1、 1 附录 A 外文资料翻译 Five Tips to Reduce Measurement Noise Ensuring measurement accuracy often means going beyond reading raw specifications in a data sheet. Understanding an application in the context of its electrical environment is also important for securing success, particularly in a noisy or industr
2、ial setting. Ground loops, high common-mode voltages, and electromagnetic radiation are all prevalent examples of noise that can adversely affect a signal. There are many techniques for reducing noise in a measurement system, which include proper shielding, cabling, and termination. Beyond these com
3、mon best practices, however, there is more you can do to ensure better noise immunity. The following five techniques serve as guidelines for achieving more accurate measurement results. A. Reject DC Common-Mode Voltage Making highly accurate measurements often starts with differential readings. An i
4、deal differential measurement device reads only the potential difference between the positive and negative terminals of its instrumentation amplifier(s). Practical devices, however, are limited in their ability to reject common-mode voltages. Common-mode voltage is the voltage common to both the pos
5、itive and negative terminals of an instrumentation amplifier. In Figure 1, 5 V is common to both the AI+ and AI- terminals, and the ideal device reads the resulting 5 V difference between the two terminals. Figure 1 An ideal instrumentation amplifier completely rejects common-mode voltages. The maxi
6、mum working voltage of a data acquisition (DAQ) device refers to the signal 2 voltage plus the common-mode voltage and specifies the largest potential that may exist between an input and earth ground. The maximum working voltage for most DAQ devices is the same as the input range of the instrumentat
7、ion amplifier. For example, low-cost M Series DAQ devices such as the NI 6220 devices have a maximum working voltage of 11 V； no input signal can exceed 11 V without causing damage to the amplifier. Isolation can dramatically increase the maximum working voltage of a DAQ device. In the context of a
8、measurement system, “isolation” means physically and electrically separating two parts of a circuit. An isolator passes data from one part of the circuit to another without conducting electricity. Because current cannot flow across this isolation barrier, you can level-shift the DAQ device ground re
9、ference away from earth ground. This decouples the maximum working voltage specification from the input range of the amplifier. For example, in Figure 2 the instrumentation amplifier ground reference is electrically isolated from earth ground. Figure 2 Isolation electrically separates the instrument
10、ation amplifier ground reference from earth ground. While the input range is the same as that in Figure 1, the working voltage has been extended to 60 V, rejecting 55 V of common-mode voltage. The maximum working voltage is now defined by the isolation circuitry instead of the amplifier input range.
11、 Fuel cell testing is an example application that requires high DC common-mode voltage rejection. Each individual cell may generate approximately 1 V, but a stack of cells may produce several kilovolts or more. To accurately measure the voltage of a single 1 V cell, the measurement device must be able to reject the high common-mode voltages generated by the rest of the stack.
