ECG for home use

[Olihou]

Hello every body!

I am new to this forum.

I wish to build a simple ECG (Electrocardiograph, for one of my family members not having a good heart). This project does not seem to have been a topic of discussion in this forum.

Can any one advise me how I should start. If any one can provide me with a circuit, it would be great.

Many thanks.

Oli

 

[Olihou]

I have found a circuit at the website of Scientific America:

http://www.sciam.com/article.cfm?articleID=000C74E4-5172-1C74-9B81809EC588EF21&pageNumber=1&catID=2

The ECG circuit seems to be surprisingly simple.

Would it really work -- I wonder.

Any one has tried this circuit? Or any others?

Oli

 

[audioguru2]

Hi Oli,
That ECG circuit is too simple. It is like only part of circuits on the web and in the IC's datasheet.
It doesn't even have a shield on its cable, which is very important to have on a circuit with such a high gain. Most ECG circuits use another opamp to drive the cable's shield. The lowpass filter in that ECG circuit cuts-off frequencies above only about 34Hz, but still a lot of mains hum will get through.
Maybe the article's author used his in a field that was far from electrical interference.

I am curious. Are you going to have a warning alarm circuit, with a missing or too-fast pulse detector?
What are you going to do if there is an alarm, zap the person with a defib unit?
Do you have a defib unit and are you trained to use it? Some of them even "tell" you what to do!

 

[Olihou]

Hi Audioguru

Thank you very much for your advice. You have always been so nice and helpful.

My interest in ECG arises from my curiosity, and most probably due to my childhood desire to become a medical doctor, but that has been only a dream for me.

I just wish to find out if a ECG can reveal one's cardiovascular health. The commercial unit must be very expensive. I just wish to have a cheap and simple unit to show if there is some early indication of changes in the heart condition at home, whenever we want. Undoubtedly I have no intention to have it replacing an actual medical check.

May be the circuitry of a working ECG is very complicated and beyond the capability of an amateur -- a dream for me again. ???

 

[audioguru2]

Hi Oli,
I was worried that you wanted an early indication of an inheiritance!

If you can afford the high price of that instrumentation-amp IC and can obtain one, it would be interesting to experiment with building an ECG. Try the author's circuit and see how much hum is on the output. Then try it with a shielded-to-circuit-ground cable, then with an opamp driving the shield. Try the author's simple LowPass Filter, then a Sallen and Key LPF, then a switched-capacitor IC LPF.
Lotsa fun! Of course you must be wearing a white doctor's gown!

 

[Olihou]

Hi Audioguru

Thanks for your advice. The only thing I do not understand is that why I should be wearing a white doctor's gown. :

I have started searching for the instrumentation amp, but am not successful yet from local suppliers I have checked. I will continue to search for it.

I do need help please!!! Can any one supply more complete circuits/guides for this project, please? ???

Oli

 

[Olihou]

Here is one circuit I have found from :

Has any one tried it out? How good can it perform ?

I would like to have some assurance of the circuit before trying to search for the parts, the op amps in particular, which seem to be unavailable locally.

Will any one please advise.

Oli

View attachment 36093

 

[audioguru2]

Hi Oli,
You don't need to use those particular IC's, many manufacturers make inamps and opamps that would be OK.
Where on this planet are you at? Maybe I can direct you to a good international supplier.

On another electronics web-forum, a guy is having trouble with exactly the same circuit that you recently posted. He doesn't understand the concept of proper ground reference and shielding so has added a poor low-pass-filter and a notch filter to reduce hum from the mains.
In another post he asks why his poor filter and notch don't work, and I came up with the idea of replacing them with a simple 8-pole switched capacitor low-pass-filter IC. Another "geek" simulated it and reports that it should work really well. The forums are here:
http://www.electro-tech-online.com/viewtopic.php?t=11735

http://www.electro-tech-online.com/viewtopic.php?t=11936

 

[Olihou]

Dear Audioguru

I am in a small place south of China, ruled by British for one hundred years until 1997 when it is returned to China. Got it?

I have been searching for the inamps. They are available locally only in large quantities! So I am still thinking if I can use opamps readily available, as you have suggested. However I find opamps, such as TL074, LM741 or LM709 are normally of low CMRR, around 20 to 30 DB lower than the inamps. Besides, to build inamps from these opamps, high matching resistors (specified to .1% in circuits I have seen) are needed. No easy job to me!

Or you can advise me where to get the inamps. I will be prepared to buy just two to three pieces, as they seem to be very expensive.

