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Hi,
I'm working on an ECG. Its not going very well at the moment...
I'm basically using an Instrumentation amplifier to pick up the electrical signal causing the heart to function.
So far I'm not getting anything.
Have any of you guys done this before? if so how did you go about it?
Thanks.
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Newcastle, UK
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Have you checkout :

http://www.open-ecg-project.org/

lots of good information there for ECG projects

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Yes I'm going through it as we speak.
The current circuit I've been using is this one:
http://skippyrecords.wordpress.com/2007/09/01/building-an-electrocardiograph-i/#comment-69.

But it hasn't been working..
The problem is I'm trying to avoid buying new components (as I have quite a few bits and pieces already!).
I've got a bunch of 741 (which are probably far from being the best opamps for this application), and would like to use them up!

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I'm not sure I would trust a homebrew instrumentation amplifier made from 741 ICs when it comes to monitor heart (or brain) signals. If something went wrong - it could go very, very wrong. There do exist proper medical-equipment grade amplifiers out there - but they aren't cheap. Then again, I value my life, so...?
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I'm not sure I would trust a homebrew instrumentation amplifier made from 741 ICs when it comes to monitor heart (or brain) signals. If something went wrong - it could go very, very wrong. There do exist proper medical-equipment grade amplifiers out there - but they aren't cheap. Then again, I value my life, so...?

I'd want to make the device battery operated (no high voltages anywhere) and optically isolated from anything it connects to such as a PC. One possibility would be to base it on the Arduino Uno design but split it in two, with opto isolators on the Tx and Rx signals between the mcu and the USB-to-serial chip. The mcu could be run from a 9v battery via the regulator. An Arduino PWM output pin and a couple of capacitors and diodes can be used to generate -3.7v (or -7.5v using a transistor as well), so the op-amps can be run from about +/- 4v or +/-8v.

At the very least, I'd replace the 741 op-amps with something more modern with jfet or mosfet inputs.
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I'm not sure I would trust a homebrew instrumentation amplifier made from 741 ICs when it comes to monitor heart (or brain) signals. If something went wrong - it could go very, very wrong. There do exist proper medical-equipment grade amplifiers out there - but they aren't cheap. Then again, I value my life, so...?

I'd want to make the device battery operated (no high voltages anywhere) and optically isolated from anything it connects to such as a PC. One possibility would be to base it on the Arduino Uno design but split it in two, with opto isolators on the Tx and Rx signals between the mcu and the USB-to-serial chip. The mcu could be run from a 9v battery via the regulator. An Arduino PWM output pin and a couple of capacitors and diodes can be used to generate -3.7v (or -7.5v using a transistor as well), so the op-amps can be run from about +/- 4v or +/-8v.

At the very least, I'd replace the 741 op-amps with something more modern with jfet or mosfet inputs.

Isolation is a good idea - but the issue isn't so much voltage - it's current. Even a 9 volt has enough current to send you into defib (though not via skin resistance). The problem is, in order to run the circuit, you need enough current to run everything; I think the way medical inst amps work is by isolating the pads so only very low currents are on the pad side (probably way less than 100mA). I don't know how you would do this "homebrew" to make things safer...
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Isolation is a good idea - but the issue isn't so much voltage - it's current. Even a 9 volt has enough current to send you into defib (though not via skin resistance). The problem is, in order to run the circuit, you need enough current to run everything; I think the way medical inst amps work is by isolating the pads so only very low currents are on the pad side (probably way less than 100mA). I don't know how you would do this "homebrew" to make things safer...

I'm quite aware that it is the maximum current flow that has to be kept within safe limts. However, we're not talking open-heart surgery here, we're talking about electrodes on the skin - so surely skin resistance will always be present in the circuit, and 9v should be safe? If there is any concern about the maximum current flow under fault conditions, then I would have thought that adding a 100k resistor in series with each input (thereby limiting the possible fault current to around 0.1mA) would provide sufficient protection. However, I don't claim any expertise in this area.
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I'm quite aware that it is the maximum current flow that has to be kept within safe limts. However, we're not talking open-heart surgery here, we're talking about electrodes on the skin - so surely skin resistance will always be present in the circuit, and 9v should be safe? If there is any concern about the maximum current flow under fault conditions, then I would have thought that adding a 100k resistor in series with each input (thereby limiting the possible fault current to around 0.1mA) would provide sufficient protection. However, I don't claim any expertise in this area.

I don't claim expertise either - but if Steve Ciarcia was hesitant and cautionary about such a device in the 1980s (and he used a then-available commercial instrumentation amp) in his "Circuit Cellar" articles, I defer to his caution and understanding, for he is a far better EE than I. Anyone building a device like this would do well to look at his article for some guidance, perhaps...
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I just found this: http://www.analog.com/library/analogDialogue/archives/37-11/ecg.html. About half way down, it says that the medical standards require fault current to be less than 50uA. To achieve this they run the instrumentation amplifier from +/- 5v and use 220K series resistors on the inputs. They also run it from a battery and use optical isolation from the PC, just as I suggested in my original post.
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I just found this: http://www.analog.com/library/analogDialogue/archives/37-11/ecg.html. About half way down, it says that the medical standards require fault current to be less than 50uA. To achieve this they run the instrumentation amplifier from +/- 5v and use 220K series resistors on the inputs. They also run it from a battery and use optical isolation from the PC, just as I suggested in my original post.

Interesting find - I'll have to keep that in mind for the future. smiley
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There are dozens of national specifications for testing electrical safety of medical electronic equipment, but the two main international specs.  are IEC60601 and IEC62353.

IEC60601 is mainly used for type testing of equipment, while IEC62353 is used for checking equipment after repair or servicing.

Both specs. specify numerous tests to ensure the medical equipment is safe to use, but for Patient Applied parts (i.e. electrodes you might find with a ECG) the following electrical leakage current limits apply:

IEC60601 specifies numerous tests and limits: 10uA (DC) and 100uA (AC) leakage current with no fault condition and 50uA (DC) and 500uA (AC) with a single fault condition (i.e. one of the electrodes connected to ground)

IEC62353 has slightly simpler requirements and specifies 5000uA for Type BF equipment and 50uA for Type CF.

There's a nice write up on the practical testing for IEC62353 here



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Read "The Art of Electronics".  ECGs/EEGs need very high input impedance opamps (JFET or MOSFET) and you have to cancel out stray cable capacitances with special techniques from what I gather.
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You should definitly try to purchase an instrumentation ampklifier. I built one for my Emg Project and although it performed well, it would have caused less headache if I just purchased it. The main importance in choosing an instrumentation amplifier is CMRR (Common Mode rejection Ratio)
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I just built an ECG prototype with an Arduino Nano and a JY MCU bluetooth transceiver. I wrote an Android App to display the output. I'm teaching myself about biosignal processing as I go along, so it isn't perfect. But, so far so good:

https://www.youtube.com/watch?v=sH5oDiuCh5w

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