Hi and thanks for a nice project. I cloned the git project but the apk file constatnly frose. I managed to get the app from playstore, run it and get data. I also liked to try it on windows but was not able to compile the ecg_monitor project. I have installed mingw so I have gcc, but make seems to be called mingw32-make. Running mingw32-make Makefile gives errors and no exe file. Also tried several imports to Eclipse without luck. Could you please explain how to compile the windows application? (Im not fluent in gcc and make).

Hi! Thanks :)
Not sure why you can't get android app to work - it seems that github has the latest version, possibly some problems with permissions? If you won't be able solve it in a simple way, please send more detailed reports of what exact errors your are getting (and what happens in android studio if you run it in debug mode), we'll try to figure it out.

As for PC program - please don't try to get ecg_monitor running, it is outdated and I'm not sure if we'll continue that branch anytime soon. As of now, our best solution is using node.js version (also on github) - and if you want these data in another app, you can get them via POST requests, browser interface gets data this way (you can look into html code, functions poll_history_data and poll_ecg_data process requests, and call corresponding data parsers - it should be reasonably doable to convert it to other languages. Or alternatively you can look into server.js and use parsing function from it).

Please keep in mind that we will introduce support of multiple devices soon (most likely this year, possibly even this month) and thus requests will use some kind of device ID in them. We'll try to get backward compatibility though

Thanks for your reply.  I am focusing on the PC side now. Looks like I need to investigate the node.js solution, I'm unfamiliar with it. In the meantime I can see on a serial monitor that data is received on the PC side. I would like to interpret them as I am experienced in serial communication and data processing. In a comment you made January 17th you touch on the format. Is there a more precise description of the format or could you explain it?

If you want to implement serial parser, then it can be done in the following way:

USB receiver sends all packets it gets from radio as virtual COM port data at speed 921600. Packet length can be different in different modes (by default uECG sends BLE-compatible packets and receiver ignores them if not told explicitly to switch into BLE mode - this part isn't working well right now so you won't find code for this mode, although I think it will be there in the next update), with single button press it switches in direct radio protocol with 64 data bytes per packet - a few more header bytes are sent via COM port - and with second button press it switches into 32 bytes per packet, third one returns it into BLE mode).

For direct radio mode, packet structure is the following:
1. first two bytes have fixed values (79, 213) and are used to sync with data stream (we don't know at which point we start listening, and it's possible that some packets will be corrupted, but normally once you've synced with the stream, you know packet length when you parse it)
2. Starting from 3rd byte packet header starts (1 byte per each value): RSSI value, payload length, packet ID (if you are parsing data before whole payload is received, you need to wait - checksum is the last byte of the message)
3. after that, please check function parse_direct_packet_uecg from server.js - while the whole idea is the same for all packets - 4 bytes for unit ID, 1 byte for data points count, 4 bytes for less-realtime parameters (like steps count, battery, accelerometer etc - each packet has some of them, but not all together, first byte indicates which ones are present and this number cycles), and then some bytes for ECG data and checksum - but there are a several variations, I hope the code of this function is self explanatory enough. If you are in doubt what some values mean, I'm ready to explain them in detail.
 - important note: at the moment, each packet is sent several times so if you already got packet with given ID at the previous iteration, current one is duplicate and should be ignored. And it's not a rare case when all packets with given ID are lost, sometimes several in a row, so when interpreting data you want to indicate that one or more time points are missing - in that function, all lost points are filled with the first value from the new packet (you can see it in pack_dist > 1 logic branch).

Right now I'm working on a new radio protocol and packet format will be somewhat different - but I'll try to make as little changes as possible

Thank you so much for your reply. I think this is good enough information,  to start with at least. One more question. Why is there a base station with PC? Can PC's not receive Bluetooth directly? 

In BLE mode, device sends much less data (122 Hz ECG vs 976 Hz ECG in direct radio mode), and even for that rate we are on the edge of violating BLE standard - possibly some devices won't accept this. In part it is because of BLE advertisement mode we use (we didn't implement direct BLE connection yet).