One more question for your help please. For the INA118 circuit I have posted previously, I do not quite understand what is the purpose of the amplifier output of which is connected via a 390 Kohm resistor to the right leg. Also why the gain is made to 39 (390k over 10 kohm)? Do you have a good explanation?

The links you have provided to other discussion forum are very useful to me.

Thanks and best regards

Oli

from Hong Kong

 

[trigger]

Hey Oilhou,

I am same as you, come from Hong Kong. ;D
There are some places where you can buy components, like in Sham Shui Po Ap liu Street, IC master in ShanPoKwong, Leader in Kwun Tong, etc.
If you can't get required components, you can try local RS or Fernell, there you should get some..... but expensive.
Or you can go to Shenzhen for components..... but less reliable...

Hope this can help.

Dear Audioguru

I am in a small place south of China, ruled by British for one hundred years until 1997 when it is returned to China. Got it?

I have been searching for the inamps. They are available locally only in large quantities! So I am still thinking if I can use opamps readily available, as you have suggested. However I find opamps, such as TL074, LM741 or LM709 are normally of low CMRR, around 20 to 30 DB lower than the inamps. Besides, to build inamps from these opamps, high matching resistors (specified to .1% in circuits I have seen) are needed. No easy job to me!

Or you can advise me where to get the inamps. I will be prepared to buy just two to three pieces, as they seem to be very expensive.

One more question for your help please. For the INA118 circuit I have posted previously, I do not quite understand what is the purpose of the amplifier output of which is connected via a 390 Kohm resistor to the right leg. Also why the gain is made to 39 (390k over 10 kohm)? Do you have a good explanation?

The links you have provided to other discussion forum are very useful to me.

Thanks and best regards

Oli

from Hong Kong

 

[audioguru2]

Hi Oli,
Greetings to Hong Kong. Your English is excellent!
I buy parts on the web from the local division of Farnell and they guarantee next day delivery. Millions of parts at pretty good prices. At www.farnell.com I can see Hong Kong's flag! Try them.

Notice that the amplified signal is fed to the right leg by a 390K resistor, which is probably part af a voltage-divider using the body's resistance as part of its attenuation.

 

[Olihou]

Thanks Trigger and Audioguru !

With your advice I have now been able to locate the suppliers locally.

The AD624AD cost $38. The INA118PB costs $10. Surprisingly INA118PB seems to have better specifications! I will certainly use INA118PB!

I just wonder if AD624 is something out of production? -- scarcity causes expensiveness ....

Oli

 

[audioguru2]

Hi Oli,
I would just use a single cheap, common and low-noise TL072 dual opamp to replace both of those rare expensive ones.

 

[trigger]

I think the quoted price of those chips are in HK dollor, right?
If yes, very good price.....
Oli, can you tell me where you get them? Local stores?
Thanks.

 

[Olihou]

They prices are in US$, and are quoted by Farnell in HK$297 and HK$79 respectively. It is expensive !

I think I will take Audioguru's advice and try the TL072 or other popular opamps first. However I can foresee that much circuit skill would be needed to get it perform, in say achieving good CMRR values. I have to do further searching for good circuit designs before switching on my soldering iron ....

Cheers,

Oli

 

[Olihou]

They prices are in US$, and are quoted by Farnell in HK$297 and HK$79 respectively. It is expensive !

I think I will take Audioguru's advice and try the TL072 or other popular opamps first. However I can foresee that much circuit skill would be needed to get it perform, in say achieving good CMRR values. I have to do further searching for good circuit designs before switching on my soldering iron ....

Cheers,

Oli

 

[Kevin Weddle]

It is my opinion that in real ECG monitors, the filtering is suited to filter out the part of the ECG that is not wanted.

 

[audioguru2]

Kevin,
What is not wanted in an ECG is hum pickup and its harmonics from nearby mains wiring.
The patient's body acts like a high impedance antenna and the wanted signal is only microvolts. A very high gain is required to make the signal useable. This high gain also amplifies hum. Hum is reduced by using the following methods:
1) Using a differential Instrumentation Amplifier IC instead of ordinary opamps and resistors. The inamp's built-in tightly matched resistors allows it to have superb common-mode rejection (the same signal that is fed to both inputs is cancelled).
2) Using a complicated cable shield that has a bootstrap signal applied and part of that signal is fed to the body as a reference.
3) Any hum that remains that could be caused by having one probe closer to a hum field than the other probe, is lowpass filtered out.