NI High-Speed Digitizers Help (NI-SCOPE)

Edition Date: January 2017

Part Number: 370592AB-01

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Symbol Prefix Value
p pico 10 -12
n nano 10 -9
µ micro 10 -6
m milli 10 -3
k kilo 10 3
M mega 10 6
G giga 10 9
T tera 10 12


AC alternating current
AC-coupled Allowing the transmission of AC signals while blocking DC signals.
AC coupling cutoff The boundary between DC and AC signals.
accuracy A measure of the capability of an instrument or sensor to faithfully indicate the value of the measured signal. This term is not related to resolution; however, the accuracy level can never be better than the resolution of the instrument.
active probe An oscilloscope or instrument probe with built-in, active electronics. The built-in electronics are used to condition the signal—for example, to give a well-defined impedance or to connect the probe to a very high impedance circuit, whose signal would be degraded if transmitted over a longer signal lead. Active probes are especially used with high-frequency oscilloscopes, logic analyzers, and RF instruments.
ADE, application development environment A software environment incorporating the development, debug, and analysis tools for software development. LabVIEW, Measurement Studio, and Visual Studio are examples.
AGCautomatic gain control—Applies gain to small signals to increase their bit resolution, so the signals can be processed by the DDC's resampling/interpolating halfband filters and output formatter.
amplitude The voltage amplitude of a signal. When speaking of the amplitude of a signal, it is usually assumed the rms value for an AC signal. However, amplitude can also refer to the instantaneous amplitude, or the peak, peak-to-peak, or average amplitude, if so specified.
API application programming interface
  1. A standardized set of subroutines or functions along with the parameters that a program can call.
  2. A set of functions exported by a library.
  1. Hardware—A property of an event that occurs at an arbitrary time, without synchronization to a reference clock.
  2. Software—An action or event that occurs at an unpredictable time with respect to the execution of a program.
attenuation The reduction of a voltage or acoustical pressure. Measured referenced to the original voltage.
attenuator An electronic device that reduces (attenuates) a voltage by a given amount. Attenuation is normally given in dB.
averaging Averaging is a technique to improve frequency domain analysis accuracy. The principle is to average continuous frequency spectrums calculated out of continuous time domain waveform records. Averaging reduces noise and enhances frequency domain features for a tested system over time. Different averaging modes and averaging weighting are normally used in signal processing. For more information on the basic concepts of averaging, refer to Improving Accuracy through Averaging at


backplane An assembly, typically a printed circuit board (PCB), with 96-pin connectors and signal paths that bus the connector pins.
bandwidth The range of frequencies present in a signal, or the range of frequencies to which a measuring device can respond.
bit One binary digit, either 0 or 1.
block diagram A pictorial description or representation of a program or algorithm. In LabVIEW, the block diagram consists of executable icons called nodes and wires that carry data between the nodes; the block diagram is the source code for the VI.
buffer Temporary storage for acquired or generated data (software).
bus The group of conductors that interconnect individual circuitry in a computer. Typically, a bus is the expansion vehicle to which I/O or other devices are connected. Examples of PC buses are the PCI bus, AT bus, and EISA bus.
byte Eight related bits of data, an 8-bit binary number. Also used to denote the amount of memory required to store one byte of data.


calibration A means of verifying and adjusting the accuracy of a device.
conventional mode For the NI 5911 and NI 5620/5621, an operating mode in which the digitizer is used like a conventional oscilloscope.
CMRR common-mode rejection ratio—a measure of the capability of an instrument to reject a signal that is common to both input leads. For instance, if you measure a thermocouple in a noisy environment, the noise from the environment appears on both input leads. Therefore, this noise is a common-mode voltage signal that is rejected by an amount equal to the CMRR of the instrument. The CMRR is defined by the following equation:

CMRR = 20 log(Differential Gain/Common Mode Gain)

The ratio is important because it indicates how much of the common mode signal appears in your measurement. The value of the CMRR depends on signal frequency as well and must be specified as a function of frequency. An equivalent equation to represent CMRR is as follows:

20 log(Measured Common Voltage/Applied Common Voltage)

coercion The automatic conversion LabVIEW performs to change the numeric representation of a data element.


