【正文】 n have reading rates as high as 100,000 readings per second. Counts of 8 to 16 channels are mon, and acquired data is stored directly into the puter, where it can then be analyzed. Because the card is essentially part of the puter, it is easy to set up tests. PC cards also are relatively inexpensive, in part, because they rely on the host PC to provide power, the mechanical enclosure and the user interface. Data acquisition options In the downside, PC plugin cards often have only 12 bits of resolution, so you can39。t perceive small variations with the input signal. Furthermore, the electrical environment inside a PC tends to be noisy, with highspeed clocks and bus noise radiated throughout. Often, this electrical interference limits the accuracy of the PC plugin card to that of a handheld DMM .These cards also measure a fairly limited range of dc voltage. To measure other input signals, such as ac voltage, temperature or resistance, you may need some sort of external signal conditioning. Additional concerns include problematic calibration and overall system cost, especially if you need to purchase additional signal conditioning accessories or a PC to acmodate the cards. Taking that into consideration, PC plugin cards offer an attractive approach to data acquisition if your requirements fall within the capabilities and limitations of the card. Data loggers Data loggers are typically standalone instruments that, once they are setup, can measure, record and display data without operator or puter intervention. They can handle multiple inputs, in some instances up to 120 channels. Accuracy rivals that found in standalone bench DMMs, with performance in the 22bit, accuracy range. Some data loggers have the ability to scale measurements, check results against userdefined limits, and output signals for control. One advantage of using data loggers is their builtin signal conditioning. Most are able to directly measure a number of different inputs without the need for additional signal conditioning accessories. One channel could be monitoring a thermocouple, another a resistive temperature device (RTD) and still another could be looking at voltage. Thermocouple reference pensation for accurate temperature measurement is typically built into the multiplexer cards. A data logger39。s builtin intelligence helps you set up the test routine and specify the parameters of each channel. Once you have pleted the setup, data loggers can run as standalone devices, much like a recorder. They store data locally in internal memory, which can acmodate 50,000 readings or more. PC connectivity makes it easy to transfer data to your puter for indepth analysis. Most data loggers are designed for flexibility and simple configuration and operation, and many provide the option of remote site operation via battery packs or other methods. Depending on the A/D converter technique used, certain data loggers take readings at a relatively slow rate, especially pared to many PC plugin cards. Still, reading speeds of 250 readings/second are not unmon. Keep in mind that many of the phenomena being monitored are physical in nature — such as temperature, pressure and flow — and change at a fairly slow rate. Additionally, because of a data logger39。s superior measurement accuracy, multiple readings and averaging are not necessary, as they often are in PC plugin solutions. Data acquisition front ends Data acquisition front ends are often modular and are typically connected to a PC or controller. They are used in automated test applications for gathering data and for controlling and routing signals in other parts of the test setup. Front end performance can be very high, with speed and accuracy rivaling the best standalone instruments. Data acquisition front ends are implemented in a number of formats, including VXI versions, such as the Agilent E1419A multifunction measurement and control VXI module, and proprietary card cages.. Although frontend cost has been decreasing, these systems can be fairly expensive, and unless you require the high performance they provide, you may find their price to be prohibitive. On the plus side, they do offer considerable flexibility and measurement capability. Data Logger Applications A good, lowcost data logger with moderate channel count (20 60 channels) and a relatively slow scan rate is more than sufficient for many of the applications engineers monly face. Some key applications include: ? Product characterization ? Thermal profiling of electronic products ? Environmental testing。 environmental monitoring ? Component characterization ? Battery testing ? Building and puter room monitoring ? Process monitoring, evaluation and troubleshooting No single data acquisition system works for all applications. Answering the following questions may help you decide which will best meet your needs: 1. Does the system match my application? What is the measurement resolution, accuracy and noise performance? How fast does it scan? What transducers and measurement functions are supported? Is it upgradeable or expandable to meet future needs? How portable is it? Can it operate as a standalone instrument? 2. How much does it cost? Is software included, or is it extra? Does it require signal conditioning addons? What is the warranty period? How easy and inexpensive is it to calibrate? 3. How easy is it to use? Can the specifications be understood? What is the user interface like? How difficult is it to reconfigure for new applications? Can data be transferred easily to new applications? Which application packages are supported? Conclusion Data acquisition can range from pencil, paper and a measuring device, to a highly sophisticated system of hardware instrumentation and software analysis tools. The first step for users contempla