biopotentialmeasurementoscilloscope
I was at my doctor's office and noticed an EKG machine that looked like a standard oscilloscope. In fact, its case was almost identical to my dual channel Tektonix 1104b. The main difference was that it had more probes, but I didn't check if they connected to different channels.
Can I, for instance, use my a scope as an EKG? If so, would it need any modifications?
Well, it wasn't a bad scope or loose wiring or a floating ground... it was the PS/2 cable I used to connect the keyboard connector to the rest of the equipment! It's a case of not knowing your salvaged equipment very well, along with some buried surprises in the cabling itself.
Thanks for all the discussion and suggestions, though it was ultimately a non-member who caught the similarity to diode behavior and made me suspect something unexpected was actually present within this cabling.
I had an old cable from a CueCat. There's some interesting teardown info here, though that's not exactly what I was trying to do (I don't even have the 'Cat anymore... just the wiring. Figured someday there'd be a website I could post to after I screwed up while using it. :-)
Thinking incorrectly that these connectors were wired in parallel, I diligently tested the bare wiring what once connected to the 'Cat against the male plug (it was handy.. had I had ANY suspicion I'd have tested the correct connector and none of this would've happened. Assumptions...)
I accounted for the correct pin numbering and got everything wired just right based on that... except neither DATA nor CLK actually feed through to the female connector (I just figured the two extra wires were unused since they didn't probe to anything). So Vcc and Gnd were fine everywhere, but the actual DATA and CLK from the keyboard were not wired to anything!
So, by rights I should've gotten nothing useful at all on the output side, since those wires weren't even connected (they were snipped short and a little staggered. However, I clearly got two non-trivial and distinct (but greatly attenuated) signals, which I diligently read and which people here tried in vain to understand. It dawned on me they might indicate a reverse-biased diode was doing the attenuation, but I'm not sure (I actually did throw a diode into the mix after it was working and it had a similar effect). I think there's some circuitry in the male connector (which has two wire bundles, one from the female PS/2 plug and one from the CueCat side). If it were simple crosstalk I'd expect the signals I saw to be even smaller, and for these signals to be jumbled together, but they were VERY distinct (had they simply been dead I would've eventually realized I needed to test the female plug and would have found my error, but the apparent presence of protection diodes in that intermediate plug threw everything off. I'll post a followup if I manage to nail that detail down - I may even cut open the connector to do it.
Edit: I just cut it open... Vcc and Gnd were indeed bonded to all three targets as expected, while DATA and CLK were run over distinct lines from the female and male connectors to the endpoint. However, NO active or passive components were present, debunking that theory! It must've been just enough cross-talk. Given how they were crammed into the connector it's possible they were juuuuust close enough to their respective wires to bleed only their signals, though it's weird that I was getting attenuated though distinct clock and data signals despite the fact their wires were terminating on unused ping and their wires weren't even routed to the keyboard (that part of the cord didn't even have these two wires I was reading!) Bizarre...
I posted this thinking it was a very simple cable... clearly it was not and I used the wrong lines but this is a good example of what can happen when there are active components hidden in an apparently innocuous connector or cable (if anyone remembers APC's serial cables, they had hidden surprises in the form of special resistors to keep you from just using any old serial cable for communicating with your UPS... and the old iPod cables did that as well. Proprietary Cabling 101!) That's why the signals I saw rang a bell. Hopefully this helps others who are troubleshooting weird results with an apparently normal cable assembly.
tl;dr - Always keep the possibility of hidden components in mind when using salvaged connectors (especially if you get bizarre but strangely repeatable and well-isolated signals), and test the pins you'll actually be using, not just the ones you think are wired through!
Here's the output I got in the end. It was identical whether the keyboard was plugged into the microcontroller, with or without pull-up resistors, or even without anything at all (just straight into the scope). It should be a nice 5V p-p signal straight out... I was wrong about these open-collector outputs having any bearing on the signal strength or shape. They were just red herrings thanks to whatever is in that splitter jack...
I that think you need to add a driven right leg circuit at the first for patient safety and protection to bypass the excessive current in case of contact with live wire. Also for proper reproduction of waveforms on PC make sure to isolate your analog ground and your controller ground.
Also for Ecg, gain of around 10000 is enough.You can split this gain between two stages to avoid saturation and add a 60hz notch filter between the two.
Eg. stage1 amplifier(Instrumentation amp)----notch filter------stage2 amplifier.
Best Answer
Absolutely, yes. But you'll need a frontend.
An EKG is usually just an instrumentation amplifier or a multistage amplifier with as many dB of common mode rejection as you can get.
The problem is that people are bags of electrolyte-rich fluids and meat with that right combination of resistance on the outside (skin) and relatively lower impedance to AC signals that all sorts of radiated EMI couples into our skin all the time.
If you've ever accidentally touched the tip of a speaker jack with your finger, or heck, just try probing your finger with your oscilloscope (isn't that the first thing one does with a new scope? I know it is for me!), you'll see what I mean. There is probably a volt or 2 of 50Hz or 60Hz ripple just from the wiring in your house.
This presents a problem when trying to detect electrical signals originating deep inside the skin-enclosed meat sack with all that noise coupling right at the surface trying to drown it out.
Where that noisy garbage coupling into your skin is on the order of a volt or two, the signals from your heart (at the point of detection on your skin at least) are about 1mV peak to peak. To get a good waveform, you'll want to be able to clearly see peaks as low as 20µV.
So you can't use an oscilloscope unaided, you can certainly use one as a very effective ECG/EKG with a front-end.
Such a front-end is not particularly difficult to make. The bandwidth and frequency of signals from the human heart are... quite slow in the context of analog electronics. This makes a frontend particularly forgiving, and you can even build one on a breadboard with just one IC.
Essentially, it all boils down to two things: You need a lot of gain, and a lot of common mode rejection.
Common mode rejection is achieved by, at the simplest, coupling the op amp's ground reference to your own body through a low (but not too low) resistor, like 100Ω. This is the lead that tends to connect far away from your chest, like at the angle or leg. This ensures only the noise gets picked up and rejected as common mode, leaving the cardiac signal (which are far too weak to matter that far away in your ankle or where ever).
A higher performance way of achieving this is to actually use a second op amp to drive the ground reference (your skin) and actively cancel out most of the common mode noise.
If you search for 'ECG frontend circuit', you'll find quite a few complete schematics of varying simplicity or complexity.
Any of them can work, but it ultimately depends on what level of performance is acceptable.
I have personally made this one, and a different similar one that also used a right leg driver that I can't seem to find, and both exceeded my expectations. They worked quite well:
Safety Note
Do not attach anything connected to your body like this to an oscilloscope that plugs directly into the wall. You must use an isolation transformer to fully isolate the oscilloscope so no ground path can be taken through you (and probably the electrical conduction system of your heart). Real ECGs are fully isolated if they plug into the wall at all.
Final note: Your body doesn't run on electric currents.
It runs on ionic current. Electric current has electrons as the charge carriers making up that current, but inside your body, it is positive ions rather than electrons that flow and do things like make your muscles move.
For that reason, you need to use some sort of electrolyte between an electrode and your skin, forming a half cell and allowing the ionic currents to be converted into electric currents for use with your ECG frontend.
Buying proper medical electrodes is ideal, but I can personally attest that tinfoil with some shampoo smeared on it can work in a pinch if you are really impatient to test what you made. Your results may vary.
Use an isolation transformer and don't get hurt or killed accidentally. Beyond that, this is actually a great project that isn't too hard or expensive to build if you're sufficiently interested and motivated. Good luck!