So far, we have discussed considerations specific to voltage, current, resistance, and charge measurements. The following paragraphs examine considerations that apply to all types of electrometer and SMU measurements on high resistance sources.
至此，我们已经讨论了专门针对电压、电流、电阻和电荷测量所要考虑的问题。以下各段考察所有类型的静电计和SMU在测量高阻源时要考虑的问题。
2.6.1 Making Connections 恰当的连接
To avoid measurement errors, it’s critical to make proper connections from the electrometer, SMU, or picoammeter to the device under test. Always connect the high resistance terminal of the meter to the highest resistance point of the circuit under test.
为了避免测量误差，关键之点在于将静电计、SMU或皮安计和被测装置进行适当的连接。总是要把仪表的高阻端和被测电路的最高电阻点相连。
Figure 2-39 shows an electrometer connected to a current source that consists of a voltage source in series with a resistor. An AC powered source usually has a significant level (often several volts) of line frequency common mode voltage. As shown in Figure 2-40, this will cause a current (i) to flow through the low to ground capacitance of the electrometer (IM). This circuit is connected properly, so this current doesn’t flow through the electrometer measurement circuitry and, therefore, doesn’t cause any measurement errors. However, when the HI terminal of the electrometer is connected to the low impedance power supply, this AC current (i) flows through the electrometer (IM), as illustrated in Figure 2-41. This current may affect the measurement accuracy, especially at low signal levels.
图2-39所示为静电计和由电压源及电阻器串联组成的电流源相连接。交流供电的源通常具有很高电平的电源频率共模电压（常常有几伏）。如图2-40所示，这将会引起电流（i）流过静电计（IM）的低端到地的电容。此电路连接恰当，该电流不流过静电计的测量电路，所以不产生任何测量误差。然而，如果静电计的HI端连到电源的低阻端，则该交流电流（i）就流过静电计（IM），如图2-41所示。此电流就可能影响测量的准确度，在低信号电平时尤为显著。
See Section 2.6.6 for details on appropriate cabling and connector types for electrometer measurements.
有关静电计测量时恰当的电缆和连接器类型的详细情况，请见第 2.6.6节。
2.6.2 Electrostatic Interference and Shielding 静电干扰和屏蔽
Electrostatic coupling or interference occurs when an electrically charged object approaches the input circuit under test. At low impedance levels, the effects of the interference aren’t noticeable because the charge dissipates rapidly. However, high resistance materials don’t allow the charge to decay quickly, which may result in unstable measurements. The erroneous readings may be due to either DC or AC electrostatic fields, so electrostatic shielding will help minimize the effects of these fields.
带电物体接近被测电路的输入端时，就会发生静电耦合和干扰。在低阻抗之下，由于电荷迅速消散，所以干扰的影响不明显。然而，高阻材料不允许电荷迅速衰减，就可能产生不稳定的测量结果。由于错误的读数可能由直流或交流静电场引起，所以静电屏蔽有助于尽量降低这种电场的影响。
DC fields can produce noisy readings or undetected errors. These fields can be detected when movement near an experiment (such as the movement of the person operating the instrument or others in the immediate vicinity) causes fluctuations on the electrometer’s display. To perform a quick check for interference, place a piece of charged plastic, such as a
comb, near the circuit. A large change in the meter reading indicates insufficient shielding.
直流电场可能产生有噪声的读数或无法探测的误差。实验电路附近的运动（例如，操作仪器人员的运动或者在临近区域里的其它运动等）引起静电计显示读数发生波动，就反映出这种场的存在。为了迅速检查干扰的存在，在电路附近放置一个带电的塑料物体，如梳子等。仪表的读数发生大的变化就说明屏蔽不够完善。
AC fields can be equally troublesome. These are caused most often by power lines and RF fields. If the AC voltage at the input is large, part of this signal is rectified, producing an error in the DC signal being measured. This can be checked by observing the analog output of the electrometer or picoammeter with an oscilloscope. A clipped waveform indicates a need to improve electrostatic shielding. Figure 2-42 illustrates a clipped waveform taken from the 2V analog output of an electrometer. In this example, the amount of clipping reduced the DC current reading by nearly 50%.
