Customers should consider using VCM as a third electrode when using the MAX30001, MAX30002, or MAX30004. This third electrode tied to VCM (instead of being bypassed to ground) will help increase ECG performance.
Using a third electrode connected to VCM helps in these ways:
- It won’t lower the electrode to skin impedance which is higher with dry electrodes.
- It will help with electrode impedance mismatch because when using the third electrode connected to VCM, the lead bias resistors internal to the part can be turned off, and this will keep the input impedance to the part high. Higher impedance will reduce the common mode to differential conversion (by keeping CMRR high) if there is a mismatch in the impedance of the two electrodes connected to ECGP and ECGN.
- Using VCM can also help with 50 or 60 Hz line noise by attenuating the capacitive coupling between the body and our part.
The performance difference is night and day. A two-electrode solution will not work with dry electrodes, as arm hair, low perspiration (reduces contact impedance), half-cell voltages, contact impedance mismatches, etc..all make it extremely difficult to give a generalized number. To keep it simple, we advise using three electrodes to every customer whose application is wrist-based using dry electrodes.
The MAX3000x parts are technically made for use in a hospital environment (read: wet electrodes), although they can work very well with dry electrodes, but that requires the third electrode for the same reasons mentioned above.
Reference schematic: Simply connect VCM to the third electrode. No filters or caps are needed. Please see below figure or attached picture (manually edited application circuit, that has been edited from the MAX30003 Datasheet). The ECGP vs ECGN polarization doesn’t matter, as it can be inverted easily based on right or left hand. Engineers traditionally put VCM and ECGN on the Bottom (touching wrist) and connect ECGP on top (finger).