A friend and I joined forces to build a simple ECG (or EKG) Amplifier for our Signal Processing class at University of Applied Sciences Regensburg. We searched the web, magazines and books for suitable circuit diagrams and instructions on how to build one. We found that the instrumentation amplifier INA121 (datasheet) was a suitable IC and also the precision instrumentation amplifier AD624 (datasheet) mentioned in this Scientific American article seem to be viable option. However as it turned out those ICs are very hard to come by and rather expensive ranging from about $10 to over $50.
We also found a quite professional solution in Funkamateur, edition 12/93, pages 794-796. The article is in German and can be found here. We found another very simple circuit diagram for an amplifier in Elektor, edition 7-8/2000 that we ended up building. Here’s the diagram:
The components cost about 10 Euro. We built it on two breadboards one for the amplifier itself and one for the power supply. With two 9V batteries connected it supplies -9V and +9V. Here’s a picture of the amplifier board:
Green red and black cables are connected to the power supply, the thick black cables are connected to the subject and at the bottom there are two pins to connect the oscilloscope probe or an AD-converter.
We used real ECG pads from the hospital to collect the signal from the subject. At first we used regular unshielded copper cables to connect the electrodes which resulted in a lot of noise. We then replaced the copper cables with shielded thin microphone cables and connected the shield to ground. This hugely reduced the signal noise. Here’s an image showing an EKG taken with our amplifier:
The electrodes are connected to right hand, left hand and right foot. As you can see it’s pretty much what you would expect. You can see the P-wave, the QRS-complex and the T-wave. We’re quite happy with the results.
If you try to build something like this yourself please be advised that you should be careful when connecting this amplifier to anything that carries high voltage (line voltage). The amplification circuit and measuring devices are not separated from each other. So you should only use battery powered devices like a USB oscilloscopes connected to a laptop running on battery. This is important as you consciously connect the subject to your circuit in a very well conducting manner (because that’s what you want) and at very dangerous points. So you should take care that no high voltages can flow between the electrodes at any time.