交流电场同样会产生麻烦。交流电场常常由供电电源和RF场引起。如果输入端的交流电压很大，其一部分信号被整流，于是在被测的直流信号中产生了误差。用示波器观察静电计或皮安计的模拟输出，可以对此进行检查。限幅的波形表明需要改进静电屏蔽。图2-42示出在静电计的2V模拟输出端观察到的限幅波形。在这个例子中，限幅作用使直流读数降低大约50%。
For an SMU, check for AC pickup by connecting the oscilloscope between the guard terminal and common.
对于SMU来说，在其保护端和公共端之间连接示波器就可以观察交流干扰情况。
Figure 2-43 shows an example of AC electrostatic coupling. An electrostatic voltage source in the vicinity of a conductor, such as a cable or trace on a PC board, generates a current proportional to the rate of change of the voltage and of the coupling capacitance. This current can be calculated with the following equation:
图2-43示出一个交流静电耦合的例子。在导体（例如电缆或印制电路板上的线）附近的静电电压源会产生正比于电荷变化率和耦合电容变化率的电流。该电流可以按下式来计算：
i = C dv / dt + V dC / dt
For example, two conductors, each with 1cm2 area and spaced 1cm apart by air, will have almost 0.1pF of capacitance. With a voltage difference of 100V between the two conductors and a vibration causing a change of capacitance of 0.01pF/second (a 10% fluctuation between them), a current of 1pA AC will be generated.
例如，两个面积为1厘米2，在空气中相距1厘米的导体，具有大约0.1pF的电容。若两个导体之间的电压差为100V，由于振动引起该电容的变化为0.01pF/秒（10%的电容变化量），这时就能产生1pA 的交流电流。
To reduce the effects of the fields, a shield can be built to enclose the circuit being measured. The easiest type of shield to make is a simple metal box or meshed screen that encloses the test circuit. Shielded boxes are also available commercially.
为了降低电场的影响，可以制作屏蔽将被测电路包围起来。最容易制作的屏蔽形式为包围被测电路的简单的金属盒子或金属网。屏蔽盒也可以在市场上买到。
Figure 2-44 illustrates an example of shielding. Made from a conductive material, the shield is always connected to the low impedance input of the electrometer or picoammeter or to the output LO (or common) terminal of the SMU. If circuit LO is floating above ground, observe special safety precautions to prevent anyone from touching the shield. These safety precautions
are discussed in Section 2.6.8.
图2-44是一个屏蔽的例子。用导电材料制成的屏蔽总是连到静电计或皮安计的低阻抗输入端，或者连到SMU的LO输出端（或公共端）。如果电路LO端对地浮空，则要采取特别的安全措施，避免人员触及该屏蔽。这些安全措施将在第2.6.8节讨论。
The cabling between the HI terminal of the meter and the device under test also requires shielding. Capacitive coupling between an electrostatic noise source and the signal conductors or cables can be greatly reduced by surrounding those conductors with a metal shield connected to LO, as shown in Figure 2-45. With this shield in place, the noise current generated by the electrostatic voltage source and the coupling capacitance flows through the shield to ground rather than through the signal conductors.
仪表HI端和被测装置之间的电缆也需要屏蔽。用连接到LO端的金属屏蔽将信号导体包围起来，可以大大降低静电噪声源和信号导体或电缆之间的电容耦合，如图2-45所示。有了这种屏蔽，由静电电压源和耦合电容产生的噪声电流就经过屏蔽流到地，而不再流过信号线。
To summarize, follow these guidelines to minimize error currents due to electrostatic coupling:
总的说来，遵守下列指导原则能够尽量降低静电耦合产生的电流：
- 使所有带电物体（包括人员）和导体远离测试电路的敏感区域。
- Avoid movement and vibration near the test area
- 在测试区域附近避免运动和振动。
- When measuring currents <1nA, shield the device under test by surrounding it with a metal enclosure and connect the enclosure electrically to the test circuit common terminal.
- 当测量电流小于1nA时，将被测装置用金属闭合物包围屏蔽起来，并将该闭合物连到测试电路的公共端。
Shielding vs. Guarding 屏蔽和保护
Shielding usually implies the use of a metallic enclosure to prevent electrostatic interference from affecting a high impedance circuit. Guarding implies the use of an added low impedance conductor, maintained at the same potential as the high impedance circuit, which will intercept any interfering voltage or current. A guard doesn’t necessarily provide shielding. Guarding is described further in Section 2.2.1 for voltmeters, Section 2.3.1 for ammeters, and Section 2.4.2 for ohmmeters.