In direct radio mode, we use non-BLE protocol so PC can't receive it. Instead, 32-byte mode of direct radio is compatible with popular nRF24 module for Arduino, and thus can be used for projects not involving PC (I guess it is hardly usable right now due to poor documentation and lack of examples, but one day it will be there)

Thanks for this wonderful effort .. I hope to receive a piece for me, how can I get it .. Thank you again

Hi Ahmed! We actually saw your email first, and answered there, and only now saw your comment - sorry about that! Anyway, you can get uECG from our Tindie store: https://www.tindie.com/products/18040/ - we also sent this link in our email, plus some info on shipping options.

Thanks for getting in touch, and hope you're staying safe!

Got mine yesterday! Woohoo!

...and then the app crashes. Repeatedly. So often the One UI wants to put it into deep sleep. Android 10 (Kernel 4.14.113-18615768) will only run the app once, from a cold boot. Even then if I pause or stop recording -> app crashes....Not sure if the reporting it wants to do pipes back to Google or you...

Ideas?

Edit: It's a Samsung A51

Hi!
We are rewriting the app right now :) Android 10 handles things quite differently, although we had even current version working there (it won't survive turning the screen off, so not very useful, but still it works until then).

If it doesn't work at all - then please manually give location permission to the app, that is most likely the reason - it should have asked for it, but that doesn't work

G'day! I did go through and make sure it has all permissions (storage & location). The storage works b/c the /Documents folder is full of .csv files from when it works from the cold boot. I agree A10 handles things very differently - for me it's really different stepping up from a phone running pie. Also - an old moto e2 running A6 does the very same thing. Cheers!

Updated app is in the play store - please check (it might show an old version though, try to reload play app if you don't see a new one)

Sorry for the delay - Updated the app tonight (here, anyway) and works great so far. (edit) too fast. The UI mgr complained after about 12 minutes that the app keeps stopping [Process Name: com.ultimaterobotics.uecgmonitor4_2]

Crash log:

Exception class name:

java.lang.illegalStateException

Source file: Surface.java

Source class: android.view.Surface

Source method: checkNotReleasedLocked

Line Number: 662

I have captured the stack trace and send it to you via e-mail.

Cheers!

Thanks for the report! There was really a reckless behavior in my code - I was drawing on the surface not checking if it's already created (which happens all the time when you switch tasks) - so if OS wasn't fast enough to create it before I first access it, the app crashed. Now fixed - should be live in play store sometime during next 3-12 hours (reload play app if it's been quite a while and it still doesn't show updated version)

Hi, I'm getting stuck with flashing the firmware. After many attempts I finally managed to get openocd to connect (I think in the beginning I was too careful with the pogo pins), but I get stuck on the second telnet command. When I try to execute "nrf5 mass_erase" it complains that "nrf5" is an invalid command name. Just in case it is of any use, I'm using Ubuntu 18.04 with openocd from the package manager and below is the terminal output for both commands:

$ openocd -f interface/stlink-v2.cfg -f target/nrf52.cfg
Open On-Chip Debugger 0.10.0
Licensed under GNU GPL v2
For bug reports, read
        http://openocd.org/doc/doxygen/bugs.html
Info : auto-selecting first available session transport "hla_swd". To override use 'transport select <transport>'.
Info : The selected transport took over low-level target control. The results might differ compared to plain JTAG/SWD
adapter speed: 10000 kHz
Info : Unable to match requested speed 10000 kHz, using 4000 kHz
Info : Unable to match requested speed 10000 kHz, using 4000 kHz
Info : clock speed 4000 kHz
Info : STLINK v2 JTAG v29 API v2 SWIM v7 VID 0x0483 PID 0x3748
Info : using stlink api v2
Info : Target voltage: 3.155039
Info : nrf52.cpu: hardware has 6 breakpoints, 4 watchpoints
Info : accepting 'telnet' connection on tcp/4444
target halted due to debug-request, current mode: Handler External Interrupt(6)
xPSR: 0x01000016 pc: 0x000074f6 msp: 0x2000fe80
invalid command name "nrf5"

