[Design Application]
Choosing Data Acquisition Boards And Software
Speed, Resolution, Reliability, And Flexibility Are All Factors To Consider When Trying To Determine The Best Solution.
Making the right choice from the wide array of data acquisition boards and software available can be a daunting task. Ideally, you want to maximize your PC-based system now, and ensure future flexibility as your needs change.
Data acquisition boards translate real-world analog and discrete signals into digital data for processing by a computer. The primary feature of the majority of these boards is the analog-to-digital converter (ADC). Many boards also include a digital-to-analog converter (DAC) for control capabilities, configurable counter/timers, and digital I/O lines for communication with digital devices (Fig. 1). By correctly choosing from the myriad of variations on these features, you can fully optimize your system and avoid paying for features you don't need. Careful review of product specifications is the key to configuring the optimum system.
Input Ranges One of the first things to consider is the voltage range you will be measuring. Begin by choosing the sensor (for example, strain gauge, thermocouple, microphone, or pressure transducer). Determine the range of output voltage the sensor will provide. The data acquisition board should provide a range that matches the maximum output range of the transducer. This results in the ADC utilizing the greatest number of data points in the range to be measured, thereby providing the highest possible resolution.
For example, if the sensor's output varies from 1 to 3 V, choosing a board with a 0-to 5-V input range, rather than a board with a range of ±5 V (0 to 10 V), yields twice as many valid data points and a higher resolution.
Many boards provide multiple input ranges by using software-programmable-gain amplifiers. While this is a flexible solution, a software approach may incur a performance penalty, especially if you require a different gain on every channel. For applications requiring high-speed data acquisition with multiple gains and channels, a channel-gain list is a better alternative. This on-board memory buffer is preloaded with channel numbers and associated gains. It automatically selects channel and gain values, without compromising throughput.
Input Types The number of input channels on data acquisition boards typically ranges from four to 64. Input channels can be single-ended (SE) or differential (DI), and many boards allow you to choose between the two types. Differential inputs offer noise immunity (common mode rejection), and can improve accuracy when long cables, low-level input voltages (less than 1 V full-scale), or high-resolution converters (greater than 16 bits) are used, or when input signals are at different ground potentials.
A third type of input, known as pseudo-differential, offers enhanced, common-mode rejection in designs with a single, external common ground. This type is typically used with external signal conditioning.
Accuracy The accuracy of a data acquisition board is defined by how closely the binary code matches the true value of the incoming or outgoing analog signal. Yet, manufacturers' specifications rarely tell the whole story. Several different methods are used to specify this important measurement. One common specification is system error, stated as a percent of full-scale range. It typically includes all sources of error—analog noise, system nonlinearities, and reference variation.
The major component of accuracy, and its limiting factor, is resolution. Stated in bits, resolution determines the number of counts or binary numbers used to represent the analog signal. Twelve-bit converters, for example, divide ranges into 4096 parts. This translates into digital code, which tracks the analog signal to within 0.024% of the range. More bits yield exponentially higher resolution.
The limiting factor is the least significant bit (LSB). The LSB is the smallest change in the analog signal which can be represented by digital code. LSBs are specified both as both a percent of range and the smallest voltage change that can be resolved on a particular range.
Total harmonic distortion (THD) is the ratio of the sum of the harmonics of the fundamental frequency of the input signal to the fundamental frequency itself. THD is a good indicator of the quality of the circuit design. A high THD measurement, for example, may indicate a flawed analog-input design.
When evaluating a data acquisition board, it is critical to understand the relationship between resolution and accuracy. Resolution is simply one factor affecting accuracy. Many board manufacturers' specifications, however, use these terms interchangeably.
Speed The throughput of a board—specified in megasamples per second (Msamples/s) or kilosamples per second (ksamples/s)—is a crucial measurement for high-speed applications such as audio, radar, and destructive testing. If multiple ADCs are used on a single board, the specified throughput represents the sum total of the individual converter throughputs. For example, to sample four channels at 40 ksamples/s each, you need a throughput of at least 160 ksamples/s. Delta-sigma (Δ-Σ) ADCs are an exception, since each channel has its own ADC.
Aliasing can be avoided by following the Nyquist Theorem guideline (sampling an input signal at least twice as fast as the input's highest frequency component). When the input's frequency content is unknown, many users sample at the highest frequency available, or use a low-pass filter to remove very high frequencies.
ADC throughput is determined by three elements: conversion time (the time needed to do actual conversion), acquisition time (the time needed by associated acquisition circuitry—the multiplexer, amplifier, and sample and hold—to acquire a signal accurately), and transfer time (the time needed to transfer data from the board to system memory). Normally, a board first acquires a signal, then converts it. Some high-speed boards increase throughput by overlapping the acquisition time on one sample with the analog-to-digital conversion time of the previous sample, in effect handling two signals at one time.
Most analog output circuits have a separate DAC and data buffer for each channel. The major portion of DAC throughput is settling time—the time needed to reach rated accuracy after receiving an output change. Settling time varies proportionately with the size of the output change, and is specified in microseconds.
To provide a cleaner analog output signal, some boards use a low-pass reconstruction filter. A key consideration is the capacitive drive capability when using a typical cable with 30-pF/ft. capacitance load.
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