屏蔽通常意味着使用金属的闭合物来避免静电干扰影响高阻抗电路。而保护意味着使用保持在与高阻抗电路相同电位的附加的低阻抗导体来阻止可能的干扰电压或电流。保护措施不一定提供屏蔽。有关电压表、安培计和欧姆计的保护技术将在第2.2.1、2.3.2和2.4.2节进一步介绍。
2.6.3 Environmental Factors 环境因素
A stable test environment is essential when making accurate low level measurements. This section addresses important environmental factors that may affect the accuracy of low level measurements.
稳定的测试环境是进行准确的低电平测量的基本条件。本节阐述可能影响低电平测量准确度的重要环境因素。
Temperature and Temperature Stability 温度和温度稳定性
Varying temperatures can affect low level measurements in several ways, including causing thermal expansion or contraction of insulators and producing noise currents. Also, a temperature rise can cause an increase in the input bias current of the meter. As a general rule, JFET gate leakage current doubles for every 10°C increase in temperature, but most electrometers are temperature compensated to minimize input current variations over a wide temperature range.
温度变化能在几个方面影响低电平测量工作，包括：引起绝缘体的热膨胀或收缩，产生噪声电流等。此外，温度上升还会引起仪表偏置电流的增加。一般规律是温度每上升10℃，JFET门极泄漏电流增加一倍。但是大多数静电计都进行了温度补偿，在很宽的温度范围内尽量降低了输入电流的变化。
To minimize errors due to temperature variations, operate the entire system in a thermally stable environment. Keep sensitive instruments away from hot locations (such as the top of a rack) and allow the complete system to achieve thermal stability before making measurements. Use the instrument’s zero or suppress feature to null offsets once the system has achieved thermal stability. Repeat the zeroing process whenever the ambient temperature changes. To ensure optimum accuracy, zero the instrument on the same range as that to be used for the measurement.
为了尽量减小温度变化引起的误差，应当使整个系统在温度稳定的环境中工作。让灵敏的仪器远离热的位置（例如机架的顶部），并在进行测量之前使整个系统达到热稳定状态。系统达到热稳定状态之后，使用调零或零点抑制功能消除偏置量。每当环境温度变化时，都要重复消零操作。为了确保最佳的准确度，应当在要进行测量的量程上进行消零操作。
Humidity 湿度
Excess humidity can reduce insulation resistance on PC boards and in test connection insulators. A reduction in insulation resistance can, of course, have a serious effect on high impedance measurements. In addition, humidity or moisture can combine with any contaminants present to create electrochemical effects that can produce offset currents.
过高的湿度会降低印制电路板和测试连接绝缘子的绝缘电阻。当然，绝缘电阻的降低会严重影响高阻抗测量工作。此外，湿度或湿气还能够与存在的污染物结合起来，产生电化学效应，并产生偏置电流。
To minimize the effects of moisture, reduce the humidity in the environment (ideally <50%). Be sure all components and connectors in the test system are clean and free of contamination. When cleaning, use only pure solvents to dissolve oils and other contaminants, then rinse the cleaned area with fresh methanol or deionized water. Allow cleaned areas to dry for several hours before use.
为了尽量降低湿气的影响，应当降低工作环境中的湿度（理想情况为<50%）。确保测试系统中的所有元件和连接器清洁、无污染。进行清洁处理的时候，只使用纯净的溶剂来溶解油脂和其它的污染物,然后用新的甲醇或去离子水冲洗清洁过的区域。使清洁过的区域干燥几个小时后再使用。
Light 光
Some components such as diodes and transistors are excellent light detectors. Consequently, these components must be tested in a light-free environment. To ensure measurement accuracy, check the test fixture for light leaks at doors and door hinges, tubing entry points, and connectors or connector panels.
