Current: The rate that charge passes through an electrically conductive medium. One Ampere is equal to one Coulomb per second. In a wire, current is carried by electrons. In extracellular fluid, current can be carried by any ion in solution. Voltage: The force per unit of charge. Voltage causes charge (i.e. current) to flow in a particular direction. One Volt is equal to one Joule per Coulomb.
Resistance: The property of a material to resist current flow. One Ohm is equal to one Volt per Ampere.
Reactance: The property of an elelectical circuit element to resist changes in voltage or current. Like resistance, it is measured in Ohms. The reactance of a circuit changes with frequency. Reactance also alters the phase relationship between current and voltage. It applies to all time-variant signals. See als impedance, resistance, capacitive reactance.
Impedance: A complex quantity representing the property of a circuit to resist both steady-state current flow (resistance) and changes in current or voltage (reactance). See als reactance, resistance, capacitive reactance.
Complex numbers: Quantities that are expressed as a vector having both magnitude and phase information.
Johnson noise: Noise generated by the random movement of charge in a resistive medium. This noise is often described by electrophysiologists as "hiss" or "hash". It has a wide frequency spectrum, thus, it can be limited by electronic filters. Johnson noise increases with temperature and is sometimes called "thermal noise". Johnson noise also increases in proportion to resistive impedance.
Inductive pickup: Noise induced by external sources of electromagnetic radiation in unshielded connections between elements of the recording system. Electrophysiologists often describe this noise as "hum" or "buzz".
Input bias current: The current that an op-amp must draw through its inputs to operate properly.
Movement artifact: When the cable connecting the electrode to the preamplifier is bent, the change in cable capacitance causes charge to move. This charge movement generates an appreciable voltage across high impedances. This type of noise is experienced as an abrupt change in baseline that can sometimes result in amplifier saturation.
Gate leakage current: Current drawn through the input (gate) of a junctional FET circuit for proper operation.
Ground loop: A condition that occurs when the components of your recording system use different references. A ground loop is most often experienced as a prominent 60 Hz "hum" in your recording. Thus, it is indistinguishable from some types of inductive pickup. Ground loops are avoided by making sure each component of your recording system is tied to one (and only one) reference point.
Capacitive Reactance: The component of impedance that is associated with capacitance. Capacitve reactance (Xc) is inversely proportional to frequency and capacitance. When driven by a sinusoidal source, capacitive reactance is given by:
Xc = 1 / 2 p f C
where f is the frequency of the sinusoid in Hertz (Hz), C is the capacitance in Farads (F) and Xc is the capacitive reactance in Ohms (W). See als reactance, impedance. Operational Amplifier (op-amp): A general-purpose building-block for constructing analog circuits. Common circuits that can be built with op-amps are amplifiers, filters and comparators. Op-amps are integrated circuits and are usually grouped in packages of one, two or four. The design of op-amp circuits is beyond the scope of this document.
Field-effect transistor (FET): A discrete semiconductor component used to build integrated circuits such as operational amplifiers. A FET is a 3-terminal device with terminals labelled gate, source and drain. Generally, the input signal is connected to the gate and the output is taken from either the source or the drain. The design of FET circuits is quite involved and beyond the scope of this document.
