低电平测量术语.doc

低电平测量手册专栏第4节 Understanding Instrument Specifications 正确理解仪器的技术指标 低电平测量 技术指标 Knowing how to interpret instrument specifications properly is an important aspect of making good low level measurements. Although instrument accuracy is probably the most important of these specifications, there are several other factors to consider when reviewing specifications, including noise, deratings, and speed. 正确理解仪器的技术指标对于做好低电平测量工作是十分重要的。虽然在众多技术指标中准确度占据主要地位，但是诸如噪声、漂移、速度和其它可能降低技术指标的参数亦不容忽略。1.4.1 Definition of Accuracy Terms 准确度术语的定义 This section defines a number of terms related to instrument accuracy. Some of these terms are further discussed in subsequent paragraphs. Table 1-1 summarizes conversion factors for various specifications associated with instruments. 本节给出了有关仪器准确度的若干术语的定义。其中一些术语将在后续段落中予以讨论。表1-1汇总了与仪器各项技术指标有关的参数间的换算关系。 SENSITIVITY - the smallest change in the signal that can be detected.灵敏度(SENSITIVITY) 仪器所能检测得到的信号中的最小变化量。 RESOLUTION - the smallest portion of the signal that can be observed. 分辨率(RESOLUTION) 仪器所能检测到的最小信号所占的比例分量。 REPEATABILITY - the closeness of agreement between successive measurements carried out under the same conditions.重复性(REPEATABILITY) 在相同测试条件下相邻的成功测量结果之间的一致性。 REPRODUCIBILITY - the closeness of agreement between measurements of the same quantity carried out with a stated change in conditions.再现性(REPRODUCIBILITY) 在给定的变化条件下，相同量值测试结果之间的一致性。 ABSOLUTE ACCURACY - the closeness of agreement between the result of a measurement and its true value or accepted standard value. Accuracy is often separated into gain and offset terms. 绝对准确度(ABSOLUTE ACCURACY) 测量结果与被测量真值或其标准认定值之间的接近程度。准确度常被分成增益和偏置二项。 RELATIVE ACCURACY - the extent to which a measurement accurately reflects the relationship between an unknown and a reference value. 相对准确度(RELATIVE ACCURACY) 用被测值与参考值之间的相对关系表示出的测量准确度。 ERROR - the deviation (difference or ratio) of a measurement from its true value. Note that true values are by their nature indeterminate.误差(ERROR) 与被测量值的真值相比测量的偏差（差值或比值）。请注意真值就其本质而言是不可确定的。 RANDOM ERROR - the mean of a large number of measurements influenced by random error matches the true value. 随机误差(RANDOM ERROR) 受随机分布误差影响的大量测量结果的平均值与真值的偏差。 SYSTEMATIC ERROR - the mean of a large number of measurements influenced by systematic error deviates from the true value. 系统误差(SYSTEMATIC ERROR) 受系统自身分布误差影响的大量测量结果的平均值与真值的偏差。 UNCERTAINTY - an estimate of the possible error in a measurement, i.e., the estimated possible deviation from its actual value. This is the opposite of accuracy. 不确定度(UNCERTAINTY) 对测量中可能产生的误差的评估，即估计与被测量的实际量值可能产生的偏差范围。不确定度与准确度相反。 “Precision” is a more qualitative term than many of those defined here. It refers to the freedom from uncertainty in the measurement. Its often applied in the context of repeatability or reproducibility, but it shouldnt be used in place of “accuracy.” 精度(Precision)是一个与这里定义的许多术语相比更加定性的指标，指测量中不确定性的程度。适用于可重复性或可复现性测试等场合，而不适用于在其他应该用准确度（ACCURACY）的场合。 1.4.2Accuracy 准确度 One of the most important considerations in any measurement situation is reading accuracy. For any given test setup, a number of factors can affect accuracy. The most important factor is the accuracy of the instrument itself, which may be specified in several ways, including a percentage of full scale, 直在任何测量场合中最重要的考虑因素之一就是读数的准确度。对任何给定的测试设置来说，有几个因素能够影响准确度。