Data Acquisition Glossary

A - B - C - D - E - F - G - H - I - J - K - L - M - N - O - P - Q - R - S - T - U - V - W - X - Y - Z

A (return to top)

accelerometer: a sensor that responds to acceleration, often using a piezo crystal that generates a voltage when distorted
accuracy: a stated maximum deviation from the NIST standard
acquisition speed: analog-to-digital conversion speed
active filter: a filter that incorporates active components, usually as amplifiers, so that it more closely approximates an ideal filter
actuator: a device that converts an electrical signal into a physical quantity
A/D conversion: analog-to-digital conversion
ADC: analog-to-digital converter
alias frequency: a false signal component in a stream of data corrupted by sampling frequencies above Nyquist
aliasing: the presence of alias frequencies in acquired data
analog-to-digital conversion (A/D conv.): sampling voltage at a given moment and digitizing the value
analog-to-digital conversion time: the time taken to convert one sample, the inverse of the sampling rate (acquisition speed), with 4 microseconds (time), for example, corresponding to 250k samples per second (rate).
analog-to-digital converter ( ADC): hardware component that samples voltage and digitizes the measured value
antialiasing filter: analog filter employed to remove frequencies above Nyquist, because these high-frequency signal components corrupt acquired data and, although digital filters perform better than analog filters at many tasks, digital processing cannot remove frequencies above Nyquist

B (return to top)

bipolar: either terminal may have a higher voltage than the other
bridge: a resistor bridging the junctions of two pairs of voltage-dividing series resistors, and that carries no current when the two pairs divide the voltage equally
buffer: temporary storage for data values in a process - with the additional sense, in hardware, of isolation or protection
burst mode: processing at high speed, usually to or from a buffer

C (return to top)

calibration: adjusting equipment to improve its accuracy
calibration period: the interval between calibrations, usually one year for data acquisition equipment
channel: a signal or data path
CJC: cold-junction compensation
CMR: common mode rejection
CMRR: common mode rejection ratio
cold junction: the constant-temperature reference junction of a thermocouple, classically in water kept at freezing point by thawing ice
cold-junction compensation (CJC): circuitry that allows for an ambient-temperature reference junction
common mode input voltage: how far signal ground may differ from system ground for differential inputs
common mode rejection (CMR): the rejection of a common mode signal, often noise, applied equally to both inputs and that ideally results in no differential signal
common mode rejection ratio (CMRR): a measure of common mode rejection, and therefore, ideally, of the ability to reject noise
common-mode signal: a signal applied simultaneously and equally at both differential inputs
continuous acquisition: acquiring data without having to pause to prevent buffer overflow
conversion rate: the inverse of conversion time, with a conversion rate of 250k samples per second corresponding to a conversion time of 4 microseconds, for example
conversion time: analog-to-digital conversion time or digital-to-analog conversion time
counter: a circuit that counts pulses; also, in software, the address of a count
counter/timer Input: a digital input that channels a signal to high-speed hardware that counts pulses or measures frequencies
cut-off frequency: the highest frequency that can pass through a filter without attenuation

D (return to top)

D/A conversion: digital-to-analog conversion
DAC: digital-to-analog converter
data acquisition: converting an analog signal into a time series of digital values, with each value recording the signal voltage at that instant
more data acquisition terms...
data reduction: summarizing a stream of data, often in real time, to extract useful information - such as blocks of data surrounding peaks or triggers
datalogger: a device that records acquired data for later collection
differential amplifier: a cicuit that amplifies the difference between two inputs
differential inputs: a signal input and its individual ground
digital signal processing (DSP): processing analog signals in the digital domain: performing mathematical operations on digitized samples of a signal to achieve results -- filtering, phase-shifting -- previously obtainable only by passing an analog signal through special circuits
digital-to-analog conversion (D/A conv.): recreating an analog signal from a series of digital values
digital-to-analog converter (DAC): a device that creates an analog signal level from a digital value
DMA: direct memory access: a data transfer between memory and a peripheral that bypasses the CPU
DSP: digital signal processing
dynamic range: the ratio of full-scale to the smallest measurable unit: 96dB in a 16-bit instrument (6dB per bit of resolution)