$ telnet 127.0.0.1 4444
Trying 127.0.0.1...
Connected to 127.0.0.1.
Escape character is '^]'.
Open On-Chip Debugger
> halt
target halted due to debug-request, current mode: Handler External Interrupt(6)
xPSR: 0x01000016 pc: 0x000074f6 msp: 0x2000fe80
> nrf5 mass_erase
invalid command name "nrf5"

It could be a bit different version of openocd, and instead of nrf5 you should use nrf52 command prefix.
Also it probably would work even without mass_erase command at all (flash write command overwrites content) but it's safer after mass_erase

I'm having an issue with the firmware update; the update process itself seemed to go through without issues, but after resetting and disconnecting the device, it doesn't seem to turn on anymore. Anything I could do to reverse this?

Sorry, missed your comment! If you have uploaded the correct file and it went well, it should work - but it could be that process wasn't actually finished. Have you tried to repeat it?

The incorporation of stlink support as described in your response to the questions below seems to have worked.

But returning to the instructions for updating the firmware, things move only a half step ahead. In response to openocd -f interface/stlink.cfg -f target/nrf52.cfg

I get:

Info : auto-selecting first available session transport "hla_swd". To override use 'transport select <transport>'.
Info : The selected transport took over low-level target control. The results might differ compared to plain JTAG/SWD
Info : Listening on port 6666 for tcl connections
Info : Listening on port 4444 for telnet connections
Info : clock speed 1000 kHz
Info : STLINK V2J29S7 (API v2) VID:PID 0483:3748
Info : Target voltage: 3.176608
Error: init mode failed (unable to connect to the target)

If all wires are connected the right way - then it's a problem of contact. Try moving pogo pins a bit and run openocd command repeatedly (pressing "up" key repeats last console command so you don't need to type it again) - it's perfectly normal to get connection only after 10+ attempts, especially before you'll find the right way to position them. But if within 20 tries you still can't connect - check wires (and quality of their contact)

lots of tries later...

A few observations,

the USB controller has two rows of connections, attaching to the upper (nearer the printed side) I consistently received the above error message,

Moving all the jumpers to the lower row,  it did not error and started listening on port 3333 first try. Also, the led on the programmer went from solid red to alternating red and blue

I suggest moving the instruction to open a new terminal to its own line and putting it in bold text (I missed it a couple of passes...)

It might be fine to simply detach the connection and close the terminals, but maybe indicate "exit" on the telnet session and ctrl+C (?) for the openocd session to close out nicely.

for me - working connection to android phone and decent trace displayed

Interesting! I never realized there is a difference in programmer rows - but somehow always connected them to the lower one, just randomly did it the first time and always followed the same pattern. Maybe there actually is.

Ok, will make new terminal part more visible ))

Exit on telnet isn't really needed (it auto closes with openocd), although ctrl+c in openocd probably is worth mentioning.

Glad it worked! :)

I'm one of the unlucky ones with an android phone that doesn't see the uecg, nor does BLE scanner.

It seems installation of openocd went fine (on linux), but the command:

openocd -f interface/stlink.cfg -f target/nrf52.cfg
gives me an error that interface/stlink.cfg isn't found.

Where should I get it from and where does it need to go?

Is there something else that needs to be with it?

while I'm here...

Does the white nub / button of plastic sticking out of the back do something?

I would suggest moving the second photo before instruction 1  in the firmware update, and redoing the photos in instruction 3 to show both the programmer end and the uecg end more clearly, and  using a uecg with a white case so the holes are visible. Switching the orientation so green is near the snap connector in one and away in the other is a bit confusing since the other end is connected to the same pin on the programmer.

Instructions for use with the usb basestation?

