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Learn about Data Acquisition

MCC Data Acquisition Handbook

Data Acquisition Glossary of Terms

Absolute Accuracy
Used to describe the maximum or worst case error of a system when all potential system errors are considered. Absolute accuracy of a data acquisition system includes gain error, offset error, quantization error, noise, and non-linearity.

Acquisition Rate
The rate at which samples are acquired. Typically described in terms of samples per second (S/s), kilosamples per second (kS/s), or Megasamples per second (MS/s). Also may be referred to in terms of Hz, kHz, or MHz, where 1 Hz represents 1 sample per second.

ActiveX Control
(a.k.a. OLE Controls)
A special function object that can readily be used by an OLE-enabled application, tool, or web browser. Examples include the functions in Measurement Computing and SoftWIRE products. Active X controls were replaced by .NET components and controls in version 7 of Visual Studio, which was released as Visual Studio .NET in 2001.

Analog-to-Digital. (typically conversion)

Analog-to-Digital Converter. An electronic device that converts an analog input voltage into a digital value.

A false or erroneous low-frequency waveform or component that can appear in acquired data when sampled at too low a sampling rate. (See also Nyquist Theorem.)

Analog Trigger
A trigger that is based on an analog threshold level. For example, you may wish to start your data acquisition scan when the input voltage first passes through 3.5 volts. To accomplish this you would set your analog trigger level to +3.5 V.

Application Programming Interface.

Application Specific Integrated Circuit
A custom semiconductor component developed to perform a specific set of functions. ASICs allow a large amount of digital logic to be combined into a single chip.

A process or event that occurs without synchronization to a reference clock.

A DAQ system task (such as acquiring data) that occurs without interruption while another program routine is running.

Bandwidth, Small-Signal
A description of the highest frequency signal component that will pass through an amplifier and/or filter. Bandwidth is specified as the frequency where the attenuation is 3 dB.

Bandwidth, Large-Signal
Large signals are generally slew rate-limited before they are bandwidth-limited. Large signal bandwidth can be determined by the following equation: BW= Slew Rate/(2p x Vp) where Vp is the peak amplitude of the signal in question.

Base Address
The I/O or memory address that serves as the primary address for programmable registers. All other addresses are referenced to (or offset from) the base address (e.g., Base +2, or Base +8).

Baud Rate
A serial communications data transfer rate that is expressed in bits per second (b/s).

An input or output range that includes both positive and negative voltages (e.g., ±5V).

A single binary digit, either 0 or 1. The bit is the basis for all digital logic.

Block Transfer
Transfer of data to memory in blocks, typically from a FIFO. Block transfer of data is faster and more efficient than programmed I/O or interrupt transfer. (See also DMA and REPINSW.)

Break Before Make
A relay or switch configuration in which one connection is completely broken from before the next is connected.

Breakdown Voltage
The voltage at which a breakdown of optical isolation, semiconductors, or dielectric materials can occur.
See also Working Voltage.

An operation in which a device is powered up for a period of time to stabilize and remove devices subject to infant mortality prior to moving them to the next manufacturing stage, or shipment to customers.

Burst Mode
Sample data is acquired on adjacent channels at the maximum sample rate, while a longer time interval is inserted between scans. This mode is often referred to as pseudo-simultaneous sample and hold or “interval” sampling. It is used to minimize ‘channel skew’ or time between samples on different channels.

Bus Master
A type of a plug-in board or controller with the ability to take control of the computer’s data bus and perform reads and/or writes without requiring the use of the host CPU.

Eight bits of data, representing an eight-bit binary number. A byte has a value from 0 to 255 (decimal).

Channel Main Queue
A user-defined arbitrary sequence of channels with corresponding gains or input ranges. Utilizing a Channel Gain Queue, you may create complex scan sequences that exactly match your application’s requirements.

CMR or Common Mode Range
Describes the maximum voltage values that may be applied to both the positive and negative terminals of a differential input and still maintain rejection of that signal within the CMRR levels specified.

CMRR or Common Mode Rejection Ratio
A measure of a differential input's ability to reject signals common to both the positive and negative terminals. CMRR is usually expressed in decibels (dB).

CMV or Common Mode Voltage
A voltage common to both the positive and negative terminals of a differential input. Because a differential input accepts only difference signals, any signal common to both inputs would ideally be ignored.

