外文翻译---降低测量噪声的五个技巧.docx

附录A 外文资料翻译Five Tips to Reduce Measurement NoiseEnsuring 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 industrial 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 common 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 VoltageMaking highly accurate measurements often starts with differential readings. An ideal 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 positive 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 maximum working voltage of a data acquisition (DAQ) device refers to the signal 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 instrumentation 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 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 reference 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 instrumentation 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.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.14B. Reject AC Common-Mode VoltageRarely do common-mode voltages consist of only a DC level. Most sources of common-mode voltage contain an AC component in addition to a DC offset. Noise is inevitably coupled onto a measured signal from the surrounding electromagnetic environment. This is particularly troublesome for low-level analog signals passing through the instrumentation amplifier on a DAQ device.Sources of AC noise may be broadly classified by their coupling mechanisms capacitive, inductive, or radiative. Capacitive coupling results from time-varying electric fields, such as those created by nearby relays or other measurement signals. Inductive or magnetically coupled noise results from time-varying magnetic fields, such as those created by nearby machinery or motors. If the electromagnetic field source is far from the measurement circuit, such as with fluorescent lighting, the electric and magnetic field coupling is considered combined electromagnetic or radiative coupling. In all cases, a time-varying common-mode voltage is coupled onto the signal of interest, most often in the range of 50-60 Hz (power-line frequency).An ideal measurement circuit has a perfectly balanced path to both the positive and negative terminals of an instrumentation amplifier. Such a system would completely reject any AC-coupled noise. A practical device, however, specifies the degree to which it can reject common-mode voltage with a common-mode rejection ratio (CMRR). The CMRR is the ratio of the measured signal gain to the common-mode gain applied by the amplifier, as noted by the following equation:Choosing a DAQ device with a better CMRR over a broader range of frequencies can make a significant difference in your systems overall noise immunity. For example, Figure 3 shows the CMRR for a low-cost M Series device compared with that of an industrial M Series device.Figure 3 The NI 6230 provides a much higher CMRR than the NI 6220 relative to earth ground.At 60 Hz, NI 6230 industrial M Series devices have 20 dB greater CMRR than NI 6220 low-cost M Series devices. This is equivalent to a 10 times better attenuation of 60 Hz noise.Any application may benefit from rejecting 60 Hz noise. However, those with large rotating machinery or motors require noise immunity at higher frequencies. At 1 kHz, NI 6230 devices reject noise 100 times better than NI 6220 devices, making them ideal for industrial applications.C. Break Ground LoopsGround loops are arguably the most common source of noise in data acquisition systems. Proper grounding is essential for accurate measurements, yet it is a frequently misunderstood concept. A ground loop forms when two connected terminals in a circuit are at different ground potentials. This difference causes a current to flow in the interconnection, which can produce offset errors. Further complicating matters, the voltage potential between signal source ground and DAQ device ground is generally not a DC level. This results in a signal that reveals power-line frequency components in the readings. Consider the simple thermocouple application in Figure 4.Figure 4 A differential thermocouple measurement with a grounded signal source can create a ground loop.Here, an otherwise