What is Data Acquisition Card?
To measure signals and transfer data to the computer is done by using Data Acquisition Cards, commercially called DAQ cards. These cards have Analog / Digital Converters (ADC) and Digital / Analog Converters (DAC) that allow the input/output of analog and digital signals.
The data acquisition card do the signal conditioning, Analog to digital and Digital to analog data conversion processes.
The figure below shows, typical scheme of a Data Acquisition Card. You can see all the most important components of a Data Acquisition card.
The data is captured by an ADC using a sampling process. Sampling an analog signal requires taking samples of said signal at certain times, discrete times. The frequency in which the signal is sampled is known as Sampling Frequency. The sampling process generates values of the signal at known time intervals.
The sampling frequency determines the quality of the analog signal that is converted. The higher the sampling frequency, the better the conversion of the analog signal will be, the greater the resemblance to the real signal. The minimum sampling frequency required to adequately represent the signal must be at least twice the maximum frequency of the analog signal to be converted (Nyquist Principle). As shown below:
An example of how the converted signal is altered based on the sampling frequency is shown. It can also be seen that if the signal is sampled at a Sampling Rate less than twice the maximum frequency of the signal, the Aliasing effect occurs.
Analog / Digital Converters (ADC)
Once the signal has been sampled, your samples need to be converted to digital code. This process is called Analog / Digital conversion. The figure below shows the conversion:
Most cards also have a multiplexer that acts as a switch for the different ADC channels. This makes it possible to capture different analog signals in parallel, the drawback is that the sampling frequency must be divided by the number of parallel channels that the card has.
Resolution of the ADC
The accuracy of the analog input signal converted to digital format depends on the number of bits that the ADC uses. The resolution of the converted signal is a function of the number of bits that the ADC uses to represent the digital data. The voltage range between maximum voltage and minimum voltage with which the actual signal can be represented analogically is subdivided according to the number of digital resolution bits.
For example, an 8-bit ADC can give up to 256 levels of digital representation (2 ^ 8 = 256). This means that the voltage range of the real signal will be divided into 256 levels, in which each of them will take a certain digital value.
Transfer of Data to the Computer
Normally, DAQ cards are installed on the high-speed buses of the PC, such as PCI buses. Depending on the speed of the PC motherboard, the maximum data transfer speed between components of the motherboard is usually between the microprocessor and the memory with values ranging from 20Mhz to 40Mhz.
To improve data transfer, Bus Mastering is implemented, which allows DAQ cards to transfer data directly to memory, thereby accelerating the data acquisition process. See the following examples of cards without and with Bus Mastering implemented.
The figure shows, the microprocessor is participating in the data transfer, using in it a time when it could be doing other tasks.
On the other hand it is observed that in the Figure above shows, data transfer is direct to the memory, with which the microprocessor is free to be used for other tasks, this is achieved through Bus Mastering technology.
Digital / Analog Converters (DAC)
The multifunction cards also have integrated, usually, a Digital / Analog Converter (DAC). A DAC can generate an analog signal based on a digital data.
This allows cards of this type to generate an analog output signal, with voltages in direct current (DC) or alternating current (AC). Like the ADC, the DACs are limited by the number of samples they can process and the number of bits they use to convert the digital data into analog signals.
The figures, shows how a sine wave is converted by a 3-bit DAC.
It is also important in a DAC a small Settling Time, as this will be able to generate high-frequency signals because the time used to set the output signal for a new voltage level will be low.