Glossary, Data acquisition glossary, Accuracy – Measurement Computing DataShuttle User Manual
Page 123: Alias frequency, Analog, Analog to digital converter, Ascii, Byte, Cold junction, Common mode
DataShuttle and DynaRes
11-12-01
Glossary - 1
Glossary
Data Acquisition Glossary
Accuracy
In data acquisition applications, accuracy refers to how close reported data is to actual data. The difference
in reported and actual data is known as the error. Data acquisition error has many components, including
offset, gain, drift (repeatability), linearity, and noise. Reducing any of these error influences increases the
accuracy.
Alias Frequency
An erroneous frequency constructed from sampled data that was measured too slowly (slower than the
actual frequency).
Analog
A quantity that has a continuous range of values between the highest and lowest. Temperature and length
are analog quantities. See “Digital.”
Analog to Digital Converter
An electronic circuit that converts an analog quantity to a digital representation of it.
ASCII
American Standard Code for Information Interchange. The standard file format for alphanumeric files (text,
numbers, and special symbols). ASCII files are readable by virtually all word processing, spreadsheet, and
database programs.
Bit
Binary Digit. The smallest amount of data that is stored or processed by a computer (0 or 1).
Byte
A sequence of eight bits. A byte is the most common unit of data storage.
Cold Junction
The reference junction of a thermocouple. A thermocouple generates a voltage between two junctions where
two dissimilar metals are in contact. The temperature of one of the junctions must be measured to calculate
the absolute temperature of the other. This is cold junction compensation.
Common Mode
The average of the voltage applied to both wires of a two wire (differential) input.
Common Mode Rejection
A measure of how closely an A/D circuit measures identical common mode voltages generated by dissimilar
differential voltages. For example, a 4 and 6 volt differential input and a 3 and 7 volt differential input both
have a common mode of 5 volts. Any measurement deviation from 5 volts is common mode error. The
metric for measuring common mode error is the common mode rejection ratio (CMRR). A low CMRR
indicates better common mode rejection than a high CMRR.
Current Loop
A technique for transmitting analog data accurately over two wires. The measured signal is the value of the
changing current. The signal source or transmitter is powered from the signal current.