最重要的因素是仪器本身的准确度它可以用几种方法来表示，包括满度的百分数、读数的百分数或这二者的组合。仪器准确度的要素将在下面的段落中加以介绍。TABLE 1-1: Specification Conversion Factors 表 1-1 技术指标变换因子 Other factors such as input loading, leakage resistance and current, shielding, and guarding may also have a serious impact on overall accuracy. These important measurement considerations are discussed in detail in Sections 2 and 3. 其它的因素如输入负载、泄漏电阻和泄漏电流、屏蔽和保护等，也可能对总的准确度有重大的影响，这些重要的测量考虑因素将在第二章和第三章讨论。 Measurement Instrument Specifications 测量仪器的技术指标 Instrument accuracy is usually specified as a percent of reading, plus a percentage of range (or a number of counts of the least significant digit). For example, a typical DMM accuracy specification may be stated as: (0.005% of reading + 0.002% of range). Note that the percent of reading is most significant when the reading is close to full scale, while the percent of range is most significant when the reading is a small fraction of full scale. 仪器的准确度通常规定为读数的百分数，加上量程的百分数（或者最低有效位的字数）。例如，典型数字多用表的准确度技术指标可以表达为：（读数的0.005% + 量程的0.002%）。注意，在读数接近满度时读数的百分数最重要，而当读数为满度的一小部分时，量程的百分数更重要。 Accuracy may also be specified in ppm (parts per million). Typically, this accuracy specification is given as (ppm of reading + ppm of range). For example, the DCV accuracy of a higher resolution DMM might be specified as (25ppm of reading + 5ppm of range). 准确度也可以用ppm（parts per million，即百万分之一）来表示。通常,这种准确度技术指标表示为（读数的ppm + 量程的ppm）。例如高分辨率数字多用表的直流电压准确度可能表示为（读数的25ppm + 量程的5ppm）。 1.4.2 准确度 Resolution 分辨率 The resolution of a digital instrument is determined by the number of counts that can be displayed, which depends on the number of digits. A typical digital electrometer might have 51.2 digits, meaning five whole digits (each with possible values between 0 and 9) plus a leading half digit that can take on the values 0 or 1. Thus, a 51.2-digit display can show 0 to 199,999, a total of 200,000 counts. The resolution of the display is the ratio of the smallest count to the maximum count (1/200,000 or 0.0005% for a 51.2-digit display). 数字仪器的分辨率由可以显示的数字值决定，而该数字值又决定于位数。典型的数字静电计可能有51/2位，意思是由5个完整的位（每一位的数值可以从0到9），加上首位的半位（可以取值为0或1）。这样51/2位显示可以指示出0到199999，共200000个字。显示的分辨率为最小字数与最大字数之比（对51/2 位显示来说为1/200000或0.0005%）。For example, the specification of (0.05% + 1 count) on a 41.2-digit meter reading 10.000 volts corresponds to a total error of (5mV + 1mV) out of 10V, or (0.05% of reading + 0.01% of reading), totaling 0.06%. Generally, the higher the resolution, the better the accuracy. 例如：41/2位表的技术指标为（0.05% + 1个字），10.000V 的读数相当于总误差（5mV+1mV）或（读数的0.05%+读数的0.01%），总共为0.06%。一般地说，分辨率越高准确度越好。Sensitivity 灵敏度 The sensitivity of a measurement is the smallest change of the measured signal that can be detected. For example, voltage sensitivity may be 1V, which simply means that any change in input signal less than 1V wont show up in the reading. Similarly, a current sensitivity of 10fA implies that only changes in current greater than that value will be detected. 测量的灵敏度是可以探测出的被测信号的最小变化量。例如，电压灵敏度为1V表示输入信号小于1V的变化将不会在读数中反映出来。与此类似，电流灵敏度为10fA，则意味着只有大于该数值的电流变化才能探测出来。 The ultimate sensitivity of a measuring instrument depends on both its resolution and the lowest measurement range. For example, the sensitivity of a 51.2-digit DMM with a 200mV measurement range is 1V. 测量仪器最终的灵敏度决定于其分辨率和最低测量量程。例如，51/2位数字多用表在200mV量程时的灵敏度为1V 。