It seems that your openocd build doesn't have ST-link support :( It was the case for most Linux distributions a couple of years ago, now some has it, some still don't. I'm afraid you'll have to compile it from source code in order to make it work. For that, download openocd version 0.10.0 or higher (master branch should work too, here: https://sourceforge.net/p/openocd/code/ci/master/tree/ ), then inside it run
./bootsrap
./configure --enable-stlink
make
sudo make install

 - that should make it work.

As for the base station, PC program is still in progress but it is kinda usable already. You can download it here: https://github.com/ultimaterobotics/uECG/tree/master/ecg_monitor - then just run make and then you can launch it with
sudo Release/ecg_monitor (sudo might be required for USB access - depends on system config, but by default it typically is needed). App can switch between BLE and direct link modes (should be more or less clear from the interface). Detailed description will be ready a bit later - right now we are trying to compile it for Windows.

White button switches uECG between BLE and direct link mode (and long press switches it into EMG mode, in the PC app you can see the result, but it's far from ready).

Thanks for the feedback on the instruction - will try to make it more clear! Yes, those two photos contradict each other, the best reference is schematic image at the beginning of part 7.

for any other git novices - better to use:

git clone https://git.code.sf.net/p/openocd/code openocd-code

instead of downloading a snapshot (using the convenient button) at: 

https://sourceforge.net/p/openocd/code/ci/master/tree/

as the latter leaves you having to track down a number of uncooperative modules and libraries.

I see you've eliminated one of the confusing photos,

But now the text for the Firmware update is in Russian, which dare I say is even more of an obstacle.

Sorry about that! Apparently one of our team members had auto translate turned on while editing the post. The good news is, we added some improvements while writing it back (didn't have a backup copy) :)

for the base-station on Ubuntu18.0.4 I needed sudo to make the connection work

then open connection and tick the BLE box to get the trace up.

Yes, sudo is required... BLE box is needed for bluetooth mode - without it, it will receive data via direct protocol (for switching uECG into direct protocol mode, press button on it once for a short time. Second press switches it back into BLE)

I haven't found anything on your site showing or explaining where to attach the electrodes to the human body.  Do the electrodes need to attach to the chest to get a reliable signal while exercising?  Is the device ready to use when I receive it?  Do I have to install firmware on the ECG sensor part before I can use it?  Does the device come with the battery and electrode pads.  Do I order replacement pads and batteries from you?  Can I read the received heart rate data from a laptop's USB port and use the data in my own custom program in real-time?  How do I read your data, what format is it in?  Is it just a stream of bytes representing a series of consecutive heart rate readings?  How often are the readings taken and transmitted?

Sorry about that - really didn't have time yet to write anything comprehensive, even though first batch is ready, we couldn't stop improving it firmware-wise, only 2 weeks ago we became satisfied with its state (not fully yet, but it's something we are not ashamed to share now).

It is optimized for standard gel electrodes (10 pcs are supplied with the unit, and almost any readily available ones would fit - we plan to sell them too for convenience, after we establish regular serial production of the units). Also it is possible to use chest strap, but as of now you would need to manually solder adapter connector (we will supply them later too). Electrodes must be on the chest, yes - placing them in different areas would highlight different aspects of the heart activity, and some places work well during exercising, some don't (skin shouldn't move too much in electrode area, or it won't hold well).

Device is ready to use, right now you will get raw data, BPM, HRV stats, skin conductivity (also step count and body position). It is quite useful if you know how to look at it, and of limited use if you don't. We definitely will add a server side for automatic analysis - but that would take months.

Firmware is installed - we supply programmer for further updates and custom builds, but it is fully functional as is (over the air update is not supported yet though). Battery (rechargeable LiPo) is installed, and charging is done via usb. You can replace it with any other lipo cell if you want more capacity (would need to solder 2 wires for that, for much larger capacity - also replace one resistor to speed up charging).