An unavoidable, (and undesired) thermocouple junction that occurs where a thermocouple is connected to a measurement system.

Cold-Junction Compensation
A system developed to remove the error caused by the thermocouple’s cold-junction.

Component Object Model – A Microsoft developed object-oriented system for creating binary software components that can interact. COM is the foundation technology for Microsoft's OLE (compound documents), ActiveX® (Internet-enabled components), as well as others.

A bus configuration based on the popular PCI bus, but modified and enhanced for industrial applications.

Contact Bounce
The temporary, undesirable opening and closing of mechanical contacts that occurs when a relay or switch is closed. (See also Mercury Wetted Relay.)

Contact Life
The number of openings/closures of a relay or switch that can be expected before failure.

Contact Rating
The maximum voltage, current, and/or power capacity of relay or switch contacts.

Contact Resistance
The resistance across a set of closed contacts, measured in ohms.

A circuit or device that can be used to count or generate digital pulses or edges. Counter/timers are often used to set sample timing.

An unwanted error signal created on one channel by a varying input on an adjacent channel.

Current Drive Capability
The current that a digital or analog output is capable of sourcing or sinking while still maintaining its voltage range specifications.


DAC or D/A Converter
Digital-to-Analog Converter. An electronic device that converts a digital number into an equivalent analog voltage or current.

Common abbreviation for Data Acquisition.

dB or Decibel
A common unit used to express the logarithm of the ratio of two signal levels:
dB = 20 log10 V2/V1, for signal voltages or dB = 10log10 P2/P1, for signal powers.

Dynamic Data Exchange. A software protocol in Microsoft Windows for inter-application communication. DDE allows a data acquisition application to share data real-time with Windows applications such as Microsoft Excel. DDE is an older protocol that has been replaced with COM and .Net protocols.

Delta-Sigma A/D Converter
A highly flexible type of A/D converter that allows the user to trade off accuracy versus sample rate. A single converter can be used for high accuracy, low sample rate applications and then reconfigured for lower accuracy, higher speed applications.

Derivative Control
The D in PID control, a derivative control acts on the rate of change of an input or error and provides a very quick control response to a rapidly changing process (m = Ddi/dt or m = Dde/dt). Can be used with proportional and/or integral controls but, except in feed-forward control schemes, is rarely used alone.

Differential Input
A differential analog input consists of two input terminals as well as a ground connection. The input measures the difference between the two inputs without (see CMRR) regard to ground potential variations. For example, the two inputs may be labeled Hi and Low, the difference between those two inputs is the measurement returned by the system. The third input, labeled Ground is a reference ground and must be connected to a reference ground at the signal source. A differential input can reject some signal difference between the Low input and the reference ground.

Digital Trigger
A trigger that is based on a standard digital threshold level.

Dynamic Link Library. A Windows-based file containing executable code that is run by other Windows applications or DLLs.

Differential Nonlinearity. A measure in LSB of the worst-case deviation of code widths from their ideal value of 1 LSB. A DNL lower limit specification of -1 LSB guarantees no missing codes. A maximum code width can be determined via [abs(lower limit DNL) + upper limit DNL].

DMA or DMA Mode
Direct Memory Access. A method by which data can be transferred to/from computer memory from/to a device or memory on the bus while the processor does something else. DMA is the most efficient method of transferring data to/from computer memory. (See also REPINSW.)

A gradual change of a measurement with no change in the input signal or operating conditions. Common contributors to drift are temperature and time, and are usually expressed in ppm or % of unit measure. (Also, see Temperature Coefficient.)

Software that controls hardware devices, such as DAQ boards, GPIB. interface boards, PLCs, RTUs, and other I/O devices.

Digital Signal Processing.

Dual-Ported Memory
Memory that can be accessed by multiple controllers or processors. Dual-ported memory is commonly used on “intelligent” boards, enabling the on-board processor and the PC’s CPU to asynchronously share data.

Duty Cycle
The ratio of pulse width to overall signal repetition period.

Dynamic Range
The ratio of the largest signal an input will measure to the smallest signal it can detect. Normally expressed in dB (20 log10 V2/V1).

