STM CircleOS forum / Tests

Francis · 2008-02-21 13:10:39

Jingxi,
We are trying your hardware. It works (after resoldering some components), but the signal seems too weak to trigger the detection (always under the threshold line). Is there any simple way to change the threshold level?
Thanks,
Francis

jingxizhang · 2008-02-21 21:06:02

Hi Francis,
The ECG amplifier hardware sample I mailed to you has problem which I did not have time to fix it before I sent it to you (the ECG amplifier embedded in my Primer works fine). I sent to you for just for the physical sample. I am glad you made it work. The amplifier gain on the sample I sent to you is set to x500 (the R6 is 10K). In later design I changed the gain to x1000 to get larger ECG signal. What you need is to solder another 10K resister (0402 SMD) piggyback on top of R6 and parallel the two resisters to get to R6 to 5K. This will make the 2nd amplifier gain from x100 to x200 (1M/5K = 200) and double the signal strength. The following picture is the screenshot of the ECG board component layout for you to identify the R6.
The threshold is defined in Application.c file by 2 macros “UpperThresheld” and “LowerThreshold”. Because I wanted package the application fitted into the 8K boundary so I did not make a GUI for user to change the threshold (I should do it in later version). The UpperThresheld determines the trigger level while the LowerThresheld determines the hysteriesis. If the signal is still too small after you change the R6, you can modify these two macros.
The dry skin can affect the ECG signal (in clinic the patients are applied conducting gel on the contact electrodes to lower the contact resistance). For Primer ECG I find if I wet the finger before touch the ECG input lead there are usually no problem for get large ECG signal.

jingxizhang · 2008-02-21 22:01:22

Hi Francis,
Another note I should mention for using ECG Primer is the power supply noise from the host PC. The ECG Primer is working fine when uses its battery. If you connect it to a laptop by the USB port, make sure the power cord for the laptop power adaptor has 3 conductors (two power wires plug the grounding wire). I have tried connecting ECG primer to some laptop which has 2 conductors in its power cord and I find there is a large 60 Hz (in the United States and 50 Hz in Europe I think) voltage from laptop to the earth. This common mode noise saturates the ECG amplifier and makes the amplifier not function. Another reason to use power cord has grounding wire is the safety issue.

Jingxi

Francis · 2008-02-22 09:30:49

Thanks. We will modify the gain.
An idea would be modify the threshold by tilting the Primer.
Francis

jingxizhang · 2008-02-22 18:44:48

Hi Francis,
Yes, I thought about using the accelerometer to set the heart rate trigger threshold. Ideally it would be nice the user can see the ECG trace when the threshold level is adjusting. It needs a button action when the threshold level indicator is moved to the right place. However, both thumbs of the user are tired up on the ECG electrodes. There is no free hand to push the button at that moment.
It would good if there is a push button at back of the STM32 Primer (just like the trigger at bottom of those game controllers for Sony playstation or X-Box) so the user can use the middle finger to push the button at back of the device for setting the threshold while both thumbs are touching the ECG electrodes. I also like a function to freeze the screen temporary. So, if the user can hold the button at back of the device, the threshold level indicator can be moved by tilting the device until the user release the button. If a single toggle at the button, it will stop or start the ECG trace. Because adding a button may violate contest rule so I did not do it. But it can be easily done. Do you have any better idea?
Regards,

Jingxi

# 1 2008-02-21 13:10:39 Tests

Tests

# 2 2008-02-21 21:06:02 Tests

Re: Tests

# 3 2008-02-21 22:01:22 Tests

Re: Tests

# 4 2008-02-22 09:30:49 Tests

Re: Tests

# 5 2008-02-22 18:44:48 Tests

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