Default pack includes USB base station that receives device data in both BLE and direct connect modes (direct connect has 8 times higher time resolution - might be useful for research). Data includes raw ECG signal, RR interval, BPM and HRV detected by on-board algorithm, skin conductivity, steps counter, device orientation, battery level. Everything is packed as a byte array (and not all fields are present in every packet in order to save bandwidth) - you can take function that unpacks it from our PC tool code and use for your project (if you will be doing that anytime soon - contact me, I'll try to clean it up and write comments).

In BLE mode, ECG data are sent at 122 Hz rate (packet with ~25 meaningful bytes is sent every ~10 milliseconds, so there is a lot of redundancy - in order to increase reliability). In direct connection mode, ECG data are sent at 976 Hz rate (32-byte packet is sent every few milliseconds, so still a lot of redundancy).

We made quite a few improvements since last github code update, soon will upload it there - but right now our main problem is finding money to run a production of the next batch. First batch was only 50 units and most of them are for Indiegogo bakers, in a few days we'll make remaining ones available on Tindie (wanted to make it before Christmas, unfortunately it took much longer than we could anticipate), but it's by far not enough to cover our waitlist.

Thanks for answering all my questions and providing such clear and comprehensive answers.   I want to buy a unit as soon as possible, so I signed up to be notified when they become available to the public.  Will the price remain under a hundred U.S. dollars for the full package?    I've bought so many ECG units that aren't accurate at all, especially when jogging or exercising.  I have high hopes that your device will work better based on the information you've provided.    If it really works, then I'll incorporate it into my product.   I may need some custom software to interface and integrate your data with my software, which is currently running on a laptop.   I can hire people to do that, but I'd prefer to hire your team to do it if that's  something you have the manpower to handle?    By the way, I read somewhere that your company is located in the Ukraine, but your English is better than most Americans.

We intend to keep price the same - in the worst case, if we'll meet more problems with money processing, it might increase like 10% to compensate additional fees, but that's it (it turned out that paypal not only doesn't work with Ukraine as a country, but with Ukrainians as well even if they have a legal entity in Estonia and bank in Finland - finding a way around may cost us).

Well, as far as we tested - the signal is accurate. When jogging, it has motion artifacts of course, quality depends a lot on exact placement and electrodes (that's where difference between electrode manufacturers really shows - with some it practically doesn't work) - but I was observing very clearly how my T wave changed shape under load. P wave might be not clear enough for analysis due to motion.

As for customization - definitely yes. Basically, we funded this whole effort by working as freelancers for other electronics project :) And for a while we'll need to continue to operate this way. Also, one of the reasons to create uECG at all was to make it a starting point for other projects - so we'd really like to see it working for that purpose.

Thanks :) Well, we really are from Ukraine (and live here), but by our team standards, my English is, to say the least, not outstanding )) As freelancers, we use it basically every day, for quite a few years now.

My VISA credit card works world wide, why can't you use that for payment?   And, you can always accept payment by Bitcoin.  

When we sell on Tindie, they transfer payments to us only via paypal (we specifically asked for any other option - they don't support it as of now). Setting up our own online shop is possible and probably we'll do it - but Tindie is a specialized platform for maker-friendly hardware, we'd like to work with them...

Now I understand, it's a PayPal issue between you and Tindie, because there's no other way for the public to purchase from you.  Hopefully, if I use your team for consulting work in the future (to integrate your device with my project), then I can pay you directly and the Tindie/PayPal problem won't make any difference.   

Very interesting project! Do you guys think this could be miniaturized even more and set up to measure a cats ECG continually?

There are so many people having cats with HCM or other cardio-illnesses that would love to have an option for continuous monitoring.