Electrically Erasable Programmable Read-Only Memory. A nonvolatile memory device that can be programmed and erased with electrical control signals.

A device that converts linear displacement or rotation into a stream of digital/pulse signals.

Erasable Programmable Read-Only Memory. A nonvolatile memory device that can be programmed, and will retain its memory until erased by exposure to high intensity UV. After erasure, the EPROM can be reprogrammed.

External Trigger
A signal used to start or stop an event such as a scan of A/D channels. (Also, see Analog Trigger and Digital Trigger).

Fall Time
The time required for a signal to change from a high level (usually 90%) to a low level (usually 10%) of its base line-to-peak or peak-to-peak amplitude. (Also, see Rise Time.)

First-In First-Out Memory. A specialized type of memory buffer. Data is entered on one side of the buffer, is stored for a time, and exits on the other side. The unique function of the FIFO is that the data order is preserved, and data leaving the memory leaves in the same order it arrived. FIFO buffers are used extensively between A/D converters and the PC bus.

Flash ADC
An extremely high-speed analog-to-digital converter. The output code is determined in a single step by comparing the input signal to a series of references via a bank of comparators.

A condition in which there is no electrical connection between the ground of the signal source and the measurement system.

An operation that is directly controlled by the active software application. (see Background.) When a foreground operation is running, no other foreground operations can be active in that window.

A factor by which a signal is amplified, typically expressed in terms of “times” a number. Examples include X10 and X2 where the signal amplitude is multiplied by factors of 10 and 2, respectively.

Gain Error
A measure of difference between the actual gain of an amplifier and the specified gain.

A signal that, depending on the condition, either enables or disables an operation to occur.

Glitch Energy
Glitches are undesirable transients that occur as the output of a digital-to-analog converter moves from one value to another. The glitch energy provides a measure of the magnitude of the glitch, both in amplitude and in duration.

General Purpose Interface Bus, originally developed by Hewlett-Packard and designated HP-IB. Also known as IEEE-488, this bus is an industry standard used to interface a wide assortment of instruments to computers.

Graphical User Interface. A computer user interface in which the user interfaces with a computer via simple graphic displays or icons rather than text. GUIs often resemble common objects (e.g., stop signs) and are frequently developed to provide a “virtual” representation of a variety of instrumentation and data acquisition objects.

A reference potential in an electrical system.

IEEE 488

The ratio of the voltage across a device or circuit to the current flowing in it. In AC circuits, the impedance takes into account the effects of capacitance and inductance. In most data acquisition specifications, the impedance listed is actually the DC impedance, which is the same as the resistance (in ohms).

Integral NonLinearity. A measure of the worst-case deviation of a data converter's transfer function from an ideal straight line with offset and gain zeroed out.

Input Bias Current
The undesirable current that flows into or out of the inputs of an analog input device or system.

Input Impedance
The impedance between the input terminals of a circuit. Although impedance implies AC characteristics that are affected by inductance and capacitance, most data acquisition specifications simply list the DC component of resistance (see Impedance).

Input Offset Current
The difference in the input bias currents of the two inputs an analog input device or system.

Instrumentation Amplifier
An amplifying circuit whose output voltage with respect to ground is proportional to the difference between the voltages at its two inputs.

Integral Control
Integral control, the “I” in PID Control, is a control action that balances supply to a process with the load on it. In so doing, it keeps the measured variable at the desired point and error at zero. The integral function acts on the error signal multiplied by the time it would take to reset the error to zero. If the error changes, a larger or smaller integral control signal is created over time to return the error signal to zero. Integral control (m(e) = I?edt) can be used alone or in conjunction with Proportional and/or Derivative control actions.

Integrating ADC
A slow but highly accurate and noise-immune analog-to-digital converter.

Internal Trigger
A condition or event that serves to start an operation or function. (See External Trigger.)

A hardware interrupt is a signal on a computer bus requesting that the CPU suspend its current task, perform another task, and then return to the original task or program. Interrupts alleviate the need to continuously poll a device to determine if a task needs to be performed.

Interrupt Level
A priority hierarchy that allows the computer to prioritize which Interrupt to service first when more than one Interrupt request has been issued.

Interrupt Service Request (IRQ)
A hardware notification to the CPU that an interrupt has been generated and CPU control should be transferred to the corresponding Interrupt Service Routine.

