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
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.
(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
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
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
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.
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.
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
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).
A serial communications data transfer rate that is expressed in bits per second
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
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.
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.
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
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
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.
A system developed to remove the error caused by the thermocouple’s
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
The temporary, undesirable opening and closing of mechanical contacts
that occurs when a relay or switch is closed. (See also Mercury Wetted Relay.)
The number of openings/closures of a relay or switch that can be
expected before failure.
The maximum voltage, current, and/or power capacity of relay or switch
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
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
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.
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
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.
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
Software that controls hardware devices, such as DAQ boards, GPIB. interface
boards, PLCs, RTUs, and other I/O devices.
Digital Signal Processing.
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.
The ratio of pulse width to overall signal repetition period.
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
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.
A signal used to start or stop an event such as a scan of A/D channels. (Also,
see Analog Trigger and Digital Trigger).
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.
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.
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.
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
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.
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
Input Bias Current
The undesirable current that flows into or out of the inputs of an analog input
device or system.
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.
An amplifying circuit whose output voltage with respect to ground is
proportional to the difference between the voltages at its two inputs.
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
A slow but highly accurate and noise-immune analog-to-digital
A condition or event that serves to start an operation or function. (See
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.
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.
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
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
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.
An undesirable electrical signal.
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
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.
See ActiveX Controls. See also Visual Studio .NET Components and Controls.
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
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
An electrical device with an output related to the level of light falling on
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
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
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.
A data transfer method where the data is read or written by the CPU.
The amount of time required for a signal or disturbance to pass
through a circuit or process.
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
The uncertainty that is inherent when digitizing an analog value due
to the finite resolution of the conversion process.
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.
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.
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.
A flat cable in which the conductors lie side-by-side.
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.
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
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
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.
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
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.)
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.
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.
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
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
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.
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.
The rate that data, measured in bytes per second or words per second,
can be continuously acquired or output.
A signal that is used to start or stop an operation. Triggers can be an analog,
digital, or software event.
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.
A combination of hardware and software elements that emulates a stand-alone
instrument both in electrical function and in the computer screen
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
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
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.
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.