Two-stage EKG

In EMG and EKG works, I described an EMG and EKG that worked, but I was not completely happy with.  The output was too small and too noisy to feed into an Arduino for data logging.

For this post, I used the extra op amp in the MCP6002 package to add another stage of amplification.

Circuit for 2-stage amplification of EKG signal. All capacitors are 4.7µF.

Because the output of the instrumentation amp was not centered on Vref (maybe because of a slight DC difference in the two electrodes), I had to AC couple the output of the instrumentation amp to the op amp, using C3 and R7 as a high-pass filter with time constant 155 msec.  The op amp provides an additional gain of 5.76, then the final low-pas filter removes extraneous signals, with a time constant of 18.3 msec.

The trace on the oscilloscope shows peaks of about 0.6v and Vout seems to be well centered on the Vref voltage.

I had a hard time seeing the pulse wave form on the scope, since both the phosphor and my retina fade faster than the signal evolves.  I took some long exposures (over 3 seconds) to try to get a full pulse on the screen.

Two pulses (triggering on the first one, so a little of it is missing). The big R spike goes up to about 0.5 V, and the subsequent T to about 0.1v.  The “Q” and “S” parts of the waveform (on either side of the R spike) are hard to make out from the noise.  The time scale is 0.2 seconds per division, so the two R spikes are a little more than a second apart.

I think that these waveforms are now clean enough and large enough to try feeding the into my son’s data logger.  I think that will be tomorrow’s project.