Interrupt Service Routine (ISR)
A software program that is engaged by an interrupt. This program typically performs a specific time-sensitive function and then returns control of the computer to the original application or program.

Input/Output. The transfer of data to/from a computer system involving communications channels, operator interface devices, and/or data acquisition and control interfaces.

Two circuits or devices are isolated when there is no electrical connection between them. Isolated circuits can be interfaced to each other via optical or electromagnetic circuits. A signal source is sometimes isolated from the measurement device in order to provide protection to the measurement device.

Isothermal Block
A block of thermally conductive material (typically copper or aluminum) that is used to help ensure that all contacts or screw terminals on a board are maintained at the same temperature. Isothermal blocks are very useful in minimizing cold-junction errors in thermocouple measurements.

k or K
Kilo, the metric prefix for 1,000. When used with units of measure such as volts, amps or hertz, the “k” is typically not capitalized and stands for exactly 1000. When used to describe memory size (e.g., Kilobytes or Kbytes), the “K” is usually capitalized and actually stands for a factor of 1024 (e.g., 1 Kilobyte is 1024 bytes).

A data transfer specification equal to 1024 bytes/sec.

The measure of a device's transfer function relative to a perfect Y = mX straight-line response.

A device in a GPIB system addressed by the controller for ‘listening’ or receiving data. (See also Talker.)

Least Significant Bit.

An abbreviation for the Low-power Schottky-clamped TTL logic family.

Mega, the metric prefix for 1,000,000. When used with units of measure such as volts, amps or hertz, the “M” stands for exactly 1,000,000(106). When used to describe memory size (e.g., Megabytes or Mbytes), the “M” actually stands for a factor or 1,048,576 (220).

Milli is the metric prefix for one-thousandth
(1/1000). Thus, mA is thousandths of an amp.

A data transfer specification equal to 1,000,000 bytes/sec.

Mercury Wetted Relay
A relay in which the contact surfaces are actually coated by a film of liquid mercury. This wetting virtually eliminates the contact bounce associated with dry relays as well as often offering lower “on” resistance and longer contact life.

A switch that allows one of multiple inputs to be selected and connected to a single output. Multiplexers are commonly used in DAQ products to allow a single A/D converter to acquire data from multiple analog input channels.

See Multiplexer.

An undesirable electrical signal.

Nyquist Theorem
A sampling theory law that states that to create an accurate digital representation of a sampled waveform you must sample the waveform, at least twice as fast as the highest frequency component contained in the waveform. Note that this is a minimum condition. In most applications, it is preferable to sample at a minimum of 3 to 4 times the highest expected frequency component.

An abbreviation for an OLE Custom Control.

Object Linking and Embedding
A protocol that allows multiple applications to seamlessly interact. Based on the Microsoft’s Component Object Model (COM), OLE allows compatible objects to operate in a variety of applications and environments.

OLE Control
See ActiveX Controls. See also Visual Studio .NET Components and Controls.

Optical Isolation
The use of one or more light transmitters and receivers (typically in the form of an LED and a photo detector) to transfer digital signals between devices or systems without any electrical connection between them.

Output Settling Time
The time required for an analog output to change and stabilize at its final value. Final value is specified as a range in LSB or % of full scale.

Output Slew Rate
The rate of change of analog output voltage as it changes from one output voltage to another. Slew rate is typically specified in volts per microsecond.

Pacer or Pacer Clock
An on-board or external timing source that sets the timing for events such as analog-to-digital and digital-to-analog conversions.

Parts Per Million (PPM).
A relative term typically used in drift specifications. One part per million is equivalent to 0.0001%

A credit-card-sized expansion card that fits in a PC-CARD slot, often referred to inaccurately as a PCMCIA card.

A very high-performance expansion bus architecture developed by Intel to replace ISA and EISA. It has achieved widespread acceptance as a standard for PCs. It supports a theoretical maximum transfer rate of 132 Mbytes/s.

The Personal Computer Memory Card International Association. An international standards body and trade association that was founded in 1989 to establish standards for PC-CARD expansion cards used primarily in laptop computers. (See also PC-Card.)

Photoelectric Sensor
An electrical device with an output related to the level of light falling on it.