1.4.2 准确度Absolute and Relative Accuracy 直绝对准确度和相对准确度 As shown in Figure 1-4, absolute accuracy is the measure of instrument accuracy that is directly traceable to the primary standard at the National Institute of Standards and Technology (NIST). Absolute accuracy may be specified as (% of reading + counts), or it can be stated as (ppm of reading + ppm of range), where ppm signifies parts per million of error. 如图1-4所示，绝对准确度是仪器直接溯源到NIST（美国国家标准技术研究院）的一级标准的准确度。绝对准确度可以规定为（读数的百分数+字数），或者也可以表达为（读数的ppm+量程的ppm）。这里ppm表示百万分之一。FIGURE 1-4: Comparison of Absolute and Relative Accuracy图1-4：绝对准确度与相对准确度Relative accuracy (see Figure 1-4) specifies instrument accuracy to some secondary reference standard. As with absolute accuracy, relative accuracy can be specified as (% of reading + counts) or it may be stated as (ppm of reading + ppm of range). 相对准确度（见图1-4）规定了仪器相对于某一个二级参考标准的准确度。和绝对准确度一样，相对准确度可以规定为（读数的百分数 +字数），或者表达为（读数的ppm+量程的ppm）。 Transfer Stability 传递稳定度 A special case of relative accuracy is the transfer stability, which defines instrument accuracy relative to a secondary reference standard over a very short time span and narrow ambient temperature range (typically within five minutes and 1C). The transfer stability specification is useful in situations where highly accurate measurements must be made in reference to a known secondary standard. 相对准确度的一种特殊情况是传递稳定度。它定义了在很短的时间段内，很窄的温度范围内，仪器相对于某一二级参考标准的准确度（通常在5分种和1之内）。在必须相对于某一已知的二级标准进行高准确度的测量时，传递稳定度特别有用。 Calculating Error Terms from Accuracy Specifications 根据准确度技术指标计算误差项 illustrate how to calculate measurement errors from instrument specifications, assume the following measurement parameters: 为了说明如何由仪器的技术指标计算测量误差，我们假定下列的仪器参数：Accuracy: (25ppm of reading + 5ppm of range) 准确度：（读数的25ppm+量程的5ppm） Range: 2V 量程：2VInput signal: 1.5V 输入信号：1.5V The error is calculated as: 其误差计算如下： Error误差 =1.5（25x10 -6 ）+ 2(5x10 -6 ) =(37.5x10 -6 ) + (10x10 -6 ) =47.5x10 -6 Thus, the reading could fall anywhere within the range of 1.5V 47.5V, an error of 0.003%. 所以，读数可以落在1.5V 47.5V的范围内，误差为0.003%。1.4.3 Deratings 指标降低 Accuracy specifications are subject to deratings for temperature and time drift, as discussed in the following paragraphs. 准确度的技术指标，由于温度和时间漂移等因素的影响会有所降低。如下所述： Temperature Coefficient 温度系数 The temperature of the operating environment can affect accuracy. For this reason, instrument specifications are usually given over a defined temperature range. Keithley accuracy specifications on newer electrometers, nanovoltmeters, DMMs, and SMUs are usually given over the range of 18C to 28C. For temperatures outside of this range, a temperature coefficient such as (0.005 % + 0.1 count)/C or (5ppm of reading + 1ppm of range)/C is specified. As with the accuracy specification, this value is given as a percentage of reading plus a number of counts of the least significant digit (or as a ppm of reading plus ppm of range) for digital instruments. If the instrument is operated outside the 18C to 28C temperature range, this figure must be taken into account, and errors can be calculated in the manner described previously for every degree less than 18C or greater than 28C. 工作环境的温度能够影响准确度。因此，仪器的技术指标通常是在规定的温度范围内给出的。吉时利公司新的静电计、纳伏表、数字多用表和SMU的准确度技术指标，通常都在18到28的温度范围内给出。在这个温度范围之外，规定了温度系数，如（0.005%+0.1 字）/，或者（读数的5ppm+量程的1ppm）/。和准确度技术指标一样，对数字显示仪器来说，温度系数表达为读数的百分数加上最低有效位的字数（或者读数的ppm+量程的ppm）。 