Gas station without pumps

2017 December 18

EKG without amplifier

Filed under: Circuits course,Data acquisition — gasstationwithoutpumps @ 18:24
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I have done a lot of EKG projects on this blog, mostly for the Applied Electronics course, where an EKG amplifier is Lab 12, but some just for fun. Today I decided to see whether one could do an EKG with just the Teensy LC board or just an Analog Discovery 2 USB oscilloscope.

The project was partly inspired by the Digilent post DIY ECG using a Analog Discovery 2 and LabVIEW, which I saw the title for and assumed that they were using just the USB oscilloscope.  It turns out that wasn’t what they meant—it is just a simple 6-op-amp EKG amplifier looked at with the USB oscilloscope.

The setup I tried was about as simple as you can get—I put on three electrodes wired up as Lead I (see earlier post for this configuration), connected the body electrode to ground, and the other two electrodes to the plus and minus leads of channel 1 of the Analog Discovery 2.

The signal I got was quite small (about 1mV) and buried in 60Hz noise:

The raw signal from the oscilloscope, sampling at 1200 Hz, shows some spiking from the pulse, but a lot of noise. The big R spike is only about 3 LSB, so quantization noise is a problem.

I ran the recording through a digital filter to bandpass filter it to 0.1Hz–100Hz and put in a notch from 58Hz to 62Hz. The 100Hz low-pass had the effect of averaging out the noise, producing a signal with much finer resolution than the raw ADC values:

After filtering, the EKG signal is fairly clear. I don’t recommend trying to use only a couple of the lower-order bits of an ADC, but it is surprising how much information can be recovered by the filter.

I also tried using a Teensy LC board running PteroDAQ, using the A10–A11 differential channel. I had to bias my body between 0V and 3.3V, so I used a pair of 120kΩ resistors (one to GND, one to 3.3V) to connect to the body electrode.

Once again the raw signal was not great:

The signal had less noise than the signal to the Analog Discovery 2, but the signal was smaller also, negating the value of the finer steps of the ADC on the Teensy LC.

Once again, digital filtering restored the signal:

The signal-to-noise ratio here looks a little worse than for the Analog Discovery 2, despite the raw signal looking cleaner.

I managed to get a cleaner signal for the Analog Discovery 2 by turning off the surge protector, so that there was no 60Hz current anywhere nearby. The results after filtering were no better (and possibly worse) than from the signal with the 60Hz interference, so I did not bother plotting them for this post.

My conclusion is that it is possible to get EKG signals without adding an amplifier, but you can only see the signal clearly if you do some filtering.  I’ll have to decide whether to recommend to students that they record signals directly from the EKG electrodes to get an idea what their amplifiers have to work with.

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