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Biopotential Amplifiers
Abstract
The vast majority of biomedical amplifiers have been specifically designed to detect, measure, and record biopotentials, i.e., voltages and electrical fields generated by nerves and muscles. The measurement of these biopotentials is not a trivial task as their level is low (around 1 mV although they can be as low as 1 µV) and is also immersed in noisy environments including internal and external interference.
This article starts with a short description of the different types of biopotential signals, followed by the requirements for amplifiers to detect biopotential signals and their design. After discussing the different interference sources for biopotential amplifiers, the entry analyzes the methods to mitigate their effects. The final section in this article describes how the generic biopotential amplifier described earlier is used for the measurement and detection of the different types of biopotential signals described at the beginning of this article.
Keywords: Biopotentials; Amplifiers; Interference; Isolation; Electrical safety; Signal; Bioelectrical signals; Electrocardiography; Electroencephalography
Applications:
Biopotential amplifiers are a crucial component in many medical and biological measurements, and
largely determine the quality and information content of the measured signals. The extremely wide range
of necessary specifications with regard to bandwidth, sensitivity, dynamic range, gain, CMRR, and patient
safety leaves only little room for the application of general purpose biopotential amplifiers, and mostly
requires the use of special purpose amplifiers.
Electrocardiography (ECG, or EKG [from the German Elektrokardiogramm]) is a transthoracic interpretation of the electrical activity of the heart over time captured and externally recorded by skin electrodes.[1] It is a noninvasive recording produced by an electrocardiographic device. The etymology of the word is derived from the Greek electro, because it is related to electrical activity, cardio, Greek for heart, and graph, a Greek root meaning "to write". In English speaking countries, medical professionals often write EKG (the abbreviation for the German word elektrokardiogramm) in order to avoid confusion with EEG.[citation needed]
The ECG works mostly by detecting and amplifying the tiny electrical changes on the skin that are caused when the heart muscle "depolarises" during each heart beat. At rest, each heart muscle cell has a charge across its outer wall, or cell membrane. Reducing this charge towards zero is called de-polarization, which activates the mechanisms in the cell that cause it to contract. During each heartbeat a healthy heart will have an orderly progression of a wave of depolarisation that is triggered by the cells in the sinoatrial node, spreads out through the atrium, passes through "intrinsic conduction pathways" and then spreads all over the ventricles. This is detected as tiny rises and falls in the voltage between two electrodes placed either side of the heart which is displayed as a wavy line either on a screen or on paper. This display indicates the overall rhythm of the heart and weaknesses in different parts of the heart muscle.
Usually more than 2 electrodes are used and they can be combined into a number of pairs (For example: Left arm (LA), right arm (RA) and left leg (LL) electrodes form the pairs: LA+RA, LA+LL, RA+LL). The output from each pair is known as a lead. Each lead is said to look at the heart from a different angle. Different types of ECGs can be referred to by the number of leads that are recorded, for example 3-lead, 5-lead or 12-lead ECGs (sometimes simply "a 12-lead"). A 12-lead ECG is one in which 12 different electrical signals are recorded at approximately the same time and will often be used as a one-off recording of an ECG, typically printed out as a paper copy. 3- and 5-lead ECGs tend to be monitored continuously and viewed only on the screen of an appropriate monitoring device, for example during an operation or whilst being transported in an ambulance. There may, or may not be any permanent record of a 3- or 5-lead ECG depending on the equipment used.
It is the best way to measure and diagnose abnormal rhythms of the heart,[2] particularly abnormal rhythms caused by damage to the conductive tissue that carries electrical signals, or abnormal rhythms caused by electrolyte imbalances.[3] In a myocardial infarction (MI), the ECG can identify if the heart muscle has been damaged in specific areas, though not all areas of the heart are covered.[4] The ECG cannot reliably measure the pumping ability of the heart, for which ultrasound-based (echocardiography) or nuclear medicine tests are used. It is possible to be in cardiac arrest with a normal ECG signal (a condition known as pulseless electrical activity).

The measurement of temperature on small structures requires probes of correspondingly small dimensions. With the TCAM Module and a suitable thermocouple, there are virtually no limitations. Thermocouples (Type T, copper-constantan) are available in a wide range of forms. From the thinnest wires with a diameter of 0.23 mm (IT-23) to the largest rectal probe with 4 mm probe diameter (RET-1); a whole range of probes in different forms and dimensions can be supplied. The temperature probes are directly interchangeable and do not require individual calibration.
The range of the TCAM module covers 0°C to 100°C. In the physiological temperature range (30 to 45°C) the basic accuracy is 0.1°C; outside this range the accuracy is 0.2°C. The built-in digital display has a resolution of 0.1°C. The TCAM module has an analogue output for connection to a recorder or data acquisition system. The recorder or acquisition system can easily be calibrated through a build-in simulation device with two adjustable temperature values. In addition, there is a zero suppression facility for recorders which permits recording temperatures within a limited range (e.g., 36 to 38°C) at a high resolution.