某些元件，如二极管、三极管等都是很好的光探测器。所以这些元件都必须在闭光的环境中测试。为了确保测量的准确度，应当检查测试夹具的门、门铰链、管道进口点、连接器和连接器面板等处是否漏光。
Ionization Interference 电离干扰
Current measurements made at very low levels (<100fA) may be affected by ionization interference from sources such as alpha particles. A single alpha particle generates a track of from 30,000 to 70,000 positive and negative ions per cm, which may be polarized and moved about by ambient electric fields. Also, ions that strike a current-sensing node may generate a “charge hop” of about 10fC per ion.
极低电平（<100fA）的电流测量可能会受到诸如阿尔法粒子源等的电离干扰的影响。一个单个的阿尔法粒子能够产生每厘米30000到 70000个正、负离子的踪迹，这些粒子可能受到周围电场的作用而极化或运动。另外，撞击电流敏感节点的粒子能够产生大约每个粒子10fC 的“电荷跳变”。
There are several ways to minimize noise in the test system due to ionization interference. First, minimize the volume of air inside the shield around sensitive input nodes. Also, keep sensitive nodes away from high intensity electric fields.
有几种办法可以尽量降低测试系统中电离干扰引起的噪声。首先，尽量减少灵敏输入节点周围屏蔽装置内的空气体积。而且，要使灵敏的节点远离高强度电场。
RFI (Radio Frequency Interference) RFI（射频干扰）
Interference from radio frequency sources can affect any sensitive electrometer measurement. This type of interference may be indicated by a sudden change in the reading for no apparent reason.
来自射频源的干扰能够影响各种灵敏的静电计的测量工作。这种类型的干扰表现为读数的突然变化而无明显的理由。
A non-linear device or junction in the input circuit can rectify the RF energy and cause significant errors. Sources of such RFI are nearby transmitters, contactors, solenoid valves, and even cellular telephones and portable two-way radios.
输入电路中的非线性器件或结能够将RF能量整流，并引起很大的误差。这类射频干扰源可能是附近的发射机、接触器、电磁阀门、甚至蜂窝电话和便携式对讲机。
Once the source is identified, the RF energy may be reduced or elimi nated by shielding and adding snubber networks or filters at appropriate points. Consult Section 3.2.1 for further discussion of RFI.
一旦确定了干扰源，可以使用屏蔽和在适当的地点增加缓冲电路网络或滤波器等措施来降低或消除射频干扰。有关射频干扰的进一步讨论请参见第3.2.1节。
2.6.4 Speed Considerations 速度问题的考虑
Time and Frequency Relationships 时间和频率的关系
Although this handbook stresses DC measurements, an analysis of noise and instrument response speed requires a brief discussion of time and frequency relationships in electronic circuits.
虽然本书的重点是直流测量，但是要分析噪声和仪器响应速度的问题，就需要简单地讨论一下电子电路中时间和频率的关系。
A steady-state DC signal applied to a voltmeter presents no conceptual difficulty. However, if the signal has a time-varying component such as an AC signal superimposed on the DC signal, the meter will tend to follow the varying signal and show the instantaneous magnitude of the input. As the frequency of the AC component increases, the DC meter response decreases, until at some frequency only the average input voltage will be displayed. The frequency at which the voltmeter’s response to an AC signal drops to 70% is often denoted as the “3dB point” (f3dB). Digital multimeters have a bandwidth of roughly half the conversion rate (readings per second) at the display. The analog output has a much wider bandwidth unless it’s reconstructed from digital information.
理解稳态直流信号加到电压表上的情况在概念上没有什么困难。然而，如果该信号具有时变的分量，例如在直流信号上叠加了交流信号，仪表就将跟随该变化的信号，并表示出该输入信号的瞬时幅度。当交流分量的频率增加时，直流仪表的响应就会变得不够快，直到在某一频率时，仪表只能显示出输入电压的平均值。电压表对交流信号的响应降低 到70%时的频率常常称为“3dB点”（f3dB）。数字多用表的带宽粗略地为其显示读数的变换速率（每秒钟的读数次数）的一半。除了将数字量重新变换为模拟信号的情况，仪表模拟输出的带宽一般要宽得多。
Bandwidth describes the instrument’s ability to respond to time varying signals over a range of frequencies. Another measure of the instrument’s response is its ability to respond to a step function; the typical measure of response is the rise time of the instrument. Bandwidth or rise time may be used to describe the instrument’s response to time-varying signals.