如果仪器工作在18到28的温度范围之外，必须考虑这个系数。按照前面介绍的方法可以计算出低于18或高于28时的误差。Time Drift 时间漂移 Most electronic instruments, including electrometers, picoammeters, nanovoltmeters, DMMs, SMUs, and SourceMeter instruments, are subject to changes in accuracy and other parameters over a long period of time, whether or not the equipment is operating. Because of these changes, instrument specifications usually include a time period beyond which the instruments accuracy cannot be guaranteed. The time period is stated in the specifications, and is typically over specific increments such as 90 days or one year. As noted previously, transfer stability specifications are defined for a much shorter period of timetypically five or 10 minutes. 大多数电子仪器，包括静电计、皮安表、纳伏表、数字多用表、SMU 和数字源表仪器的准确度和其它参数，在长时间内无论设备是否工作，都会变化。由于这种变化，仪器的技术指标通常都指明一个时间间隔。超过这个时间间隔就不能保证仪器的准确度。这个时间间隔在仪器的技术指标中给出，通常为90天或1年。前面我们已经注意到，传递稳定度的技术指标是在很短的时间（典型值为5到10分钟）内规定的。1.4.4 Noise and Noise Rejection 噪声和噪声抑制 Noise is often a consideration when making virtually any type of electronic measurement, but noise problems can be particularly severe when making low level measurements. Thus, its important that noise specifications and terms are well understood when evaluating theperformance of an instrument. 实际上，在进行任何电子测量工作时都要考虑噪声。但是，在进行低电平测量工作时噪声问题特别重要。所以，在评价仪器的性能时，很好地理解噪声的技术指标和术语是非常重要的.Normal Mode Rejection Ratio 串模抑制比 Normal mode rejection ratio (NMRR) defines how well the instrument rejects or attenuates noise that appears between the HI and LO input terminals. Noise rejection is accomplished by using the integrating A/D converter to attenuate noise at specific frequencies (usually 50 and 60Hz) while passing low frequency or DC normal mode signals. As shown in Figure 1-5, normal mode noise is an error signal that adds to the desired input signal. Normal mode noise is detected as a peak noise or deviation in a DC signal. The ratio is calculated as: 串模抑制比（NMRR）说明了仪器抑制或衰减高、低输入端之间出现的噪声的能力。如何抑制噪声的方法是使用积分型A/D变换器对特定频率（通常为50或60Hz）的噪声进行衰减，而让低频或直流的串模信号通过。如图1-5所示，串模噪声是叠加到希望的输入信号上的误差信号。检测出的串模噪声表现为峰值噪声或直流信号偏差，串模抑制比按下式计算： Normal mode noise can seriously affect measurements unless steps are taken to minimize the amount added to the desired signal. Careful shielding will usually attenuate normal mode noise, and many instruments have internal filtering to reduce the effects of such noise even further. 如果不采取措施将叠加到信号上的串模噪声减到最小，串模噪声会对测量产生严重的影响。仔细的屏蔽通常能够衰减串模噪声，很多仪器都有内部的滤波器以进一步降低这种噪声的影响。Common Mode Rejection Ratio 共模抑制比 Common mode rejection ratio (CMRR) specifies how well an instrument rejects noise signals that appear between both input high and input low and chassis ground, as shown in Figure 1-6. CMRR is usually measured with a 1k. resistor imbalance in one of the input leads. 共模抑制比（CMRR）说明了仪器抑制高、低输入端与机箱地之间出现的噪声的能力。如图1-6所示，CMRR通常在一条输入引线上有1k电阻器的不平衡条件下测量。虽然共模噪声的影响通常没有串模噪声严重，但是这种噪声仍然是灵敏的测量工作中的一个影响因素。为了尽量降低共模噪声，将屏蔽只连到测试系统的一个点上才行。Noise Specifications 噪声的技术指标 Both NMRR and CMRR are generally specified in dB at 50 and 60Hz, which are the interference frequencies of greatest interest. (CMRR is often specified at DC as well.) Typical values for NMRR and CMRR are 80dB and 120dB respectively. NMRR和CMRR一般都在50或60Hz以dB为单位给出，因为这种频率是我们最感兴趣的干扰频率。CMRR常常也在直流下给出。NMRR、CMRR的典型数值分别为80d和120dB。 Each 20dB increase in noise rejection ratio reduces noise voltage or curre