- Alternating Current
- Abbreviation ac. Electric current whose flow alternates in direction. The number of times the current changes direction in one second is called the frequency. The normal waveform of ac is sinusoidal.
- Alumel
- Trade name for an alloy of nickel with up to 5% aluminium, manganese and silicon, used with chromel in K-type thermocouples.
- Ampere
- Abbreviation A. SI unit of electric current.
- Amplifier
- A circuit that produces a larger output power, voltage or current than was applied at its input.
- Amplitude
- The size or magnitude of a signal.
- Analogue-to-Digital (A/D) Converter
- Converts an analogue signal (such as a voltage signal from a temperature sensor) into a digital signal suitable for input to a computer.
- Anti-Alias Filter
- An anti-alias (or anti-aliasing) filter allows through the lower frequency components of a signal but stops higher frequencies, in either the signal or noise, from introducing distortion. Anti-alias filters are specified according to the sampling rate of the system and there must be one filter per input signal.
- Backbone
- The major multi-channel link in a network, from which smaller links branch off.
- Background Noise
- Extraneous signals that might be confused with the required measurement.
- Baud
- The rate of data transmission in serial data communications, approximately equal to one bit per second.
- Chromel
- An alloy of nickel with about 10% chromium, used with Alumel in K-type thermocouples.
- Constantan
- An alloy of 40% nickel and 60% copper, with a high volume resistivity and almost negligible temperature coefficient. Used with copper in T-type thermocouples.
- Current
- Current is often used to transmit signals in noisy environments because it is much less affected by environmental noise pick-up. Before A/D conversion the current signals are usually turned into voltage signals by a current-sensing resistor.
- Differential Inputs
- Reduce noise picked up by the signal leads. For each input signal there are two signal wires. A third connector allows the signals to be referenced to ground. The measurement is the difference in voltage between the two wires: any voltage common to both wires is removed.
- Digital-to-Analogue (D/A) Converter
- Used to produce analogue output signals. These may be control signals or synthesised waveforms.
- Electromotive Force (emf)
- Difference of potential produced by sources of electrical energy which can be used to drive currents through external circuits. Unit is the volt.
- Endurance limit
- In fatigue testing, the number of cycles which may be withstood
without failure at a particular level of stress.
- EIA
- Electronic Industries Association.
- Ethernet
- A local area network to which you can connect data acquisition
devices.
- E-Type Thermocouple
- Chromel-constantan thermocouple with a temperature range of 0 to 800 oC.
- FIFO buffer
- A first in, first out, store. The first value placed in the buffer
(queue) is the first value subsequently read.
- Filtering
- Attenuates components of a signal that are undesired: reduces
noise errors in a signal.
- Frequency
- Measured in hertz (cycles per second), rate of repetition of changes.
- Frequency Counter
- Counts digital pulses over a defined gate time. A typical gate
time is between 0.1 and 10 seconds.
- Front panel
- The front surface of a unit, generally containing switches and
indicator lights.
- Gain
- Amplification of a circuit.
- GPIB
- General Purpose Interface Bus. Also known as IEEE-488 bus. The GPIB standard was designed to connect several instruments to computers for data acquisition and control. Data can be transferred over GPIB at 200 000 bytes per second, over distances of 2 metres.
- Ground
- See earth.
- Hertz (Hz)
- Cycles per second unit of frequency.
- Human machine interface (hmi)
- Also known as man machine interface. The communication between the
computer system and the people who use it.
- IEEE-488 Bus
- See GPIB
- Integration Time
- An integrating A/D converter measures an input voltage by allowing it to charge a capacitor for a defined period. The integration averages the input signal over the integration time, which if chosen appropriately will average over a complete mains cycle thereby helping to reduce mains frequency interference.
- Isolation
- Two circuits are isolated when there is no direct electrical connection between them.
Isolation to Earth or System
A high transient voltage at one input may damage not only the input circuit, but the rest of the data acquisition hardware, and, by propagating through the signal conditioning and A/D circuits, eventually damage the computer system as well. You can prevent this type of damage by isolating the input from the earth of the data acquisition and computer hardware.
Isolation Between Inputs
A transient at an input can also propagate to other equipment connected to that input. This is prevented by providing isolation between inputs.
- J-Type Thermocouple
- Iron-constantan thermocouple with a temperature range of 0 to 750 oC.
- K-Type Thermocouple
- Chromel-Alumel thermocouple with a temperature range of -200 to 1200 oC.
- N-Type Thermocouple
- Nicrosil-Nisil thermocouple with a temperature range of -200 to 1200 oC.
- Resolution
- The resolution of an A/D or D/A converter is the number of steps the range of the converter is divided into. The resolution is usually expressed as bits (n) and the number of steps is 2n (2 to the power n), so a converter with a 12-bit resolution divides its range into 212 or 4096 steps. In this case a 0-10 volt range will be resolved to 0.25 millivolts.
- rms
- Root mean square. The square root of the sum of the squares of a set of quantities divided by the total number of quantities. Used when monitoring ac (alternating current) signals. Many power supplies, for example, issue an ac signal. This needs to be converted to a dc (direct current) signal for the PC interface. The solution is a signal conditioning input that produces a dc signal proportional to the rms of the amplitude of the input signal. The rms operation means the reading will always be positive.
- Settling Time
- When an output voltage swings full-scale through the range of the D/A converter, the settling time tells how long it will take for the output to settle to its new value.
- Signal Conditioning
- Makes a signal suitable for input to an analogue-to-digital converter. For example, a signal may be filtered to remove noise, or amplified to meet the range of the A/D converter.
- Simultaneous Sampling
- When all analogue signals are read simultaneously. This is achieved by providing each input with its own A/D converter, and initiating sampling from a single clock. It ensures that there is no reduction in sampling rate when more signals are connected.
- Slew Rate
- The maximum rate of change of an output signal.
- Strain Gauge
- A device which experiences a change in resistance when it is stretched or strained.
- Thermistor
- A temperature sensor. The name comes from thermal resistor. It comprises a mixture of certain oxides with finely divided copper, of which the resistance is very sensitive to change of temperature.
- Thermocouple
- Popular temperature sensor because of its low cost, versatility and ruggedness. Consists of two different metals joined together, making a continuous circuit. When one junction has a different temperature from the other an electromotive force (voltage) occurs. There are several types of thermocouples, constructed from different metals and with differing temperature ranges and accuracies.
- Transducer
- A device which converts a physical quantity into an electrical signal. Examples include thermocouples and photocells.
- Transient
- A short surge of current or voltage, often occurring before steady-state conditions have become established.
- T-Type Thermocouple
- Copper-constantan thermocouple with a temperature range of -200 to 400 oC.
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