带宽说明了仪器在某一频率范围内响应时变信号的能力。仪器响应速度的另一种度量方法是其响应阶跃函数信号的能力。这种响应的典型度量是仪器的上升时间。带宽或上升时间可以用来说明仪器对时变信号的响应情况。
Rise time of an analog instrument (or analog output) is generally defined as the time necessary for the output to rise from 10% to 90% of the final value when the input signal rises instantaneously from zero to some fixed value. This relationship is shown in Figure 2-46. In Figure 2-46a, a step function with an assumed rise time of zero is shown, while Figure 2-46b shows the instrument’s response and the associated rise time. Rise time, frequency response, and the RC time constant of a first order system are related. The 3dB point is given by the relationship:
模拟仪器（或模拟输出）的上升时间一般定义为输入信号从零立即上升到某一固定值时，输出信号从最终值的10%上升到90%所需要的时间。此关系示于图2-46。图2-46a示出假定上升时间为零的阶跃函数，而图2-46b示出仪器的响应及相应的上升时间。单极点系统（1阶系统）的上升时间、频率响应和RC常数是有关联的。3dB点由下式给出：
Rise time (tr) is related to the RC time constant as follows:
上升时间（ 
）与RC时间常数的关系如下：
2.6.4 Speed Considerations 速度问题的考虑
For example, the rise time of a circuit with a source resistance of 1T. and capacitance of 100pF will be approximately:
例如，源电阻为1TΩ、电容为100pF的电路的上升时间大约为：
tr = (2.2) (1012) (100 × 10–12) = 220 seconds
Using this with the above relationship between RC and f3dB, we see that:
使用上述RC和f3dB的关系，可以看到：
Thus, the 1T. source resistance and 100pF capacitance limit the bandwidth to:
因此， 1TΩ的源电阻和100pF的电容将带宽限制在：
Rise time affects the accuracy of the measurement when it’s of the same order of magnitude as the period of the measurement. If the length of time allowed before taking the reading is equal to the rise time, an error of approximately 10% will result, since the signal will have reached only 90% of its final value. To reduce the error, more time must be allowed. To reduce the error to 1%, about two rise times must be allowed, while reducing the error to 0.1% would require roughly three rise times (or nearly seven time constants).
当上升时间和测量周期的数量级相同时，就会影响测量的准确度。如果获取读数前允许的时间等于上升时间，将会产生大约10%的误差，因为信号只能上升到其最终值的90%。为了降低误差，必须等待更长的时间。为使误差降低到1%，必须等待大约两倍的上升时间。而为了使误差降低到0.1%，则必须等待大约三倍的上升时间（或者接近7倍时间常数的时间）。
Beyond the 0.1% error level (and occasionally the 1% level), second- order effects come into play. For example, more than four rise times are generally required to settle to within 0.01% of final value, due to dielectric absorption in insulators and other second-order effects.
在要求的误差优于0.1%（有的时候是1%）的情况下，二极点效应开始起作用。例如，由于绝缘体的介电吸收和其它的二阶效应，为了达到最终值的0.01%，一般需要4倍以上上升时间的时间长度。
In summary, an analog instrument’s response (or the analog output response of most digital instruments) to a changing input signal is a function of its bandwidth, since frequency response and rise time are directly related. To ensure accurate measurements, sufficient settling time must be allowed for the source, the connection to the instrument, and the instrument itself to settle after the input signal is applied.
总的来说，由于频率响应和上升时间直接有关，模拟仪器（或者大多数数字仪器的模拟输出）对于变化的输入信号的响应是其带宽的函数。为了确保准确的测量结果，在加入输入信号之后，必须允许足够的建立时间，以便使源、仪器的连接以及仪器本身建立到其稳定的状态。
Effects of Input Capacitance on Rise Time and Noise 输入电容对上升时间和噪声的影响
电压测量
In voltage measurements from high impedance sources (Figure 2-47), capacitance (CIN) across the voltmeter (VM) must be charged through RS. The equation for the output voltage as a function of time is:
在对高阻抗源进行电压测量时（图2-47），电压表（VM）两端的电容（CIN）必须通过RS充电。输出电压对时间的函数关系为：
VM = VS (1-e-t/RSCIN)
where: VM = voltmeter reading at t seconds
其中：VM = 在t秒时电压表的读数
VS = step function source
VS = 阶跃函数电压源
t = time in seconds after step occurs
t = 阶跃发生后的时间秒数
RS = equivalent series resistance in ohms
RS = 以欧姆为单位的等效串联电阻
CIN = equivalent shunt capacitance in farads(instrument plus cable capacitance)
CIN = 以法拉为单位的等效并联电容（仪器的电容加电缆的电容）
Thus, the familiar exponential curve of Figure 2-48 results, in which it becomes necessary to wait four or five time constants to achieve an accurate reading. In the case of large resistors and capacitance, the rise time can range up to minutes. While increased shunt capacitance causes rise time to increase, it does filter out noise produced in the source and interconnecting cable simply by reducing the effective bandwidth of the voltmeter.
