There are many different DAQ cards and modules that can be obtained for use with a variety of data acquisition systems.
The DAQ modules and cards can be very specific in their nature, or some can be quite generalised so that the software can be used to configure them in the way that is required.
To understand a little more about the different types of DAQ modules and DAQ cards, it is helpful to understand how the measurements are made.
Functions on DAQ cards, modules & devices
Dependent upon the actual device, whether it is a card / module for a DAQ rack system, or a device that takes in a number of inputs and directly interfaces to a computer, there are a number of common blocks that it will contain.
Effectively the DAQ hardware: module card, device, acts as the interface between a computer and the signals or measurements from the outside world. The data acquisition hardware primarily functions as a device that translates incoming analogue signals into a digital format so that a computer can process them.
The data acquisition device or hardware can generally be split into three main elements:
Signal conditioning: The signal conditioning circuitry takes the input from the sensor and conditions in in a variety of ways dependent upon the type of measurement and the sensor used.
Typical forms of signal conditioning may include amplification, attenuation, filtering incorporation of sensor into a Wheatstone bridge or other input circuitry, differential input, or many other forms of conditioning.
Analogue to digital conversion: This element of the DAQ hardware is probably one of the most crucial as it converts the analogue signal into a digital format.
There are several aspects to the analogue to digital conversion including the sample rate, and the resolution. There are some key trade-offs to be made, and different sample rates and resolutions are needed for different measurements, scenarios and applications.
Computer bus: The computer bus is effectively the output of the data acquisition hardware. The digital data from the analogue to digital converter, A2D will be converted into the right interface format for the interface to the computer for processing. These formats may include everything from PXI and VXI trough interfaces such as PCI to USB, Ethernet, and more. There could even be a proprietary form of interface used.
Whatever the interface used, the data will need to be put into the right format and transferred in an orderly fashion.
The DAQ cards, modules and devices can make any one of a number of different types of measurement. However when the cards are classified, there are three main types of measurements that can be made. The interfaces on the cards, modules and devices need to be able to handle the type of input that may be required of it.
Although there are a number of main types of measurement that can be made, these basic types of measurement can be used with sensors to give derived measurements.
The main types of measurement include:
Voltage: The voltage input is one of the most common and relatively straightforward. There will obviously be signal conditioning and then the digitisation elements, but these inputs can be used for a variety of different measurements - the basic measurement of voltage, but also for any sensors that give a voltage output.
Frequency or time interval
Types of DAQ cards, modules & devices
There are many different types of DAQ hardware cards, modules and devices. Different hardware cards, modules, etc are able to make different types of measurement and they also provide different levels of performance.
Understanding what the measurement is, the conditions under which it is made and the performance required enable the optimum choice to be made.
Applying a high performance specification to the requirement for a card when only a low performance one is needed will add unnecessary cost, whereas under-specifying the card will mean that the measurements made will not fulfil their requirements.
Data acquisition is a very important area of the test and measurement industry. Data acquisition systems are required in many applications from electronics manufacturing to chemical engineering, mechanical manufacture as well as more diverse applications such as monitoring geographical data from mountains and volcanoes as well as many other interesting and diverse uses.
There are naturally many different measurements that can be made by data acquisition systems. Those mentioned above are just a few of the possibilities, but many more exist.
In view of this there are many data acquisition products available on the market that may use data acquisition cards directly included in PXI or VXI chassis, on another form of bus / rack system or in computers. In some instances bus systems may be incorporated into the data acquisition system - GPIB, USB, and RS232 ware widely used. Wireless systems are also used in many applications. With this level of flexibility it is possible to create systems that can be tailored to a given applications to make the measurements that are required.
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