How would you attach it to a cat?

thinking about anything possible like a collor, to something like a harness or something, maybe put this on a collar, and connect to an HRV-Strap? :)

It's a very interesting suggestion! Really not many options are available for pets - and, well, for uECG the only limiting factor is attaching it (size can be made 2 times smaller if we remove these large electrode connectors). I guess some areas of fur should be shaved for this, without it I don't think it could be reliable. Collar could work to some degree - but signal on the neck area might be not detailed enough for diagnostics purpose. Anyway, it is a great idea to consider, thanks :)

happy to help! its not not very un-egoistical, as we have a cat with HCM which I would love to contiouous monitor, but there is not even one product availabe for this. even a device would help with streaming data to a database - even DIY style would be pretty great to learn more about the heart of the pet. shaving wouldnt be a problem either. I even thought about something like implanted electrodes, but this might be way too invasive 0:D

That's quite impressive! Can we attach this device onto the wrist and get optimum results instead of putting it on chest? 

No, it's impossible to read ECG from the wrist (if you are not touching the device with other hand all the time). Normally wrist devices measure PPG - it's very different and has much less diagnostic value

Hi! Both electrical path from USB to skin have 100k resistors on them, so even in the worst case scenario it still would be safe. But for proper operation, uECG never should be connected to USB while in use - only when disconnected from skin for recharging

 this setup may have potential dangers due to the way that the device is not isolated electrically from the USB port.

There is potential for defective switched power supplies(or indeed "normal" switched PSU) to introduce mains power into the PCB & to the  human body under test.

simple test.. take a mac portable in aluminum
plug the mac computer into the charging power supply.

ISOLATE your body completely.
run a DRY finder across the surface of the case in strokes. feel how smooth it feels.
now with your other hand ground your body
if you repeat the experiment you should feel you finger stuttering across the surface of the case.
This is mains leakage thru the psu into the "Grounded" computer case(and by default the USB).

It's also WHY some USB cable "spark"  when the "grounded" case to case is touched to another device
I have seen voltages as high as 197V with reference to "real" ground

There are VERY strict rules relating to medical equipment and this leakage.

Hi!
You are absolutely right about potential danger. However in version 4 (which is going in production) we anticipated this (with help of Marcos Chaparro actually), and electrodes are connected to everything else via 100k resistors - so in the worst possible case current flowing through the body won't exceed 4.4 mA, which is considered safe (unpleasant feeling, but no shock).

In practice though device should never be charged via USB when it's on the body (it introduces huge electrical noise), so it's more of a safety measure.

Cool! What... exactly is an ECG?

ElectroCardioGram :) In other words, heart electrical activity. We record it (send to PC or smartphone), and also detect heartbeats with onboard algorithm, so it can be used for blinking LEDs or something without any PC/phone connection

Hi, I've uploaded the KiCAD to MacroFab and am populating the BOM. I am guessing that SW1 is from Ali[baba,Express], but can't find a good reference. Same question for the micro USB, though I might be able to make a good guess for it by looking at the footprint. Can you provide part numbers for both?

Hi! SW1 is MSS3-V-T/R from Diptronics (also known as SS-1290 or IS-1290A-W), SW2 is just standard 4.5*4.5*3.8 tact switch, but can be found under part number K2-1109SP-A4SA-04, from Korean HROParts. Micro USB is U-F-M5SW-Y-3, also from Korean HROParts. 

These should fit. But maybe there are more popular part numbers for those switches and USB that we don't know about yet. We're just preparing for factory assembly, and previously used these switches and USBs that we bought earlier. 

Does the uECG work with a HRM strap, or does it require special electrodes? If so, are they reusable?

Yes, we checked it several days ago - it gets really good signal. Although it has an opposite type of connector (uECG uses female, and straps have female on them) so we had to solder a male-to-male connector to attach it (simply by soldering two connector parts together, it was surprisingly easy)

There are a lot of op amps that I'd choose before the 8606: ADA4528 is much quieter, but slightly higher cost and current, the MCP6V62 is also quieter in the 0.1 to 10Hz band, cheaper, and much lower in current.  The nice thing about these zero drift amps is that there is no 1/f noise.  You say that you're just using the HF content so the 1/f noise might not be important.