data transfer A technique for moving digital data from one system to another.
DC Direct current
DC-coupled Allowing the transmission of both AC and DC signals.
DDC mode An operating mode in which certain digitizers can be used as a digital downconverter, allowing you to zoom in on data. This mode reduces the amount of data transferred into memory.
DDCdigital downconverter—a digital signal processing (DSP) chip that mixes, filters, and decimates sampled data. In essence, the DDC allows you to zoom in on a band of frequencies much narrower than the Nyquist band of the ADC. The lower sample rate means that signals of longer duration can be stored in the same amount of memory. For spectral analysis, a smaller, faster FFT can be used to look at only the band passed through the DDC.
dead time A hardware specification relevant to multiple record acquisitions. Dead time is the total time between records that the digitizer is not acquiring. This time is required for the digitizer hardware to set up for the next record. For SMC-based digitizers, it is the time from the last post-reference trigger sample of the previous record to the first minimum pre-reference trigger sample leading into the next record.
digital trigger A TTL level signal having two discrete levels—a high and a low level.
DLL dynamic link library
driver Software that controls a specific hardware device, such as DAQ devices, GPIB interface devices, PLCs, RTUs, and other I/O devices.
dynamic range The dynamic range of a signal is the ratio of the strongest, or loudest part to the weakest, or softest, part of the signal, expressed in dB.


EEPROM electronically erasable programmable read-only memory—ROM that can be erased with an electrical signal and reprogrammed.
equivalent-time sampling (ETS) The process of repeatedly sampling a repeating analog signal at different times in the waveform to get a high-time resolution (effective sampling rate). See also RIS.
extrema The minimum and maximum values.


filtering A type of signal conditioning that allows you to attenuate unwanted portions of the signal you are trying to measure.
foldover frequency One half of the sample rate.
frequency The basic unit of rate, measured in events or oscillations per second using a frequency counter or spectrum analyzer. Frequency is the reciprocal of the period of a signal.
fundamental frequency The intended frequency of operation.


gain The factor by which a signal is amplified, often expressed in dB. Gain as a function of frequency is commonly referred to as the magnitude of the frequency response function.
gain accuracy A measure of deviation of the gain of an amplifier from the ideal gain. Also called gain error.
GPIB General Purpose Interface Bus—Synonymous with HP-IB. With the spin-off of the instrument division of Hewlett Packard, the term HP-IB is no longer used. The standard bus used for controlling electronic instruments with a computer. Also called IEEE 488 bus because it is defined by ANSI/IEEE Standards 488-1978, 488.1-1987, and 488.2-1987.


harmonic In a complex signal, a component whose frequency is a multiple of the fundamental frequency.
hysteresis Lag between making a change and the effect of the change.


interrupt A computer signal indicating that the CPU should suspend its current task to service a designated activity.
interrupt latency The delay between the time hardware asserts an interrupt and when the interrupt service routine is activated.
ISA Denotes a common expansion bus used in PCs: the 8-bit and 16-bit bus design featured in the IBM PC/AT.
IVI Interchangeable Virtual Instrument—A software standard for creating a common interface (API) to common test and measurement instruments.


jitter The rapid variation of a clock or sampling frequency from an ideal constant frequency.


LabVIEW Laboratory Virtual Instrument Engineering Workbench—A program development application based on the programming language G and used commonly for test and measurement purposes.
library A file containing compiled object code modules, each comprised of one of more functions, that can be linked to other object code modules that make use of these functions.
loading The effect of connecting the output of one electronic device to the input of another, such as the connection of a unit under test to an instrument. The loading effect is the difference in the output value (for example, voltage) with and without the load. To prevent loading effects, the impedance of the load must be smaller than the output impedance of the source.
L-Pad Refers to the basic structure of the network: one shunt and one series branch, which gives an arrangement in the shape of an L.


master/slave A type of network connection in which a request is transmitted to one or more destination nodes, and those nodes send a response back to the requesting node. In industrial applications, the responding (slave) device is usually a sensor or actuator, and the requesting (master) device is usually a controller.
measurement The quantitative determination of a physical characteristic. In practice, measurement is the conversion of a physical quantity or observation to a domain where a human being or computer can determine the value.
minimum rearm time A term describing behavior pertinent to multiple record acquisitions, also known as retriggerable acquisitions. When triggering a digitizer as quickly as possible, minimum rearm time is the time from one detected reference trigger to the next when the record length is at minimum (for example, 1 sample). Rearm time is a deterministic hardware specification, but varies based on digitizer configuration.


