D

D/A. -- Abbreviation for digital-to-analog.

D/A (DIGITAL-TO-ANALOG) CONVERTER. -- An electronic device, often an integrated circuit, that converts a digital value to an analog voltage. D/A converters are used in many instruments to convert digital reading information into an analog signal for analog output.

DATA LOGGING -- A method of recording a process variable over a period of time. Used to review process performance.

DC (Direct current) -- An electrical current that flows in one direction.

DATA TRANSFER. -- Refers to the way data is transferred to and from memory, such as programmed I/O or DMA mode.

DEGREE -- The increments in a temperature scale, or the increments of rotation of a dial. The location of a reference point in electric or phase in a cycle, in mechanical or electrical cyclic scales. (One cycle is equal to 360 degrees).

DENSITY-- Mass per unit volume of a substance usually expressed in lbs/ft3 or grams/cm3.

Deutsche Industrial Norm (DIN) -- A set of technical, scientific and dimensional standards developed in Germany. Many DIN standards have worldwide recognition.

DEVIATION -- Any departure from a desired value or expected value or pattern. Sometimes referred to as delta.

DIFFERENTIAL INPUT ISOLATION. -- On a switching card, the isolation from signal high to low. Specified as resistance and capacitance.

DIFFERENTIAL INPUTS. -- An analog input with two input terminals, neither of which is grounded, whose value is the difference between the two terminals. See also SINGLE-ENDED INPUTS.

DIFFERENTIAL NON-LINEARITY. -- The maximum deviation of an actual quantized step width from the ideal quantized step width.

DIGITAL-TO-ANALOG-CONVERTER-- A device that converts a numerical input signal to a signal that is proportional to the input in some way.

DIN -- See Deutsche Industrial Norm.

DIGITAL I/O. -- Abbreviation for digital input/output.

DIGITAL LINES/PORTS/BITS/CHANNELS. -- In hardware, a digital line is physical hardware connection to a pin with a digital signal. A digital port is a physical grouping of digital lines. In software, a digital bit (1 or 0) is a logical representation of a digital line. A digital channel is a logical grouping of digital bits.

DIGITAL TRIGGER -- An event that occurs at a user-selected point on a digital input signal. The polarity and sensitivity of the digital trigger can often be programmed. See also TRIGGER, TRIGGER CONDITIONS, TRIGGER POLARITY, and TRIGGER SENSITIVITY.

DIRECT CURRENT -- The type of current normally supplied by batteries. It is called Direct Current (DC) because the flow of electrons (electricity) is always in the same directions; from the negative (-) pole of the power source to the positive (+) pole.

DISCRETE DEVICE. -- A class of electronic components that contain one active element, such as a transistor or diode. However, hybrids, optoelectronic devices, and intelligent discretes may contain more than one active element.

DISPLACEMENT CURRENT. -- The current that flows through a capacitor in response to the rate of voltage change across it: displacement current = C * dV/dt

DMM. -- An electronic instrument that measures voltage, current, resistance, or other electrical parameters by converting the analog signal to digital information and display. The typical five-function DMM measures DC volts, DC amps, AC volts, AC amps, and resistance.

DMA (DIRECT MEMORY ACCESS) CHANNELS.-- ISA bus PCs offer eight parallel channels for DMA mode data transfers. A number of these are reserved for exclusive use by the computer. The remainder are available for use by user-supplied I/O options, such as plug-in data acquisition cards. Also called DMA levels.

DRAM. -- Dynamic Random Access Memory. A semiconductor read/write memory chip, in which the presence or absence of a capacitive charge represents the state of a binary storage element (zero or one). The charge must be periodically refreshed.

DRIFT -- A change in reading or value that occurs over long periods. Changes in ambient temperature, component aging, contamination, humidity and line voltage may contribute to drift.

DRIVERS. -- Software that controls a specific hardware device such as a data acquisition board.

DSP. -- Abbreviation for Digital Signal Processing.

DUAL IN-LINE PACKAGE (DIP). -- A plastic or ceramic package with two rows of vertical leads.

DUTY RATIO. -- The ratio of pulse width to repetition period. Also known as Duty Cycle.

DYNAMIC DATA EXCHANGE (DDE). -- A Microsoft Windows standard mechanism for communication between programs. It allows your application to send and share data with other applications such as spreadsheets.

DYNAMIC LINK LIBRARY (DLL). -- A software module in Microsoft Windows containing executable code and data that can be called or used by Windows applications or other DLLs. DLL functions and data are loaded and linked at run time when they are referenced by a Windows application or other DLLs.

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E

EARTH GROUND -- A metal rod, usually copper, that provides an electrical path to the earth, to prevent or reduce the risk of electric shock.

EAU -- Estimated annual usage is the amount of a specific part number a customer requires in a one year period.

EFFICIENCY -- The ratio of useful output energy (work) to input energy (output plus losses, or wasted energy).

EIA -- See Electronics Industries of America.

EIA/TIA -232, -422, -423 and -485 -- Data communications standards set by the Electronic Industries of America and Telecommunications Industry Association. Formerly referred to as RS (Recognized Standard).

EEPROM OR EPROM. -- Electrically-Erasable Programmable Read-Only Memory. Similar to PROM, but with the capability of selective erasure of information through special electrical stimulus. Information stored in EEPROM chips is retained when the power is turned off.