这样就得到了图2-48所示的熟悉的指数曲线。要获得准确的读数就必须等待4到5倍时间常数的时间。在大数值电阻和电容的情况下，上升时间可能达到数分钟。加大并联电容虽然增加了上升时间，但是由于降低了电压表的有效带宽，所以就滤掉了由源和互连电缆产生的噪声。
Shunt Current Measurements 分流电流测量
The effects of input capacitance on current measurements using a shunt type ammeter (Figure 2-49) are similar to those for voltage measurements. A shunt ammeter can be modeled as a voltmeter with a resistor across the input. The circuit shows that the input capacitance (CIN) must be charged toRSCINvolts, at an exponential rate of the ISRS time constant. Note that CIN is the sum of the source, connecting cable, and meter capacitance.
使用分流型安培计（图2-49）时，输入电容对电流测量的影响与电压测量时类似。分流型安培计可以看成是在其输入端跨接了电阻器的电压表。电路表明，输入电容（CIN）必须以时间常数RSCIN的指数速率，充电到ISRS伏。注意，CIN是源、连接电缆和仪表电容之和。
Feedback Current Measurements 反馈电流测量
The effect of input capacitance on current meters employing negative feedback is different than the effect on the shunt ammeter. The circuit for this mode is shown in Figure 2-50.
输入电容对采用负反馈的电流表的影响与其对分流型安培计的影响不同。这种模式的电路示于图2-50。
If A, the gain of the amplifier, is large, then VO = –IINRFB. In such an arrangement, CIN doesn’t shunt RFB, and has only a fraction of the effect it would have with a shunt picoammeter. The resulting speed-up comes from the reduction of the input impedance of the picoammeter due to negative feedback. In other words, only VS = –VO/A volts is developed across CIN instead of the VO that would occur in a shunt picoammeter. Thus, even large values of capacitance shunting the input will have negligible effect on rise time.
如果放大器的增益A很大，则V0 = -IINRFB 。在这种情况下，CIN不会对RFB分流。其影响与分流皮安计的情况相比是很小的。速度提高的原因是由于负反馈的作用使皮安计的输入阻抗降低。换言之，在CIN上产生的电压只有VS = -V0/A伏，而分流皮安计时此电压却为V0。所以，即使并联在输入端的电容很大，其对上升时间的影响也很小。
Rise time in a feedback picoammeter is a function of the physical or stray capacitance shunting the feedback resistance (RFB). Electrometers, SMUs, and picoammeters can be used with relatively large values of source capacitance. It’s important to realize that increasing values of input shunt capacitance (the parallel combination of source, cable and input capacitances) will degrade the signal-to-noise ratio of a given measurement. See Sections 2.3.2 and 4.3.1 for more information on noise and source impedance.
反馈型皮安计的上升时间是反馈电阻（RFB）上并联的物理电容或寄生电容的函数。静电计、SMU和皮安计等都可以使用比较大的源电容数值。应当认识到增大输入并联电容（包括源、电缆和输入电容等的并联效果）的数值将会使测量的信号-噪声比降低。关于噪声和源阻抗的更多的信息请参见第2.3.2和4.3.1节。
Resistance Measurements (Constant-Current Method)
电阻测量（恒流法）
Input capacitance also affects resistance measurements (Figure 2-51) in the same manner. Again, CIN must be charged by the current (IR), hence, the same equation applies. (See Section 2.4.2 for more information on the constant- current method.)
