Thu 14 Jun 2007
First prototype of right-leg-driven electrocardiogram recording
Posted by nico under Amplifier , Circuits , Signal
The basic idea with this design is that a three electrode system is used, two electrodes for the ECG and one for grounding the person being recorded from. The common mode signal from the two recording electrodes is inverted and used to drive the ground electrode with the hope of trying to mitigate the corrupting common mode signal. This grounding is typically applied far from the recording site of interest, often the right leg.
For the design, I constructed three copper coated electrodes from U.S.A. pennies and coated them with conductive paste borrowed from the electroencephalogram area. The two recording electrodes were positioned over the left portion of the chest and the right side of the torso and fed into the inputs of an INA116 (three 0p-amp instrumentation amplifier). To get the common mode signal, I split the gain resistor of the INA116 (Rg) into two equal resistors and fed the center tap voltage into a buffer. The output of that was then fed into an inverting amplifier and connected to the third reference electrode. The optimal gain of this electrode is yet to be determined, but with a few experiments, I was able to get a pretty clean signal. The final step in the equation is to do some band-pass filtering. I am thinking of doing something along the lines of 0.5 to 30Hz.
The preliminary data is shown both on the scope and verified with an optical pulse-oxymetry unit. The shape of the ECG can be distorted due to improper electrode placement and the resting heart rate is a little high for my norm, possibly due to excessive straining to get the pictures without shaking the camera. On a final note, the safety of this system is yet to be fully evaluated. The power supply is grounded to mains ground and the drive voltage of the driven reference electrode cannot exceed 5V. I am thinking of adding a series resistor and some Zener diodes to make it more safe, but that will be included in the final design. I plan to play around with this idea for a week or two and then post full schematics and possibly a data set of my own ECG when it becomes available. As always, comments are very welcome.
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June 14th, 2007 at 9:17 am
Can you make this a 12-lead ECG? That would be cool. You gotta show me that setup sometime today.
Also, the ECG you get depends a lot on which lead you look at. You gotta use all 12 leads to get a complete picture since you could be missing things if you just look at two or so leads.
June 14th, 2007 at 9:42 am
I can but it will be much more work. I am interested in recording heart rate information and not really the shape of the waveform. It may be possible to get a few more electrodes in without doing too much more work but I can save that project for later.