straightforward temperature measurement is complicated by the device under test (DUT) being at a different ground potential than that of the DAQ device. Though both devices share the same building ground, the difference in ground potential could be 200 mV or more if the power distribution circuits are not properly connected. The difference appears as a common-mode voltage with an AC component in the resulting measurement.Recall that isolation is a means of electrically separating signal source ground from the instrumentation amplifier ground reference (see Figure 5).Figure 5 Isolation eliminates ground loops by separating earth ground from the amplifier ground reference.Because current cannot flow across the isolation barrier, the amplifier ground reference can be at a higher or lower potential than earth ground. You cannot inadvertently create a ground loop with this circuit. Using an isolated measurement device removes the ambiguity of properly grounding a measurement system, ensuring more accurate results.D. Use 4-20 mA Current LoopsLong cable lengths and the presence of noise in industrial or electrically harsh environments can make accurate voltage measurements difficult. As a result, industrial transducers that sense pressure, flow, proximity, and so on often emit current signals instead of voltage. A 4-20 mA current loop is a common method of sending sensor information over long distances in many process-monitoring applications, as shown in Figure 6.Figure 6 An instrumentation amplifier uses a shunt resistor to convert process current signals into voltage.Each of these current loops contains three components a sensor, a power source, and one or more DAQ devices. The current signal from the sensor is typically between 4 and 20 mA, with 4 mA representing the lowest signal value and 20 mA representing the maximum. This transmission scheme has the advantage of using 0 mA to indicate an open circuit or bad connection. Power supplies are typically in the range of 24 to 30 VDC, depending on the total amount of voltage dropped along the circuit. Finally, the DAQ device uses a high-precision shunt resistor between the leads of the instrumentation amplifier to convert the current signal into a voltage measurement. Because all the current that flows from one lead of the power supply must return to the other, current loop signals are immune to most sources of electrical noise and voltage (IR) drops along extensive cable lengths. Furthermore, the leads that provide power to the sensor also carry the measurement signal, greatly simplifying field wiring.An isolation barrier such as the one shown in Figure 6 provides two main benefits in current loop applications. First, because power supply voltages typically exceed the maximum input range of most instrumentation amplifiers, isolation is essential for level-shifting the amplifier ground away from earth ground to an acceptable voltage. Second, current loops operate on the principal that current never leaves the circuit. Therefore, isolating the current loop from any path to ground prevents degradation of the signal. Devices such as the NI 6238 and NI 6239 industrial M Series DAQ devices provide a built-in shunt resistor and up to 60 VDC of isolation from earth ground for 4-20 mA current loop applications.E. Use 24 V Digital LogicMeasurement noise is not limited to analog signals. Digital logic may also be affected by a noisy electrical environment, possibly indicating false on/off values or accidental triggers. There are many voltage levels and logic families associated with digital I/O, some more noise resistant than others. Transistor-transistor logic (TTL) is by far the most common logic family, powering everything from microprocessors to LEDs. Though it is widely available, TTL may not always be the best choice for all digital applications.For industrial applications, TTL has the inherent disadvantage of small noise margins. With high- and low-logic levels of 2.0 V and 0.8 V, respectively, there is little room for error. For example, the low-level noise margin for a TTL input is 0.3 V (the difference between 0.8 V, the maximum low-level