Bioelectric recordings are often disturbed by an excessive level of interference. Mainly through the main power supply. Apparently, the use of equipment with very good specifications does not guarantee interference free recordings. The most common mechanisms of electrical mains interference are described in the following paragraphs. We will mention the main noise mechanisms that occur:
• Interference currents through the body - The capacitances between the patient, the power lines and ground cause a small interference current to flow through the body. In the modeling of the measurement situation the capacitance between the body and ground (Cbody) is taken to be 300 pF and the capacitance between the body and the mains power (Cpow) is taken to be 3 pF (Huhta and Webster, 1973; Forster, 1974) which values can be assumed to be typical.
• Interference currents into the amplifier - the capacitances between the amplifier common and main supply (Csup) and between amplifier common and ground (Ciso) should also be considered. Csup causes an additional interference current to flow from the amplifier to ground.
• Interference currents into the measurement cables - capacitive coupling of the measurement cables with the main supply causes induced currents that flow to the body via the electrodes and from the body to ground.
• Magnetically induced interference - Suppression is easy in theory by reducing cable area as much as possible and using a shielding mechanism.
Common mode Voltage can cause problems due to limited CMRR of the amplifier or through "the potential divider effect" (Huhta and Webster, 1973; Pacela, 1967).
If the impedance differences of electrodes and inputs cannot be kept sufficiently low to reduce the influence of the potential divider effect - the input impedances being as high as possible - the only practical solution left is to reduce the actual common mode voltage. A proper driven right leg circuit (see AD620 datasheet) offers a large reduction of common mode voltage magnitude by actively reducing the voltage difference between patient and amplifier common; a reduction between 10 and 50 dB is usually accomplished. A proper driven right leg circuit (see Fig. 2) offers a large reduction of common mode voltage magnitude in both isolated and non-isolated measurements by actively reducing the voltage difference between patient and amplifier common; a reduction between 10 and 50 dB is usually accomplished.
Given the inherent variability of the electrode impedances and the levels of interference there is only one practical way to reduce interference currents in the wires: shielding of the measuring cables. The schematics of the DRL and the driver shield are shown in the next section.
For further analysis see:
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