Also pay some attention to the noise generated by resistors.  A 100k series resistor is quite noisy in these applications.

My work is more focused on EEG rather than ECG so I'm more focused on noise.  You might be just fine.

What I notice in projects like this (and inexpensive 'scopes) is the analog section is weak while the micro/rf/software is nicely done.  I know good analog engineers are hard to find, but try.

MCP6V62 looks good, thanks! ADA4528 is way too expensive here.
As for resistor noise - you are right, we forgot about it. Before we added DC shift for measuring skin resistance it was safe to ignore (we didn't have input 100k then and no 10M pull ups), but in the latest revision we added DC shift, and even though in measurements it didn't show any noticeable difference, it really has to be properly estimated.

*although a quick estimate gives about 1.2 uV in our frequency window, while our ADC gives 1.1 uV per meaningful bit, so it doesn't look really worth worrying about

I'm wondering where are GND and GNDA joined together. Inside the ADC chip?

Wow, you've spotted it! :) No, in fact we forgot it in this design version and had to connect via additional wire (it was convenient to connect grounds of analog and digital LDOs, they have relatively large pads for soldering a wire). But it will be fixed in the production version!

A 0 ohm resistor there lets you easily try a choke to see if noise goes down.

Also I see no ESD protection at the input terminals, the OpAmp input is exposed.

Anything else should be fixed? I may give this schematic a try these days

This opamp has in-built ESD protection so it's fine.
We missed GND bug in 2 consequent revisions because surprisingly it doesn't really affect signal - analog LDO keeps AGND at 3.3V distance from battery positive rail, and that is still within the range of ADC. I really can't tell any difference between fixed unit and the one with this bug (but it still is a serious bug, we just got lucky here).
Not sure if choke would make any difference - it seems that nearly all noise is external, digital part takes too little power with too small fluctuations to really affect the signal.

Yeah I figured it would still work, I asked because maybe it was on purpose.

Most chips have some light ESD protection, but its usually not enough. Try zapping that input with an ESD gun, or a stove lighter to prove its enough, because I'm pretty sure it isn't.

You don't need this... but I designed medical grade ECGs a few years ago and they needed to survive a defibrillator discharge -and recover in <5 seconds-. A series resistor + beefy tvs was enough protection and didn't skew the signal ;-)

I think it won't survive stove lighter discharge :) Never thought that it might need to withstand defibrillator - but you are right, something as simple as 100k resistor and TVS would do just fine (and we have resistor on one side already), so probably we should add it

Another tip: missing bypass caps on both supply pins of the ADC ;-)

Accel should have a bypass as well.

A fuse right at the battery connection is usually both wanted and required.

In schematics you can see decoupling capacitor only on analog supply of ADC, but in fact on PCB it shares digital supply decoupling cap with one of NRF's. IMU also has decoupling cap placed next to its supply (it's not specifically outlined in schematics, but I kept placement in mind during PCB design). Although analog supply cap is too far from the pin, I really should change that (it still works, frequencies are relatively low there, but it isn't optimal).

Adding a fuse is a good idea, thanks! The main problem is possibility of accidental contact of battery wires with random PCB pads during soldering process (and fuse would save only power chips in such case, sensitive analog ones won't survive anyway), but I don't think it could be improved.

I have been looking for those kind of connectors for grounding. Do you have a supplier/Part Number

Oh, these are indeed hard to find, we've been trying to find them under various names until we guessed the correct keyword :) We bought ours on taobao, shipped to us with Meest-China (it's a freight forwarder). Here: https://item.taobao.com/item.htm?spm=a1z09.2.0.0.21012e8dG3MeYB&id=563032312890&_u=b3i3ohmv647f

They're called "ECG snap connectors", the Chinese keyword is 心电扣. I believe I also saw them on aliexpress, but it's often faster (not to mention cheaper) to buy them in China directly and forward after that. Also, there is more choice, so for example I could buy our snap connectors from manufacturer directly. To find difficult stuff you can google by various keywords in Google images and then do an image search by uploading most prominent pictures.
Also here's some snap connectors on aliexpress: https://www.aliexpress.com/i/32852040439.html
(but they cost $24 here, while I bought the same amount for $5.5 on taobao)

Thanks! We are getting it ready for production, hope to start crowdfunding really soon (we almost finished development, now the main goal is to get some visibility so we'll have enough bakers to produce the first batch)

Interesting.   Where are you crowdfunding and how much are you trying to raise?