TTL input, and 0.5 V, the maximum low-level TTL output). Any noise coupled to the digital signal in excess of 0.3 V may shift the voltage into the undefined region between 0.8 V and 2.0 V. Here, the behavior of the digital input is uncertain and may produce incorrect values (see Figure 7).Figure 7 24V logic has better noise margins than TTL.24 V logic, however, offers increased noise margins and better overall noise immunity. Because most industrial sensors, actuators, and control logic already operate off 24 V power supplies, it is convenient to use the corresponding digital logic levels. With a low-level input of 4 V and a high-level input of 11 V, the digital signals are less susceptible to noise.Most measurement devices with 24 V digital I/O capability offer additional noise-reducing features. For example, National Instruments industrial M Series and digital I/O devices have programmable input filters for debouncing relay inputs. When a mechanical relay closes, there is a short period of time (on the order of milliseconds) during which the contact surfaces bounce against each other. Without filtering, the logic input may read this as a burst of on/off signals. These devices also offer isolation, an important factor to consider if parts of the overall system are powered from different supplies.F. ConclusionThere are many factors to consider when attempting to reduce noise in a measurement system. Beyond proper shielding, cabling, and termination, careful consideration of common-mode voltages, grounding, and nearby noise sources is essential for accurate results. However, understanding the electrical environment of your system is not always straightforward. Isolation is an easy means of adding another layer of confidence to your measurements, no matter the signal or application.Charles StiernbergCharles Stiernberg is a product engineer for data acquisition at NI. He holds a bachelors degree in electrical engineering, with a focus on embedded systems and VLSI design from The University of Texas at Austin.降低测量噪声的五个技巧确保测量精度通常意味着需要超越产品说明书的基本指标。理解其在电气环境背景中的应用对于确保在噪声环境或是工业环境中的成功应用是尤为重要的。接地回路、高共模电压以及电磁辐射都是将会负面影响信号的普遍实例。降低测量系统中的噪声有许多方法，其中包括适当的屏蔽、接线和中止。除了这些常见方法之外，还有许多可以提高噪声免疫的方法。以下的五个方法是达到更精确测量结果的指导方法。A. 抑制直流共模电压要进行高精度的测量首先从差分读数开始。理想差分测量设备能够读取仪器放大器正极端子和负极端子之间的电势差。然而，实际的设备在共模电压的抑制能力上是受到限制的。共模电压是仪器放大器的正极端子和负极端子之间的共同电压。在图1中，5V电压对于AI+和AI-端子而言是公共电压，理想的设备能够读取两个端子之间5V的差。图1 理想的仪器放大器完全抑制共模电压数据采集(DAQ)设备的最大工作电压是指信号电压加上共模电压，并且指定了存在于输入和地之间的最大电势差。对于大多数数据采集设备而言，最大工作电压与仪器放大器的输入范围是相同的。例如，例如NI 6220设备等低成本M系列数据采集设备的最大工作电压是11V；超过11V的输入信号将对放大器造成破坏。隔离可以大大提高数据采集设备的最大工作电压。在测量系统的环境中，“隔离”意味着在物理上和电气上将电路的两部分隔开。隔离器将数据从电路的一个部分传送到另一个部分，而无需电学的导通。由于电流无法流过隔离器屏障，您可以将数据采集设备的参考地和实际地隔离。这样就将最大工作电压的指标与放大器输入范围进行了解耦。举例而言，在图2中，仪器放大器的参考地与实际地是电学隔离的。图2 隔离将仪器放大器的参考地和实际地进行了电气分离尽管输入范围与图1中相同，工作电压已经被扩展到60V，能够抑制55V共模电压。这时，最大工作电压是由隔离电路定义的，而不是由放大器输入范围定义的。燃料电池测试是需要高直流共模电压抑制的范例应用。每个独立的电池能够产生大约1V的电压，而一组电池能够产生几千伏特，甚至更高。要精确测量一个1V电池的电压，测量设备必须能够抑制由组内其他电池所产生的高共模电压。B. 抑制交流共模电压通常共模电压不会只由直流电平组成。大多数共模电压源除了直流偏置之外，还包含了交流成分。来自周围电磁环境的噪声不可避免地被耦合到被测信号中。这对于通过数据采集设备仪器放大器的低电平模拟信号而言是特别麻烦的。交流噪声源可以根据其耦合机制大致进行分类：电容型、电感型或辐射型。电容型耦合来自于时变电场，例如由周围继电器或是其他测量信号产生的电场。电感型或磁耦合噪声来自于时变磁场，例如由周围机器或电机产生的磁场。如果电磁场源距离测量电路较远，例如荧光灯等，电气和磁场的耦合被认为是电磁或是辐射耦合。在所有情况下，时变共模电压被耦合到有用的信号中，通常它在50-60Hz的频率范围中(电源频率)。理想的测量电路，其通向仪器放大器正极和负极端子的路径是完全平衡的。这样的系统能够完全抑制任何交流耦合噪声。但是，实际仪器通过共模抑制比(CMRR)指定了它能够抑制共模电压的程度。CMRR是被测信号增益相对于放大器施加的共模增益之间的比值，可以使用下式表示：选择在更宽频率范围内具有更好CMRR的数据采集设备能够大大提高系统的整体抗噪声性能。举例而言，图3显示了将低成本M系列设备与工业M系列设备的CMRR相比较的结果。图3 NI 6230能够比NI 6220提供更高的CMRR(相对于物理地)在60Hz下，NI 6230工业M系列设备相对于NI 6220低成本M系列设备，其CMRR高出了20dB。这等效于对于60Hz噪声具有高于10倍以上的衰减。任何应用都能够从60 Hz噪声抑制中获益。然而，对于包含大型转动机械或电机的系统需要更高频率下的噪声抑制。在1kHz下，NI6230设备相比NI6220设备能够抑制100倍以上的噪声，从而使它们成为工业应用的理想选择。C. 切断接地回路接地回路通常被认为是数据采集系统中噪声最常见的来源。合适的接地对于精确测量而言是十分重要的，但它也是一个常常被误解的概念。如果电路中两个连接的端子处于不同地电势，就形成了接地回路。这个差别将会导致电流流入交叉连接点，将会导致偏置误差的出现。将问题变得更为复杂的是，在信号源的地和数据采集设备的地之间的电势差通常不是直流电平。这就导致了在读数中会出现电源频率分量的信号。考虑图4中的简单热电偶应用。图4 使用接地信号源的差分热电偶测量将会导致接地回路出现在这里，原来十分直观的温度测量由于被测设备(DUT)与数据采集设备出现了不同的地电势而被复杂化了。尽管两个设备都共享相同的地，如果电源分布电路没有正确连接，就会导致地电势差达到200 mV甚至更多。这个差在最后得到的测量中，以带有交流分量的共模电压出现。回忆一下隔离是将信号源的地与仪器放大器的参考地进行电气隔离的一种方法(见图5)。图5 隔离通过将物理地与放大器参考地进行分离消除了接地回路由于电流无法流过隔离屏障，放大器参考地可以比物理地具有更高或更低的电