Bio potential amplifiers

Bio potential amplifiers


Biosignals

Biosignals are recorded as potentials, voltages, and electrical field strengths generated by nerves and muscles. The measurements involve voltages at very low levels, typically ranging between 1 μV and 100 mV, with high source impedances and superimposed high level interference signals and noise. The signals need to be amplified to make them compatible with devices such as displays, recorders, or A/D converters for computerized equipment. Amplifiers adequate to measure these signals have to satisfy very specific requirements. They have to provide amplification selective to the physiological signal, reject superimposed noise and interference signals, and guarantee protection from damages through voltage and current surges for both patient and electronic equipment. Amplifiers featuring these specifications are known as biopotential amplifiers. Basic requirements and features of bio potential amplifiers, as well as some specialized systems, will be presented.

Requirements of basic bio potential amplifier 

The basic requirements that a biopotential amplifier has to satisfy are:

• the physiological process to be monitored should not be influenced in any way by the amplifier
• the measured signal should not be distorted
• the amplifier should provide the best possible separation of signal and interferences
• the amplifier has to offer protection to the patient from any hazard of electrical shock
• the amplifier itself has to be protected against damages that might result from high input voltages as they occur during the application of defibrillators or electrosurgical instrumentation


 The input signal to the bio potential amplifier consists of five components: 
(1) the desired biopotential,
(2) undesired biopotentials, 
(3) a power line interference signal of 60 Hz (50 Hz in some countries) and its harmonics, 
(4) interference signals generated by the tissue/electrode interface, and 
(5) noise. Proper design of the amplifier provides rejection of a large portion of the signal interferences.


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