Skin Motion Artifacts Skin artifacts can be on the order of several millivolts, which is greater than the ECG signal itself Artifacts can be due to electrode interaction with hair and the outer layer of skin, called the stratum corneum Skin artifacts are difficult to remove with an electronic filter
Preparing Skin for Electrode Placement Steps (1)Shave/trim hair where electrode will be applied (2)Clean skin with soap and water - Do not use alcohol as it can dry out the skin, increasing impedance (3) Next use a fine abrasive material to scrape off the stratum corneum - The stratum granulosum, or middle epidermis layer, must be scratched for best results - This also serves to remove oils from the skin
Applying Electrodes (1) Verify that the electrode’s conducting gel is intact (2) Attach the lead wire to the electrode, and attach the electrode by pressing around the outside of the pad - Do not press on the center of the pad as it can create gaps of air in the gel, which can lead to noise
Analysis of limb electrodes
Skin Preparation Results This greatly reduces the motion artifact by removing the main contributing factor to artifacts, the stratum corneum Also, this process reduces the contact resistance between the electrode and the skin. This reduces the likelihood of signal drift.
Power-Line Interference Signal picks up 60Hz noise from lights, power cables, etc.
Avoiding Power-Line Interference The Obvious Solutions Use shielded wires for the electrode leads. Twist wires together to decrease loop area. Shorten the leads from the body to the A-to-D converter. Once the signal becomes digital the induced noise on the wires is no longer an issue. Driven right leg circuitry
Digital Filtering of Power-Line Interference Digital filters can be implemented on a microcontroller with a small amount of latency One paper showed a digital notch filter with a notch width of approximately 4Hz. Setting the center frequency to 60 Hz allows us to remove the power-line interference Reference link for filter design:
EMG interference EMG: Hz 0 – 10 mV peak-to-peak Digital Time domain Wiener filter. etc.
Shaking Coldness musculature contracts Contraction signal from brain picked up by ECG Nervousness Tensing of muscle