Thanks Dave for taking the time to review my ECG project. From the great feedback you provided I want to give my thought-process on the following:
Pin headers on the development board
I wanted to make it so someone can also have the option of hooking up jumpers from the breakout board to a breadboard in case they want to feed the signal into more analog circuitry (additional filtering) or into a micro-controller development board such as Arduino.
Pin headers unlabeled on the other side
I understand the confusion this presents and actually spent some time thinking how to label the other side as the title engraving then take up too much space. So far the best I could come up with is to either place shortened labels on the other side or just remove the title engraving completely thus freeing up space to put the full labels.
Protection Resistors
Very good for taking due diligence on making sure the device won't kill or hurt you.
I was going back and forth in my head on whether or not you were going to hook yourself up to it. Besides the 10K
resistors on the breakout board, the inputs and the right leg drive also have 100k
resistors in series to the outside world as well. Adding resistance to the sensing input and right leg drive output reduces the CMRR of the module and adds offset (due to mismatch between resistors) but for piece of mind it is a good requirement. If defibrillation/synchronized cardioversion is a possibility then it is a
definite requirement as an ECG without input resistance (of at least 10k
) can shunt most of the energy away from the heart and thereby making the act of defibrillating a patient useless.
Input Jacks
It has been a bit difficult to source the right jacks for this application as it's a rather niche product. These are indeed standard for certain bioelectrical monitoring applications such as ECGs and EEGs. They are referred to as 1.5mm safety DINs. One source of 90 degree jacks is
here but I found this much later after I sent you the board. Like all things medically related, they are pricey as well. The reason for the cutout was I was trying to use the rings and PCB sort of as a brace for when the user would be pushing and pulling the electrode plugs in/out of the jacks. I never thought about what would happen if the user applied stress orthogonal to the intended insertion direction. Good catch. Will have to think about some sort of brace or just use the 90 degree jacks instead.
LED indicator
I was thinking about incorporating that as well when designing the board but decided against it at the time since I wanted to make it easy to measure the current consumption of just the device itself. I think I will add LED indicator regardless and just have a jumper link to disable it if someone just wants to measure the current consumption of the module. This would then let me add a LED ON indicator and LED heart beat indicator using a simple comparator circuit with hysteresis. I could probably add reverse-polarity protection as well.
Nudity
Colored Input Jacks
The colors on the input jacks follow the American Heart Association color coding for Lead 2 configuration. Of course one can easily reconfigure the module to read Lead 1 or Lead 3 by simply relocating the electrodes. I selected lead 2 since it usually provides the best signal for viewing the electrical activity of the heart. I will add something on the back showing how to hook up the breakout board for lead 2 configuration to mitigate uncertainty.
Interference
This is where most of the innovation lies is in the electrophysiologic world. There are many implementations (mostly digital) out there to try to deal with noise in the form of line interference, respiratory, and muscle. Each one has varying degrees of success but they all suffer from the requirement of limiting filter related distortion. An ECG machine is no good if the filters it uses to eliminate interference results in distorting the waveform to a degree that it causing diagnostic errors either in adding something that is not there or removing something that would be important. It's always a balancing act and most manufacturers rely on the side of preserving signal integrity hence for diagnostic-mode ECGs they require the patient to lay still. For monitoring-mode ECGs, such as Holters, signal distortion is more relaxed and so the filters are better. I designed the ECG module with diagnostic modality in mind. The obvious way to reduce interference caused by muscle would be to decrease the bandwidth of the module but it would start distorting the QRS complex (that spike you see on an ECG signal) as it looks like muscle interference. Even more so, the J-point and accompanying ST-segment would become even more distorted as well since they follow right after the QRS complex and that is dangerous to mess with because the morphology of the ST-segment in relation to other features forms one of the core findings for diagnosing a heart attack*.
*There are lots of other things on just an ECG alone for hinting at a heart attack such as T-wave peaking/inversion/QRS widening/conduction delay resulting in left-bundle branch block to name a few but the ST-segment is classic.
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