PID Control
A three-term control algorithm where the manipulated variable m is the sum of proportional, integral, and derivative control actions (m(e) = P(e)+Iƒedt+Dde/dt). See proportional control, integral control, and derivative control.

Plug and Play
Describes plug-in boards that are fully configured in software, without the need for jumpers or switches on the boards.

A communications connection of one or more inputs on a computer. Common port types include RS-232 and USB.

The technique used on a DAQ board to acquire a specified number of samples after trigger conditions have been met.

A continuously-adjustable variable resistor. They are used for adjustment of electrical circuits and as transducers for either linear or rotary position transmission.

A technique used on a DAQ board in which a buffer is continuously filled with data. When the trigger occurs, the sample includes one buffer full of samples immediately prior to the trigger. For example, if a 1,000 sample pre-trigger buffer is specified, and 20,000 post trigger samples, the final sample set has 21,000 samples in it, 1,000 of which were taken prior to the trigger event.

Programmed I/O
A data transfer method where the data is read or written by the CPU.

Propagation Delay
The amount of time required for a signal or disturbance to pass through a circuit or process.

Proportional Control
A control in which the amount of control action is determined by an error signal multiplied by a gain. The gain term may be specified directly or as the reciprocal of a theoretical band over which error causes a 100% control action (Analog algorithm is m(e) = P(e).) (See also PID, Integral and Derivative controls.)

Quantization Error
The uncertainty that is inherent when digitizing an analog value due to the finite resolution of the conversion process.

Real Time
A method of operation in which data is processed as it is acquired instead of being accumulated and post-processed. Process control is generally done in real time where data analysis is not.

Relative Accuracy
A measure of accuracy (typically in LSBs) of an A/D converter. It includes all non-linearity and quantization errors. It does not include gain and offset errors of the measurement circuitry. As a measurement, it is the deviation of the measured data from the ideal data relative to a straight line drawn through the measured endpoints.

Repeat Instruction Word: A processor-level command used to move data rapidly to memory over the bus. Although this method is less efficient than DMA (because it uses the processor), it is faster than DMA.

Reset Control
See Integral Control.

The smallest increment that can be detected by a data acquisition or measurement system. Resolution is expressed in bits, in proportions, or in percent of full scale. For example, if a system has 12-bit resolution, it equals one part in 4,096 for resolution, or 0.0244 percent of full scale.

Ribbon Cable
A flat cable in which the conductors lie side-by-side.

Rise Time

The time required for a signal to change from a low level (usually 10%) to a high level (usually 90%) of its base line-to-peak or peak-to-peak amplitude. See also Fall Time.

A value that is equal to the root-mean-square (RMS) of the input signal. It applies to all input waveforms that have components within a specified frequency range and within a crest factor limit.

Resistance Temperature Detector. A sensor probe that measures temperature based on changes in resistance.

Sample Rate
The rate at which a signal or value is sampled. It is frequently expressed as samples/sec (S/s), kilosamples/sec (kS/s), or megasamples/sec (MS/s).

A scan is a group of channels sampled in sequence. Often, the sequence is repeated. For example, a scan of channels 0 through 3 samples those four channels. If more than four samples are requested, then the fifth sample will contain data from channel 0, and so on. Scans are generally sequential. Systems that have a channel/gain queue may be used to create scans that are not sequential.

Scan Rate
The rate at which a group of channels is sampled, measured from scan to scan. For example, if 10 channels are in a scan, and a sample rate of 1,000 samples per second is specified, the scan rate is 100 scans per second.

A high-speed DMA burst-mode transfer method.

Standard Commands for Programmable Instruments . An extension of the IEEE 488.2 standard defining standard programming commands and syntax for device operations.

See Single-Ended Input.

A feature of a DAQ board that uses a stable on-board voltage reference and calibrates its own A/D or D/A circuits without need for manual adjustments.

A device that responds to stimulus such as temperature, light, sound, pressure, motion, or flow and produces an output that can be measured to learn about the particular condition being monitored.

Settling Time
The time required for a voltage to stabilize at its final value (usually within a specified error range).

Sample-and-Hold. A circuit that acquires and stores a signal (e.g., an analog voltage) on a capacitor or other storage element for a period of time.

