What Do Data Acquisition Specifications Actually Mean?

我第一次在NASA Glenn Research Center的一家学生实验室工作中获得了数据获取的味道。我正在研究一个用于光纤温度传感器的项目，并且我的任务是使用安装在IBM中的定制A/D板在运行Intel 286微处理器的计算机上安装的A/D板。它使用模拟开关来选择发送到仪器放大器的通道，然后将其发送到A/D转换器进行测量。没有数据表，将其发送出去进行校准是不可能的。这确实是数据获取的狂野西部。

当时我还不知道，但是这个单个A/D板会为我的职业提供指导，从而获得了30多年的数据采集产品经验。在这30年的时间里，我注意到有关与新产品一起发布的规格的信息 - 它们开始包括更多细节。A/D的精度为±1或2位，取而代之的是偏移旁的读数百分比。固有的噪声也伴随着灵敏度，也随之而来，其他规格（如线性和漂移）也是如此。在本文中，我想用一些细节来解释这些规格，以便您可以获得我希望在整个职业生涯中拥有的背景。我将重点关注四个一般领域：准确性，精度，分辨率和灵敏度。

准确性

准确性is how well the measurements match up to the government standard known as NIST. NIST is theNational Institute of Standards and Technology. They are the people that know how to produce an exact volt accurate to a single nanovolt or better. To verify accuracy simply use a voltage source and a voltmeter that has been recently calibrated with equipment that is traceable back to NIST. It’s best practice to use a voltage calibrator to dial in different values. Voltage Calibrators output a stable low noise voltage. Use a good voltmeter to verify the output value.

解析度

Next, let’s discuss ‘resolution’. Resolution refers to how many values can be represented on a scale. Think of a scale like a ruler. A highly accurate ruler (i.e., millimeter markings) that is of equal length to a low-accuracy ruler (i.e, centimeter markings) can be said to have a higher resolution than the latter by virtue of having more markings on it. These markings, the lines drawn on the ruler, are also referred to as “counts”. An absolute resolution refers to what each “step”, the actual physical distance between two ruler markings, means. The absolute resolution also depends on the range of the device – the length of the entire ruler.

For example, a 16-bit A/D has 65,536 (2 to the power of 16) steps in its scale. If you wanted to verify the functionality of a 16-bit A/D device, you’d need another device with a higher absolute resolution in order to measure the same data and compare. For example, a 6 ½ digit voltmeter would typically be recommended, which has a 2,000,000 step scale.

Similarly, a device’s theoretical (or ideal) accuracy refers to the percentage of the total range that a single step on the scale takes up. It’s the inverse of the bit counts multiplied by ten, so, for example, a 12-bit A/D (4096 counts) has an ideal accuracy of 0.024%. If you happen to be one of those who thinks in decibels, these percentages can also be directly translated into decibels – for example, a 16-bit A/D converter has an accuracy of 0.0015% of the input range, which translates to -96 decibels (20 * Log (0.000015)).

值得注意的是，使用高分辨率A/D设备有助于使用，因为仪器内部的电路和您正在测试的电路都可能增加测量值不准确。虽然可以通过将完美的测试电压直接注入A/D转换器IC芯片来达到理想的准确性，但通常没有直接通往您要测量的确切点的直接途径。更常见的是，模拟缓冲区，水平移动电路和放大器将在A/D转换器之前。同样，如果设备的测量范围较小，则可以通过添加放大器和电平转换电路扩展，将不实用的范围转换为可用的±5或0-10伏范围。但是额外的电路会增加错误，因此，例如，准确性（或不准确）规范不再是0.0015％，而是一些更大的值。这是（信号获取设备）设计中固有的众多设计权衡。

灵敏度

灵敏度is the degree to which the input signal change is reflected in the data. A device that has a good amount of internal noise on the signal path will respond poorly to small changes and conversely, a signal path with little to no noise will easily show changes. This is because a small signal change is lost in the noise. Averaging of the data will reduce the noise, but that comes at a tradeoff of speed and may not be a feature of the device or software. Most modern data acquisition devices feature low noise circuitry, but even ± 6 counts can look large when attempting to detect a signal change that is a few 100 microvolts.

精确

精确和准确性不一样吗？如果A/D转换器可以返回一致的测量值，则据说它是确切的。它们可能不准确地适用于所应用的内容，但它们始终处于相同的价值。将其视为目标中的一组箭头。如果分组很紧，据说即使将它们留在尔基中心也是如此。

An instrument with input noise will produce a group of measurements. The measured value is at the center or average of a group and should be same value every time. Therefore, precision is concerned with the measurement quality and not how well it matches up to a known value. Another term for this is repeatability – can it produce the same value every time? A precise instrument can often be calibrated to be an accurate instrument. Ultimately, you will want is to find a device that has good resolution, that can return precise accurate low noise measurements and one that is affordable.

Data acquisition devices are now including signal-to-noise (SNR) specifications that provide a clue to its noise-free resolution – often referred to as ‘effective number of bits’, or ENOB. The MCC USB-1808 data sheet includes dynamic performance specifications that include SNR and ENOB among others, representing specifications that are a far cry from what I needed to know for my first A/D board encounter.