EISA BUS. -- Extended Industry Standard Architecture. A 32-bit wide, upwards-compatible extension of the 16-bit wide ISA bus. Sometimes called the XT bus.

ELECTRICAL INTERFERENCE -- Electrical noise that can obscure desired information.

ELECTRICAL NOISE -- See noise.

ELECTROMAGNETIC COMPATIBILITY (EMC) -- The ability of equipment or a system to function as designed in its electromagnetic environment without introducing intolerable electromagnetic disturbances to that environment, or being affected by electromagnetic disturbances in it.

ELECTROMAGNETIC INTERFERENCE (EMI) -- Electrical and magnetic noise imposed on a system. There are many possible causes, such as switching AC power on inside the sine wave. EMI can interfere with the operation of controls and other devices.

ELECTROMAGNETIC RELAY -- See relay, electromechanical.

ELECTROMOTIVE FORCE (EMF) -- An increase in electrical potential energy measured in volts.

ELECTRONICS INDUSTRY OF AMERICA (EIA) -- An association in the US that establishes standards for electronics and data communications.

ELECTROSTATIC DISCHARGE (ESD) -- An electrical discharge, usually of high voltage and low current. For example, the shock that occurs when walking across a carpet.

EMC-- See electromagnetic compatibility.

EMF -- See electromotive force.

EMI -- See electromagnetic interference.

ELECTROMETER. -- A highly refined DC multimeter. In comparison with a digital multimeter, an electrometer is characterized by higher input resistance and greater sensitivity. It can also have functions not generally available on DMMs (e.g., measuring electrical charge, sourcing voltage).

ELECTRON. -- A negatively charged particle revolving round the nucleus of an atom.

ELECTRON, FREE. -- An electron not bound to the crystalline lattice, hence, free to conduct electricity.

EPROM -- Erasable, programmable, read-only memory inside the controller.

ERROR -- The difference between the correct or desired value and the actual measured value.

ESD-- See electrostatic discharge.

ETCHED FOIL -- The heater type that utilizes a resistance foil as the element with a print and etch method of manufacture.

EVENT -- A programmable ON/OFF output signal. Events can control peripheral equipment or processes, or act as input for another control or control loop.

EXOTHERMIC -- A process that releases heat.

EXTERNAL TRIGGER. -- An analog or digital hardware event from an external source that starts an operation. See also INTERNAL TRIGGER.

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F

FAHRENHEIT -- The temperature scale that sets the freezing point of water at 32¼F and its boiling point at 212¼F at standard atmospheric pressure. The formula for conversion to Celsius is: ¼C = 5/9 (¼F - 32¼F).

FALL TIME. -- The time required for a signal to change from a large percentage (usually 90%) to a small percentage (usually 10%) of its peak-to-peak amplitude. See also RISE TIME.

FERMI LEVEL.-- The energy level in the semiconductor device at which the probability of finding an electron is 50%. In other words, it's like the water level in a glass. States below the Fermi level are full and those above it are empty.

FIELD EFFECT TRANSISTOR (FET). -- A transistor consisting of a gate, source, and drain. The voltage applied to the gate controls the conductivity of the channel between the source and drain. Examples of FETs are the MOSFET, JFET

FIFO. -- First-in/first-out memory buffer. The first data into the buffer is the first data out of the buffer.

FLASH MEMORY. -- It is a non volatile memory technique with fast access times; rewriteable many times and uses a block erase technique as opposed to EEPROM, which erases one bit at a time.

FLOATING. -- The condition where a common mode voltage exists between an earth ground and the instrument or circuit of interest. (Low of circuit is not at earth potential.)

FPS (feet per second) -- A measure of flow velocity.

FREEZING POINT -- The fixed temperature point at which a material changes from a liquid to a solid state. This is the same as the melting point for pure materials. For example, the freezing point of water is 32¼F or 0¼C.

FREQUENCY -- The number of cycles over a specified period of time usually measured in cycles per second. Also referred to as Hertz (Hz). The reciprocal is called the period.

FUSE -- A device that protects electric circuits by interrupting power in a circuit when an overload occurs.

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G

GAIN. -- The factor by which an incoming signal is multiplied.

GIGA (G) -- A prefix that means 109 (one billion in the US). Note: The word billion refers to different numbers in Europe and the US. In the US a billion is one thousand million (1,000,000,000). In Germany, England, France and other countries, a billion is one hundred thousand million (100,000,000,000). That is a trillion in the US.

GPIB. -- Abbreviation for General Purpose Interface Bus. It is a standard for parallel interfaces.

GROUND -- An electrical line with the same electrical potential as the surrounding earth. Electrical systems are usually grounded to protect people and equipment from shocks due to malfunctions. Also called safety ground.

GROUNDED JUNCTION -- See junction, grounded.

GROUND LOOP -- A condition created when two or more paths for electricity are created in a ground line, or when one or more paths are created in a shield. Ground loops can create undesirable noise.

GROUND POTENTIAL -- The electrical potential of the earth. A circuit, terminal or chassis is said to be at ground potential when it is used as a reference point for other potential in the system.

GUARDING.-- A technique that reduces leakage errors and decreases response time. Consists of a guard conductor driven by a low-impedance source surrounding the lead of a high-impedance signal. The guard voltage is kept at or near the potential of the signal.

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