030043
@nobus

oops sorry, the video has dual sounds, one in english and one in greek i think, my soundcard is so crap it only outputted the greek one lol
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LVL 93

Expert Comment

by:nobus
ID: 37030093
well -i don't understand greek either lol
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Author Closing Comment

by:adkry
ID: 37030341
Thanks I think I'm going for digital isolators
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