Simultaneous Sample-and-Hold. A mechanism whereby multiple signals can be sampled simultaneously. Often constructed using multiple sample and hold amplifiers that are successively sampled by a single A/D converter until all channels have been sampled. Since the dramatic fall in the cost of A/D converters, it is now more common to have one A/D converter per signal (channel).

Single-Ended Input
An analog input having an input terminal that is measured with respect to a common reference, usually analog ground. It has two input connections, one for the signal being measured and one for the common reference. In multiple input configurations, all signal inputs share the common reference. Input systems that use analog ground as the reference are called Referenced Single-Ended Inputs. Systems that allow an arbitrary reference within the input common-mode range are known as Non-Referenced Single-Ended Inputs. (See also Differential Inputs as a contrasting input type.)

Slew Rate
The specified (typically maximum) rate of change of a D/A converter or amplifier/buffer output. It is expressed in volts/microsecond.

S/N or SNR
Signal-to-Noise Ratio. The ratio of the overall signal level to the noise level, typically expressed in dB.

Software Trigger
An event that is started (triggered) based on software control.

Single-Pole, Double-Throw. A switch or relay configured so that one terminal can be connected to either of two other terminals.

Samples Per Second.

Strain Gauge
A sensor with resistance that varies based on being either stretched or compressed. When attached to a solid object with known physical properties, the resultant deflection signal can be converted to units measuring force.

A group of software instructions separate from the main program that executes a specific function upon command and then returns control to the main program.

Successive-Approximation A/D Converter
An ADC that sequentially compares a diminishing series of binary-weighted values generated by a D/A converter against the analog input.

A timing configuration in which events occur in step with a reference clock or timer.

System Noise
A measure of residual noise of a circuit that is not related to the input.

A GPIB device that sends data to one or more listeners on the bus.

Temperature Coefficient
The change of value or function corresponding with a change in temperature. This is often expressed as a percentage of reading per degree or in PPM (parts per million) per degree.

Track-and-hold. A circuit that tracks a signal and holds the present value on command. Similar to sample-and-hold.

Total Harmonic Distortion. The ratio of the total signal generated by harmonic distortion to the overall signal, expressed in dB or percent.

A type of resistive temperature sensor. The thermistor resistance changes as a function of temperature.

A temperature sensor made by fusing together dissimilar metals. The junction produces a small voltage (referred to as the Seebeck voltage) that varies as a function of temperature.

Throughput Rate
The rate that data, measured in bytes per second or words per second, can be continuously acquired or output.

See Sensor.

A signal that is used to start or stop an operation. Triggers can be an analog, digital, or software event.

Trigger polarity
Trigger polarity defines whether the trigger occurs when the signal is rising in a positive direction or falling in a negative direction.

Universal Asynchronous Receiver/Transmitter. An IC that converts parallel data to serial data and vice versa. UARTs are the backbone of virtually all RS-232/485 computer interface boards.

A signal range from ground to a positive value (e.g., 0 to +5 V).

Universal Serial Bus. A high-speed serial bus.

Virtual Instrument
A combination of hardware and software elements that emulates a stand-alone instrument both in electrical function and in the computer screen representation.

Visual Studio .NET Components
A software program written in conformance with the standard for creating a Visual Studio .NET component. Components are non-GUI (not visible on the Form) software functions that may be used in Visual Studio projects.

Visual Studio .NET Controls
A software program written in conformance with the standard for creating a Visual Studio .NET component. Controls are software functions that may be used in Visual Studio projects and have a visible element that can be seen on the project Form.

Watch-Dog Timer
A hardware or software count-down timer that must be repeatedly reset at regular intervals by an operating program to prevent time-out. If the timer is not reset in time (meaning the operating program has stalled or has failed in some way), the timer times-out and alerts the system and/or operator of the failure.

White Noise
Noise with the same power spectral density at all frequencies.

The number of bits that a processor manipulates at one time. Microprocessors use 8-, 16-, or 32-bit words. However, in common usage, word refers to 16-bits, byte to 8-bits, and double-word to 32-bits.

Working Voltage
Maximum working voltage - Maximum working voltage is the highest voltage that should be applied to a device in normal use. The working voltage is normally well below the breakdown voltage for safety margin. The maximum working voltage is the sum of the actual signal voltage and the common mode voltage and is expressed in volts.