E (return to top)

event counter: a circuit that counts pulses
excitation: a voltage or current required to operate actuators and some sensors
external trigger: a pulse on a control line that triggers sampling on other channels

F (return to top)

FFT (fast Fourier transform): a transform of signal data from time to frequency domains: an algorithm that analyzes a finite time series of data points into component frequencies
filtering: attenuating unwanted frequency components of a signal
frequency counter: a circuit that counts pulses within a specified time, the gate time, often one second or less

G - H - I - J - K (return to top)

gain: amplification factor
hardware trigger: an external trigger; see also 'software trigger'
IA: instrumentation amplifier
instrumentation amplifier (IA): a high-impedance differential amplifier with high common-mode rejection
isolation amplifier: an instrumentation amplifier with electrically isolated inputs

L - M - N (return to top)

LSB: Least Significant Bit: the rightmost bit, bit zero, with weight 2 to the zeroth power (equal to one), in a binary representation of a value
LVDT: Linear Variable Differential Transformer: a sensor that requires AC excitation of a primary coil to output AC from a secondary coil with amplitude that varies with position of a movable core
MSB: Most Significant Bit: the leftmost bit, bit N-1 in an N-bit binary representation, with weight 2 to the (N-1)th power
Nyquist frequency: half the sampling rate, the frequency above which data acquisition fails because samples of higher frequency signal components appear in the data stream as if sampled from lower frequency components that do not, in fact, exist: alias frequencies

O - P - Q (return to top)

offset: an added constant, a zero-correction
optical isolation: optical signal transmission between electrically isolated networks
PID: Proportional Integral Derivative control algorithm that moderates a proportional gain with integral and derivative time factors to hold a measurement or process variable at or near a set point
programmable gain: amplification of a signal by specific gains under software control
quadrature encoder: circuit emitting two pulse streams generated by a rotating shaft that another special circuit can decode to reveal shaft position and direction of rotation

R (return to top)

real-time processing: processing a stream of data during acquisition: scaling, transforming, reducing, filtering or performing other operations on data, rather than simply streaming unprocessed data to disk for subsequent analysis
resolution: a measure of the least possible change in relation to full scale: 10000/4096=2.44mV, for example, at 12-bit resolution when 10V represents full scale

S (return to top)

sample and hold (S/H): storing a sampled analog voltage on a capacitor for a short period
sample rate: the rate of sampling in samples, kilosamples, or megasamples per second
sampling: taking the value of signal, usually as a voltage, and then, typically, digitizing it
scaling: applying a factor and an offset to a signal
sensitivity: minimum detectable signal change
sensor: a device that converts a physical quantity into an electrical signal
set point: value of a variable that a controller maintains by responding to errors
settling time: the time for an applied change in input voltage to result in a changed output voltage (stable within specified limits)
signal: information carried in some medium, often a waveform
signal conditioning: preparing an analog signal before digitizing it: amplifying the signal, removing frequencies above half the sampling rate (the Nyquist frequency), linearizing it, and so on, although digital processing now can perform linearization and other traditional signal conditioning tasks more effectively than analog circuitry
signal-to-noise ratio (SNR): rms of signal to rms of noise, expressed in decibels
simultaneous sample and hold: using a clock to strobe several channels into sample and hold circuits
simultaneous sampling: digitizing the values from simultaneous sample and hold operations
single-ended input: a signal channel that shares a common ground with one or more other signal channels
slew rate: maximum rate of change of output voltage from an op amp
software trigger: real-time processing and analysis of one or more signals to determine some action by the system
strain gauge: deformation-sensitive resistor bonded to material under test, part of a bridge circuit
successive approximation: digitizing an analog voltage in n comparisons (for n-bit resolution) as though determining the weight of an object with a balance and a decreasing series of binary weights

T (return to top)

time stamp: a count attached to a data item to record the units of time elapsed since a reference time
transducer: a sensor or actuator
trigger: a hardware control signal, or real-time processing and analysis of one or more signals by software, that triggers some system action

U - V - W - X - Y - Z (return to top)

unipolar: one of the terminals, the positive terminal, never has a lower voltage than the other