Electrocardiography (ECG), electromyography (EMG), and electroencephalography (EEG) measure heart, muscle, and brain activity over time, respectively, by measuring the electric potentials on the surface of living tissue. Nervous stimuli and muscle contraction can be detected by measuring the ionic current flow in the body. This measurement is accomplished using a biopotential electrode.
Clinicians face practical challenges when making biopotential measurements. Prepping a patient’s skin to make a good contact with the electrode is a step often skipped in the interest of time, unless the practitioner has a problem acquiring good signals. Impedance levels vary across ethnic groups and the ages of patients. Gold electrodes with paste commonly used in EEG readings yield much higher impedances than silver/silver chloride electrodes typically used in ECG and EMG. Interfering signals such as ablation, electric cautery, defibrillation, external pacing, internal pacing, and pacemaker H field telemetry, as well as a multitude of other signals, affect signal measurement accuracy.
Patient preparation challenges are relevant to system design. Foresight in designing the signal conditioning circuit can ensure reliable measurements and can save costs in terms of labor and electrode replacement. The input bias current of front-end amplifiers can polarize the electrode if there is poor skin contact. The AD8625/AD8626/AD8627 family of JFET input operational amplifiers offers input bias currents of less than 1 pA. The AD8220 and AD8224 JFET input instrumentation amplifiers offer input bias current under 20 pA.
Amplifiers that operate on wide supply voltages offer the advantage of a large input voltage range, and more gain can be applied. They are able to handle the noisy environments of emergency and operating rooms. The AD8625/AD8626/AD8627 family of ICs supports 5 V to 26 V single-supply operation. Both the AD8220 and AD8224 can operate on a ±18 V dual supply, as well as on a single 5 V supply. These two devices offer a rail-to-rail output stage that maximizes dynamic range. The AD8220 and AD8224 draw 750 µA of quiescent current per amplifier, ideal for battery-powered devices. In addition, the AD8224 can be configured, as a single-channel, differential output instrumentation amplifier, providing high noise immunity.
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