From Measurement Systems and Sensors
11.1 COMPUTER DAQ BOARDS
11.1.1 Structure and Functions of a DAQ Board
Certain measurement tasks cannot be performed better than by means of a computer measurement board. Combined with a computer and software, such measurement boards makes up a virtual instrument. Due to its extensive data acquisition functionality, a computer measurement board is referred to as Data Acquisition (DAQ) card. Card designations often include additional symbols DAQBoard designates a board that is designed for installation inside a computer (pluggable onto the PCI or ISA bus); and DAQ PCcard designates a PCMCIA card. The functions of a measurement card involve much more than just A/D conversion [1, 2]. Modern computer DAQ boards provide the following functions:
Digitize a single signal (voltage or sometimes current) from one of the multiple analog inputs;
Digitize multiple signals received on multiple analog inputs;
Perform antialiasing analog filtration of the input signal;
Set the analog input signal trigger levels and time-outs;
Present the required signals (voltage or current) onto analog outputs through D/A conversion;
Read and send digital signals from/to data inputs/outputs (DIO);
Produce signals of a preset frequency or pulses of a preset duration;
Measure the input signal frequency or pulse duration;
Synchronize with the triggering lines of computer-based RTSI real-time systems (optional);
Store measurement data and configuration settings in the card's memory.
Figure 11.1 shows a functional diagram of a DAQ board. A typical DAQ board consists of an analog multiplexer, a programmable amplifier, a sample-and-hold (S&H) or sample-and-trace (S&T) circuit, an ADC, a DAC,...
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