Biological signals are often very minute and typically contain unwanted interference or noise. Such interference has the detrimental effect of obscuring relevant information that may be available in the measured signal. Noise can be extraneous in nature arising from sources outside the body, such as thermal noise in sensors or 60-cycle noise in the electronic components of the acquisition system. Noise can also be intrinsic to the biological media, meaning it can arise from adjacent tissues or organs. ECG measurements from the heart, for example, can be affected by bioelectric activity from adjacent muscles.
In order to extract meaningful information from biological signals, sophisticated data acquisition techniques and equipment are commonly employed. High-precision low-noise equipment is often needed to minimize the effects of unwanted noise.
Throughout the data acquisition procedure, it is vital that the information and the structure of the original biological signal of interest be preserved. Since these signals are often used to aid the diagnosis of pathological disorders, the procedures of amplification, analog filtering and analog to digital conversion should not generate distorted or untraceable signals. Distortions in a signal measurement could lead to an improper diagnosis.
Sensors, Amplifiers and Analog Filters
Biosignals are first detected in the biological medium, such as a cell or the skin’s surface by using a sensor. A sensor converts a physical measurand into an electric output and provides an interface between biological systems and electrical recording instruments. The type of biosignals determines what type of sensor will be used. ECGs for instance, are measured with electrodes that have silver-silver chloride (Ag-AgCl) interface attached to the body that detects the movement of ions. Arterial blood pressure is measured with a sensor that detects changes in the pressure. It is very important that the sensor used to detect the biological signal of interest does not adversely affect the properties and characteristics of the signal it is measuring.
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After the biosignal has been detected with a suitable sensor, it is usually amplified and filtered. Operational amplifiers (op-amps) are used primarily to increase the amplitude or size of a biosignal. Bioelectric signals, for example, are often weak and need up to a thousand –fold boosting of their amplitude with such amplifiers. An analog filter may then be used to remove noise or to compensate for distortions caused by the sensor. Amplification and filtering of the biosignal may also be necessary to meet the hardware specifications of the data acquisition system. Continuous signals may need to be limited to a certain band of frequencies before the signal can be digitized with an analog-to-digital converted, prior to storing in a digital computer.
Analog to Digital Conversion
Analog-to-digital (A/D) converters are used to transform biological signals from continuous analog waveforms to digital sequences. An Analog-to-digital converter is a computer controlled voltmeter, which measures an input analog signal and gives a numeric representation of the signal as its output. The analog waveform, originally detected by the sensor and subsequently amplified and filtered, is a continuous signal. The A/D converter transforms the continuous analog signal into a discrete, digital signal. The discrete signal consists of a sequence of numbers that can easily be stored and processed on a digital computer. A/D conversion is principally important because storage and analysis of biosignals is becoming increasingly computer based.
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The digital conversion of an analog biological signal does not produce an exact replica of the original signal. The discrete, digital signal is a digital approximation of the original, analog signal that is generated by repeatedly sampling the amplitude level of the original signal at fixed time intervals. As result, the original analog signal is represented as a sequence of numbers i.e. the digital signal.
You can also read: Biomedical Signals Acquisition Instruments
