ALLPCB
Introduction
Electrocardiographs (ECG), electromyographs (EMG), and electroencephalographs (EEG) measure cardiac, muscular, and brain activity by recording potentials on the surface of living tissue. These bioelectric measurements detect ion flow directions associated with neural stimulation and muscle contraction using bioelectric electrodes. Clinicians face several practical challenges when performing these measurements; this article examines common issues and relevant solutions.
Practical Challenges
A frequently omitted step, often due to time constraints, is adequate skin preparation to improve electrode contact. Poor skin preparation leads to difficulties acquiring reliable signals. Impedance levels vary with patient age and ethnicity. Gold electrodes commonly used in ECG tend to exhibit higher impedance than the silver/silver-chloride electrodes typically used for EMG and EEG. Interfering sources such as ablation, electrocautery, cardiac defibrillation, external pacing, internal pacing, pacemaker H-field telemetry, and other signals can degrade measurement accuracy.
Solutions
Many patient-side challenges are tied to system design. Careful signal-conditioning design at the front end improves measurement reliability and reduces labor and electrode replacement costs. If skin contact is poor, the input bias current of the front-end amplifier can polarize the electrode. Choosing amplifiers with extremely low input bias current reduces this effect.
Amplifier and Device Considerations
JFET-input operational amplifiers such as AD8625, AD8626, and AD8627 offer input bias currents below 1 pA. JFET-input instrumentation amplifiers such as AD8220 and AD8224 provide input bias currents below 20 pA. Amplifiers that operate over a wide supply voltage range also offer a wide input common-mode range and high gain, which helps them cope with noisy environments typical of emergency and operating rooms.
The AD8625/AD8626/AD8627 series operates from a single 5 V to 26 V supply. The AD8220 and AD8224 can operate from ±18 V dual supplies or from a single 5 V supply, and they provide maximum dynamic range with rail-to-rail output. The static current per amplifier channel for AD8220 and AD8224 is 750 uA, making them suitable for battery-powered designs. The AD8224 can also be configured as a single-channel, differential-output instrumentation amplifier for improved noise immunity.