输入电容也会以同样的方式影响电阻测量（图2-51）。这时，CIN也必须由电流（IR）充电，因此也适用同样的公式。（关于恒流法的更多的信息请参见第2.4.2节。）
Electrometer Rise Time Summary 静电计上升时间小结
For most measurements of high resistance sources, rise time considerations require minimizing the capacitance shunting the meter input. Earlier, it was shown that doing so also minimizes noise gain. In broader terms, the source impedance should be large compared to the feedback impedance of the meter.
对于大多数高阻源的测量来说，考虑上升时间的时候，需要尽量减小仪表输入端并联的电容。前面提到，这样做同时也降低了噪声增益。广义地说，与仪表的反馈阻抗相比，源阻抗应当比较大。
The most effective method of minimizing input capacitance is to connect the electrometer, SMU, or picoammeter to the signal source with a shielded cable that is as short as possible. When measuring a voltage from a high source resistance, or when measuring high resistance, guarding can minimize the effects of input capacitance by driving the inner shield of a triax cable or an enclosure surrounding the input with a potential to minimize the effective capacitance, as discussed in Section 2.2.1.
减小输入电容最有效的方法是用尽可能短的屏蔽电缆将静电计、SMU或皮安计与信号源连接起来。在测量高阻源的电压或者测量高电阻的时候，保护技术可以尽量降低输入电容的影响。因为这时用适当的电位驱动三同轴电缆的内层屏蔽或包围输入端的屏蔽盒，从而尽量降低了有效电容，如第2.2.1节所述。
2.6.5 Johnson Noise 约翰逊噪声
The fundamental limit to measurement is Johnson noise in the source resistance. In any resistance, thermal energy produces motion of charged particles. This charge movement results in noise, which is often called Johnson or thermal noise. The power available from this motion is given by:
对测量的基本限制是源电阻中的约翰逊噪声。在任何电阻中，热能都会产生带电粒子的运动。这种电荷的运动产生了噪声，通常称为约翰逊噪声或热噪声。带电粒子运动的功率为：
P = 4kTB
where: k = Boltzmann’s constant (1.38 × 10–23J/K)
其中：k = 波尔兹曼常数（1.38×10 -23 J/K）
T = absolute temperature in K
T = 绝对温度，单位为K
B = noise bandwidth in Hz
B = 噪声带宽，单位为Hz
Metallic conductors approach this theoretical noise limit, while other materials produce somewhat higher noise. Johnson voltage noise (E) developed in a resistor (R) is:
金属导体接近此理论的噪声极限，而其它材料产生的噪声则更高一些。电阻器（R）上产生的约翰逊噪声电压（E）为：
and Johnson current noise (I) developed by a resistor (R) is:
而电阻器（R）上产生的约翰逊噪声电流（I）为：
Statistical considerations show that peak-to-peak noise will be within five times the rms noise more than 99% of the time; therefore, the rms level is commonly multiplied by five to convert to peak-to-peak. At room temperature (300K), the previous equations become:
统计分析表明，在99%以上的时间内噪声的峰-峰值都在5倍的噪声的有效值范围内。所以通常将有效值噪声电平乘以5变成峰-峰值噪声。在室温下（300K），上述公式变为：
All real voltage and current sources contain an internal resistance; therefore, they exhibit Johnson noise. Figure 2-52 shows Johnson noise voltage versus source resistance for various bandwidths (or rise times) at room temperature.
所有真实的电压源和电流源都具有内阻，所以它们都表现出约翰逊噪声。图2-52示出在室温下，对于不同的带宽（或上升时间），约翰逊噪声电压与源电阻的关系。
For current measurements, Figure 2-53 shows the current noise generated by various resistances at various bandwidths. Note that current noise decreases with increasing resistance, while voltage noise increases.
对于电流测量来说，图2-53示出在不同带宽下，不同电阻产生的电流噪声。注意，当电阻增加时，电流噪声减小，而电压噪声增大。
Johnson noise imposes a theoretical limit to achievable voltage or current resolution. The previous equations suggest several means for reducing Johnson noise. It might be possible to reduce the bandwidth, the source temperature, or the source resistance.