Nyquist frequency When an analog signal is sampled at a rate more than twice that of its highest frequency component, it can be properly reconstructed when reconverted back to the analog domain. The required sampling rate is called the Nyquist frequency.


offset The unwanted DC voltage due to amplifier offset voltages added to a signal.
offset error A constant error added to a measurement along the whole transfer curve. For example, in mx+b, the offset error is b.
ohm The measure of resistance. One ohm is the resistance through which one ampere current flows at 1 V.
OSP onboard signal processing
oscilloscope An instrument that displays the time domain waveform(s) of one or more signals.
oversampling factor Multiples of the maximum real-time sampling rate.
overshoot The amount that a square wave or transient signal exceeds the desired level when transitioning from one voltage to another.


passband The range of frequencies that a device can properly propagate or measure.
PCI peripheral component interconnect—A high-performance expansion bus architecture originally developed by Intel to replace ISA and EISA; it is a standard for PCs and workstations and offers a theoretical maximum transfer rate of 132 Mbytes/s.
peak-to-peak A measure of signal amplitude. The difference between the highest and lowest levels of the signal.
PFI programmable function input
PLL phase-locked loop—An electronic circuit which forces an output frequency to be locked to the same phase as a reference frequency.
posttriggering The technique used on a device to acquire a programmed number of samples after trigger conditions are met.
pretriggering The technique used on a DAQ device to keep a continuous buffer filled with data, so that when the trigger conditions are met, the sample includes the data leading up to the trigger condition.
PXI PCI eXtensions for Instrumentation—A modular, computer-based instrumentation platform.


quantization noise Noise introduced when a signal is digitized. Digitized signals always contain quantization noise because the resolution of the ADC is finite.


range The minimum and maximum analog signal levels that the ADC can digitize.
record A collection of samples acquired relative to a trigger. For example, if a single trigger occurs and you acquire 1,000 samples, you have a single record of 1,000 samples. You can also perform multirecord acquisitions with multiple triggers. Each trigger initiates the acquisition of a record, provided that the trigger is correctly recognized by the device.
resampler/halfband filterA polyphase filter that allows the output sample rate to have a non-integer relationship to the input sample rate. In essence, it acts as a fixed interpolation filter followed by an NCO controlled decimator.
resolution The smallest signal increment that can be detected by a measurement system; resolution can be expressed in bits, in proportions, or in percent of full scale. For example, a system has 12-bit resolution, one part in 4,096 resolution, and 0.0244 percent of full scale.
RIS random interleaved sampling—A method of increasing effective sample rate by repetitively sampling a waveform.
rms root mean square—A measure of signal amplitude. rms refers to the most common mathematical method of defining the effective voltage or current of an AC signal.
RTSI real-time system integration bus—The NI timing bus that interconnects data acquisition devices directly by means of connectors on top of the devices for precise synchronization of functions.


S sample
s seconds
sample Single analog or digital input or output data point.
sample rate Time interval between observations in a periodic sampling control system.
SNR signal-to-noise ratio; the ratio of total signal to noise expressed in decibels (dB). The larger the number, the better. SNR is calculated by SNR = 20 log (SignalRMS / NoiseRMS). This can be a peak, rms, or other amplitude that appropriately characterizes the data.
S/s samples per second. Expresses the rate at which a device measures a signal.
  1. Hardware—A property of an event that is synchronized to a reference clock.
  2. Software—A property of a function that begins an operation and returns only when the operation is complete. A synchronous process is, therefore, "locked" and no other processes can run during this time.


TDC time-to-digital conversion
timing NCOThe timing NCO provides the sample pulse and the associated phase information used in the Resampler/Halfband filter (NI 5620/5621 only).
throughput rate The data, measured in bytes/s, for a given continuous operation, calculated to include software overhead.
time stamps The use of a free running clock to accurately record the time of an event.
TIS time interleaved sampling
transfer rate The rate, measured in bytes/s, at which data is moved from source to destination after software initialization and set up operations; the maximum rate at which the hardware can operate.
  1. Any event that causes, starts, or stops some form of data capture.
  2. An external stimulus that initiates one or more instrument functions. Trigger stimuli include a front panel button, an external input voltage pulse, or a bus trigger command. The trigger may also be derived from attributes of the actual signal to be acquired, such as the level and slope of the signal.
trigger hysteresis Refer to hysteresis.


V volts


wave A pattern that repeats over time.
waveform A graphical display of a wave. A voltage waveform shows time on the horizontal (x) axis and voltage on the vertical (y) axis.


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