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S c s c ignal onditioning and ignal onditioners

Chapter SIGNAL CONDITIONING AND SIGNAL CONDITIONERS It is rare for biological physiological chemical electrical or physical signals to be measured in the appropriate format for recording and interpretation Usually a signal must be massaged to optimize it for both of these functions For example storage of recorded data is more accurate if the data are ampli ?ed before digitization so that they occupy the whole dynamic range of the A D converter and interpretation is enhanced if extraneous noise and signals above the bandwidth of interest are eliminated by a low- pass ?lter Why Should Signals Be Filtered A ?lter is a circuit that removes selected frequencies from the signal Filtering is most often performed to remove unwanted signals and noise from the data The most common form of ?ltering is low-pass ?ltering which limits the bandwidth of the data by eliminating signals and noise above the corner frequency of the ?lter Figure - The importance of low-pass ?ltering is apparent when measuring currents from single-ion channels For example channel openings that are obscured by noise in a kHz bandwidth may become easily distinguishable if the bandwidth is limited to kHz High-pass ?ltering is required when the main source of noise is below the frequency range of the signals of interest This is most commonly encountered when making intracellular recordings from nerve cells in the central nervous system There are low-frequency uctuations in the membrane potential due to a variety of mechanisms including the summation of synaptic inputs The small excitatory synaptic potentials that the user might be interested in are often smaller than the low- frequency uctuations Since excitatory synaptic potentials are often quite brief the low-frequency uctuations can be safely eliminated by using a high- pass ?lter High-pass ?ltering is also referred to as AC coupling Another type of ?lter that is often used in biological recording is the notch ?lter This is a special ?lter designed to eliminate one fundamental frequency and very little else Notch ?lters are most commonly used at or Hz to eliminate line- frequency pickup AXON GUIDE C Chapter six Fundamentals of Filtering Filters are distinguished by a number of important features These are - dB Frequency The - dB frequency f- is the frequency at which the signal voltage at the output of the ?lter falls to ?? i e of the amplitude of the input signal Equivalently f- is the frequency at which the signal power at the output of the ?lter falls to half of the power of the input signal See de ?nition of decibels later in this chapter Type High-pass Low-pass Band-pass or Band- reject notch A low-pass ?lter rejects signals in high frequencies and passes signals in frequencies below the - dB frequency A high-pass ?lter rejects signals in low frequencies and passes signals in frequencies above the - dB frequency A band-pass ?lter rejects signals outside a certain frequency range bandwidth and passes signals inside the bandwidth de ?ned by the high and the low -