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Introduction to Data Acquisition (DAQ)

Introduction

The simplest way to store data is to record it on paper with either a strip-chart recorder or an x-y plotter. For greater utility, most modern analytical instruments digitize data in digital format for subsequent data storage, analysis, and presentation. For some types of instruments, such as Fourier-transform-based spectrometers, digitized data is necessary for the data manipulation to extract the spectrum from the raw data. Many instruments digitize and store data in memory for subsequent transfer to a computer file. Other instruments will use a software program to control an instrument and digitize the analytical signal through a computer data acquisition (DAQ) interface.

Most analytical signals are an analog voltage from a transducer. This signal must be digitized to discrete data points without losing analytical information. The important parameters for digitizing data are the resolution of the analog-to-digital converter and the sampling rate.

The resolution depends on the number of bits in the digital representation. For example, an 8-bit converter has a resolution of 28 or 1 part in 256. If the input range of the analog-to-digital converter was 0 to 10 V, the smallest detectable voltage would be 0.04 V. 0.04 V is also the minimum discernable difference between two voltage measurements.

The sampling rate must be greater than two times the highest frequency in the analytical signal to avoid aliasing the data. Signals at frequencies greater than the sampling rate can appear to be at lower frequencies.


Data Transfer

Data that is stored digitally in an instrument can be transferred to a computer via an RS-232 serial port, general-purpose interface bus (GPIB), or a proprietary protocol. The advantage of using the RS-232 port is that it is available on all computers. The disadvantage is that it is not a fast transfer since it is a serial transfer mode. Newer standards such as the universal serial bus (USB) are faster but only available on newer instruments. The GPIB protocol uses 24 parallel data lines to provide fast data transfer, but it requires a dedicated plug-in interface board in the computer. Some instruments are now available with a DOS-compatible floppy disk drive. The disk drive makes file storage and transfer easy, but can be cumbersome for large numbers of files and for very large individual data files.


Computer DAQ

Voltages can be digitized into computer memory through plug-in DAQ boards that contain analog-to-digital converters or stand-alone interfaces that transfer data to the computer through a serial port. Many of the commercial DAQ boards also support digital and trigger inputs, as well as analog and digital outputs for integrated data acquisition and instrument control. A variety of software packages support the commercially available DAQ boards for data acquisition and instrument control. National Instruments, Inc. (http://www.natinst.com) sells a wide range of DAQ boards and LabViewTM software for sophisticated instrument control, data acquisition, and data analysis. Vernier Software, Inc. (http://www.vernier.com) sells a variety of probes for biological, chemical, and physical measurements, computer and calculator interfaces, and data logging software.


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