Recording Unipolar Signals With Cyton ? [resolved]
Hello everyone,
We are designing an ECG acquisition circuitry and wanted to simulate the performance of the filters beforehand. For this purpose, I was hoping to use OpenBCI Cyton to record sample signals and use them as possible potential values that could be picked up from the electrode sites.
After reading ADS1299 documentation, I now have impression that Cyton board samples only the differential signals between N and P pins.
Does it makes sense to place all electrodes to N pins and connect all P pins to analog ground ? Or is there a better way to do this?
Comments
Hi Ozberko,
Have you read the Getting Started and ECG setup docs?
https://docs.openbci.com/GettingStarted/Boards/CytonGS/
https://docs.openbci.com/GettingStarted/Biosensing-Setups/ECGSetup/
By default Cyton is setup for EEG, with all the 'top' pins (farthest from board surface, the IN_P pins, 1 to 8), bussed together and available at SRB2 (SRB closest to board). Then the IN_N pins (closest to board) are the scalp channels. Above link shows how to disconnect selected channels from the SRB bus, in cases when 'differential' mode is desired per channel.
William
Hi William,
Thanks a lot for your quick reply. Referring to the second documentation you shared, we want to record the exact potential values from LA, RA and LL instead of recording lead 1, lead 2, and lead3.
I will enable SRB2, then connect SRB2 pin and AGND together to record individual potentials.
Ozberk
Do NOT use AGND for anything. SRB2 is not used for ECG.
Dear William,
I think that I was not clear enough. Let me rephrase what I tried to say to you in a more detailed fashion again.
We are collaborating with a company which wants to develop an integrated circuit for ECG acquisition. They are designing filters for the analog conditioning part and they asked us to provide them with sample signals if possible to try out their analog filters.
We need to record unipolarly the potential values exactly at the electrode site so that they can feed these signals to their design and observe the performance of their analog filters, differential amplifiers etc.
As you should understand, I'm not interested in recording a conventional ECG .
I thought about using the board in EEG mode; however, I was confused with having a common reference connected to an electrode in my body which by nature cancels ambient noise and bad for us since I want to have those noises too.
Could you elaborate why it is a bad idea to connect SRB2 and AGND to each other?
Ozberk
ECG is always recorded BIPOLAR. Only EEG is recorded with a single reference electrode. I don't believe ECG makes any sense in a 'unipolar' context. All bio-amplifiers are differential. So even in the EEG case, the differential inputs to the amp are through the scalp electrode and the 'reference' electrode, wherever that is placed.
Please see my above comment.
ADS1299 chip used in Cyton has a very high input impedance, 1 gigaohm. This is assuming connections via channel inputs and the Bias pin (which is a also a Ground connection, and inverse mains injection.) Such high input impedance has numerous advantages, as far as skin impedance interactions.
http://wwe.eeginfo.com/research/ElectrodeImpedance.pdf
On the other hand, the AGND pin is extremely LOW impedance, and is not intended for connection to the subject. Check the schematics.
William