I really hate it when people comment negatively on my projects, but I really love it that they are interested enough to make a comment. My background includes design of medical equipment in the olden days: 1977-1983. I took a look at your schematic and have a couple of questions, or comments for improvement.

1. It doesn't appear that you need to use a unity gain amplifier (the AC8606) to buffer the ADC. U1B is biased near GND and might have problems tracking signals below GND. U1A doesn't do anything at all, except draw current via the 10Ω load of R3.

2. The MCP3911 has a PGA that should eliminate the need for the AD8606. If you biased the CH0- input a few mV above GNDA and AC-coupled CH0+ it should be all that is needed. Yes, there is some uncertainty as to the input impedance of the MCP3911, but that doesn't worry you with regards to the time constant of C17. which uses that impedance.

3. The need for the onto-isolator U7 escapes me. Please elaborate.

4. The ADC doesn't include a 50-60Hz notch filter. Many other ∑-∆ converters include a 50-60 Hz notch. A notch would make the subsequent signal processing easier.

5. D2 is a mystery to me. Please explain.

Overall I'm impressed with your progress. I like it a lot.

Hi! No problems with negative comments - but yours aren't negative at all :) Quite the contrary, thanks for taking a deep look at our schematics!

It seems that you've missed that we have not 2 but 3 grounds: GND is digital ground, GNDA is analog ground, and GNDS is a virtual "ground" which is actually at +0.6V and we measure all analog signals around it. Probably our symbols are misleading ))

1. U1A generates virtual ground - divider on its input sets level of about 0.6V and U1A repeats it (via 10R resistor just in case if something goes wrong, no real need in it since amplifier survives short circuit). That level of 0.6V is our zero point around which we work (and normally we stay within a few mV around).

2. MCP3911 has input impedance of about 30k when PGA is set to 32 amplification - and skin has impedance in a few MOhm area (measured that for gel pads and placement we are using) - so without buffering input with AD8606, it works like a voltage divider. Actually in our first iteration we made a mistake and had to omit amplifier completely - and it worked somehow, but signal was much, much worse and very sensitive to contact quality.

3. It is for connecting to external Arduino or other microcontroller: we are detecting heart beats onboard, and send them as short pulses there. Optocoupler makes external connection simple, eliminating possible problems (you can just connect it, and if it doesn't work - connect with opposite polarity). 

4. Yes - but check project logs, I'm really proud of the filter I've made! :) It's better than anything I saw before - including max30003 AFE which is specifically designed for ECG. Our result is seriously better (we work on another project based on max30003 in parallel, so I can compare them in identical conditions). And MCP3911 is the cheapest 16 bit AFE out there! )) (also uploaded illustration of how it works in extreme conditions: https://cdn.hackaday.io/files/1644867037369024/noise_removal_algo.png )

5. MCP3911 can be damaged if input voltage exceeds +2.0V on any ADC input. Normally we work around 0.6V point, but when the unit is not on the body, input can get anywhere. As soon as it exceeds approximately 1.2V, LED turns on, protecting the input and, at the same time, visually indicating that there is overload! And it actually works, when it's charging via usb, you really can turn it on by touching only one input! ))

Fair enough. I grok it now. There is a reason for everything in a schematic, if not entirely obvious. I'm an old school EE, so schematics should have wires connecting components instead of tags. It's easier for those not familiar with the design to understand what's going on. 

Thanks for the answers. Carry on. It obviously works.