Instrumentation Amplifier for ECG

[Galvanicdude]

Hello,
I am trying to build a usable ECG for some music project.
By scouting round Internet, most projects seem to be using an instrumentation amplifier.

So far I have no form of analogue filtering, as just want to make sure the instrumentation amplifier is working correctly. However, after applying considerable digital filtering I still can't monitor the peeks analogous to heart contraction.
From a previous experiment (connecting the electrodes to a simple gain stage then connecting them to my sound card, recording, and filtering) I was able to monitor the wanted signal just by applying digital filtering.

So I used this schematics
I have R=100k , Rgain = 470k potentiometer.
And am using 741s opamps.
If anyone could help the inexperienced me, that would be great.
thank you.

 

[Galvanicdude]

Also, the output of my instrumentation amplifier is negative.
Why?
I need it to vary between 0 - 5V. How do I do that?

 

[shrtrnd]

I think you're on the right track scouting the internet for ideas.
In answer to your second question, the output is a characteristic of an instrumentation
amplifier (Operational Amplifier).
It's stated: 'The output is inverted from the input signal'. That's the effect of an Op Amp.
Maybe somebody will have more info on your first question.

 

[Galvanicdude]

Thanks for your reply.
Where did you get the quote "'The output is inverted from the input signal'. That's the effect of an Op Amp."
That must of passed under my radar whilst reading about IA (instrumentation amplifiers).
If that is the case how did this guy get his to work?
What does a null offset do?

 

[shrtrnd]

I quoted myself.
There are several types of instrumentation amps. In most cases they're glorified op amps. Op amps are generally pretty cheap (like your LM741's). You design tighter
specs and you market them as 'instrumentation amplifiers'. That way the customer
doesn't feel so bad about paying $20 for something that does the same thing as a 40-cent
part with looser specs. (I'm exaggerating, some applications call for tight specs)
If you decide on a specific instrumentation amp, get the data sheet on it, and then
look-up information on the devices that comprise the IC.
You will find that in all cases I've ever seen, the input to the device is inverted at the
output.
Sorry if I'm not too clear. This is one of those things you learn when you're young, and
take for granted for many years. After a while you don't stop to question why anymore.

 

[Galvanicdude]

Cool,
Thanks, that makes sense.
However, I am not using a packaged instrumentation amplifier but am making one out of 741s.
From what I have gathered, a ECG requires tight specs. Building one, shouldn't be too hard and is a good learning process for me. Sadly its not working and still can't figure out why!

 

[shrtrnd]

I read your 'guy' article.
His observations are as a learning experiementor.
The only thing he said at the end of his article that caught my attention was his claim
that you need a more robust amp. He should have said a device with tighter specs.
His device for measuring heartbeats CAN'T work using 741's. They are far too susceptible
to outside noise and marginal specs for an application like this.
An old (obsolete) op amp that would do a lot better in this application is the old
Burr Brown 3500, or even better spec'd 2A083.
Burr Brown got bought out by Texas Instruments, and they dropped the series.
But you can still find a lot of them out there for about $5 apiece.
In fact, if you look-up Burr Brown Instrumentation Amplifiers, you'll find quite a few
devices that would work a lot better here than a 741. I suggest them because while
they are no longer made, they are EXCELLENT devices, and readily available from a
lot of sources for relatively cheap. If you intend to design for possible mass production,
you'd have to find instr amps that are presently in production.
The key to your present problem, is that the outside 'noise' around you, is overpowering
your circuit. The wiring in the walls around you, the lights, radios, everything that
creates electrical 'noise', is going to be more powerful than the signal you're trying to read.
Remember when you couldn't use a microwave or cell phone in a hospital?
New devices and designs are made to overcome that type of interference.
Just remember. You are trying to read an extemely small signal. Just touching the
test probes you would use to take readings, causes you to become an antenna for
outside electrical 'noise', that your mere touch, induces into your circuit.

 

[Galvanicdude]

Okey. Thanks
Is this still true for a project non intended for mass and where all I need is the heart rate? and not a clean looking curve?

 

[shrtrnd]

I appreciate you don't need precision.
What you DO need, is sensitivity.
Good luck with the experiment.

 

[Resqueline]

I'd also say that the 741 is not quite up to the ECG job, even for an amateur. There are many other low-cost low-noise low-offset devices to choose from.
An op-amp with a null offset can be adjusted to give zero volts out when its inputs are shorted together. One without will give a high positive or negative voltage out instead.

 

[scriblrr]

The frequency components of the ECG signal are typically ALL very low, well below 30 cps. ALSO, you have to contend with "movement artifact" due to change of contact between the electrodes and a moving body, as well as "muscle artifact" which is like a HF "hash" on the ECG signal. So, you have your work cut out for you, since your amplifier shows NO filtering!

A well-defined Lead II ECG shows a definite "p" wave, followed by a sharp QRS complex, followed by a "t" wave (signifying repolarization), and maybe a "u" wave. It is a thing of beauty! You can't expect to plot a dif amp across body areas and get that! I suggest you consult Lippman & Massie's "Clinical Scalar Electrocardiology" and you will learn all you need to know about what you hope to detect.

BTW, I'm not a cardiologist. Years ago, I worked for a wonderful company known as "Electronics for Medicine" and I learned this all by DOING.

Good luck!

Scriblrr