Bio-potential Amplifiers

Bio-signals 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. Amplifiers adequate to measure these signals have to satisfy very specific requirements. They have to provide amplification selective to the

a.   Physiological signal,
b.   Reject superimposed noise
c.    Interference signals,
d.  Guarantee protection from damages through voltage and current surges for             both patient & electronic equipment.

Amplifiers featuring these specifications are known as bio-potential amplifiers.

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Fig1: Amplitudes and spectral ranges of some important bio-signals. The various bio-potentials completely cover the area from 10 – 6 V to almost 1 V and from dc to 10 kHz.

Reference: Nagel, J. H. “Bio-potential Amplifiers.” The Biomedical Engineering Handbook: Second Edition.

1.1 Basic amplifier requirements

The basic requirements that a bio-potential 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 of 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 electro-surgical instrumentation.

a. The input signal to the amplifier consists of five components:
b. The desired bio-potential,
c. Undesired bio-potentials,
d. A power line interference signal of 60 Hz (50 Hz in some countries) and its harmonics,
e. Interference signals generated by the tissue/electrode interface,
Noise.

Proper design of the amplifier provides rejection of a large portion of the signal interferences. The desired bio-potential appears as a voltage between the two input terminals of the differential amplifier and is referred to as the differential signal. The line frequency interference signal shows only very small differences in amplitude and phase between the two measuring electrodes, causing approximately the same potential at both inputs, and thus appears only between the inputs and ground and is called the common mode signal.

Strong rejection of the common mode signal is one of the most important characteristics of a good bio-potential amplifier. The common mode rejection ratio (or CMRR) of an amplifier is defined as the ratio of the differential mode gain over the common mode gain. The input impedance of the amplifier should be at least 109 Ω at 60 Hz to prevent source impedance unbalances from deteriorating the overall CMRR of the amplifier. State-of-the-art bio-potential amplifiers provide a CMRR of 120 to 140 dB.

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Fig2 : Schematic design of the main stages of a bio-potential amplifier.
Reference: Nagel, J. H. “Bio-potential Amplifiers.” The Biomedical Engineering Handbook: Second Edition.

In order to provide optimum signal quality and adequate voltage level for further signal processing, the amplifier has to provide a gain of 100 to 50,000 and needs to maintain the best possible signal to noise ratio. The presence of high level interference signals not only deteriorates the quality of the physiological signals, but also restricts the design of the bio-potential amplifier.

Electrode half-cell potentials, for example, limit the gain factor of the first amplifier stage since their amplitude can be several orders of magnitude larger than the amplitude of the physiological signal. The preamplifier represents the most critical part of the amplifier itself since it sets the stage for the quality of the bio-signal. It eliminates DC offset.

   With proper design, the preamplifier can eliminate, or at least minimize, most of the signals interfering with the measurement of bio-potentials. In addition to electrode potentials and electromagnetic interferences, noise generated by the amplifier and the connection between biological source and amplifier has to be taken into account when designing the preamplifier. This peculiar noise is thermal voltage noise.

Additionally, there is the inherent amplifier noise. It consists of two frequency-dependent components, the internal voltage noise source en and the voltage drop across the source resistance Rs caused by an internal current noise generator in. High signal to noise ratio requires amplifiers with very low noise and limitation with bandwidth. Further the high pass and low pass filters removes interference signals like electrode half potentials, pre-amplifier offset potentials and to reduce noise amplitude by limitation of band width. Since the bio-signal should not be distorted or attenuated, higher order sharp-cutting linear phase filters have to be used. Active bessel filters are preferred filter types due to their smooth transfer function.

      Motion artifacts can be minimized by providing high input impedances for the preamplifier, usage of non-polarized electrodes with low half-cell potentials such as Ag/AgCl electrodes, and by reducing the source impedance by use of electrode gel. Motion artifacts, interferences from external electromagnetic fields, and noise can also be generated in the wires connecting electrodes and amplifier. Reduction of these interferences is achieved by using twisted pair cables, shielded wires, and input guarding.

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About YANAMALA VIJAY RAJ

Mtech in Clinical Eng Jointly offered by Indian institute of technology Madras& Christian medical college Vellore& Sree chitra tirunal institute for medical sciences and technology Trivandrum.
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