约翰逊噪声给出了可以达到的电压或电流测量分辨率的理论极限值。上述公式给出了降低约翰逊噪声的几种方法，即可以减小带宽、降低源的温度或者降低源电阻 。
Bandwidth 带宽
Johnson noise is uniformly distributed over a wide frequency range, so reducing the noise bandwidth effectively decreases the noise in the measurement. Note that noise bandwidth isn’t necessarily the same as signal bandwidth. The high frequency noise cutoff point is approximately equal to the smallest of:
约翰逊噪声在很宽的频率范围内呈均匀分布，所以减小噪声带宽能够有效地降低测量中的噪声。注意，噪声带宽不必与信号带宽相同。高频噪声介质频率点大约等于以下各数值中的最小者：
- π/2 times the upper 3dB frequency limit of the analog DC measuring circuitry
- л/2乘以模拟直流测量电路的3dB频率上限值。
- 0.35/tr where tr is the instrument’s 10%–90% rise time
- 其中， 为仪器的10%到90%上升时间。
- 1Hz if an analog panel meter is used for readout or
- 1Hz，如果使用模拟面板表来读数则使用此值。
- 0.314/tINT where tINT is the integration period of the A/D converter in a digital instrument.
- 0.314/tINT，其中，tINT为数字仪器中A/D变换器的积分时间。
In high resistance circuits, the noise bandwidth is often limited by the time constant of the source resistance and input capacitance, and this value In this case, noise bandwidth is:
在高阻电路中，噪声带宽常常受到源电阻和输入电容的时间常数的限制。这个数值为上述各种噪声带宽计算结果中的最小者。在这种情况下，噪声带宽为：
where REFFECTIVE is the source resistance in parallel with the input resistance of the measuring device, and CIN is the sum of all capacitance shunting the input to the instrument (input capacitance, cable capacitance, etc.) Note that this analysis assumes a simple first-order system with one dominant time constant.
其中，REFFECTIVE 为源电阻与测量设备输入电阻的并联，CIN为与仪器输入端并联的所有电容（输入电容、电缆电容等）之和。注意，此分析假定，该电路为具有一个主导时间常数的简单一阶系统。
To reduce noise, the bandwidth (B) may be reduced artificially by averaging an analog meter reading by eye over an extended period, or by averaging a number of digital readings with a computer, or by internal digital filtering. Using low pass filters before the readout device may also reduce bandwidth. There is a practical limit to reducing bandwidth since very long- term measurements become susceptible to other errors, such as time and temperature drift.
为了降低噪声，可以采用在很长的时间内用目测平均模拟仪表读数的方法，或者用计算机或内部数字滤波器平均多个数字读数的方法，来减小带宽（B）。在读出设备之前采用低通滤波器也可以减小带宽。减小带宽时有一个实际的限制，如果测量时间太长，测量就容易受到时间和温度漂移等其它误差的影响。
Temperature 温度
Reducing the temperature of the signal source from room temperature to –270°C (3K) decreases noise voltage by a factor of about ten. Similarly, a reduction from room temperature to liquid nitrogen levels (77K) reduces noise by a factor of two. In some applications, the inconvenience and expense of cryogenic operation may be justified and feasible. However, most experiments are designed to operate within a certain temperature range, which in turn determines the noise to be expected from the source.
将信号源的温度从室温降低到-270℃（3K），可以将噪声电压降低10倍。同样，将信号源的温度从室温降低到液氮的温度（77K），可以将噪声电压降低一半。在某些应用中，不方便的、昂贵的低温工作需求是正当的、可行的。但是，大多数的实验则是设计工作在一定的温度范围之内，这也决定了可以预期的源噪声水平。
Source Resistanc 源电阻
After the bandwidth and temperature, the remaining factor in determining the system noise is the effective source resistance. The effective source resistance includes the device under test as well as the measurement instrument. Changing the source resistance is usually impractical for noise reduction. However, if a change can be made, the equations show that R should be lowered to decrease voltage noise or raised to decrease current noise.
讨论了带宽和温度以后，决定系统噪声的另一个因素就是等效源电阻。等效源电阻包括被测设备和测量仪器两个方面。为了降低噪声，改变源电阻通常是不实际的。然而，如果可以改变源电阻的话，公式表明，要降低电压噪声应当减小R，而要降低电流噪声应当加大R。
In voltage measurements, the voltage source resistance is in parallel with the voltmeter input resistance (see Figure 2-1). The input resistance is normally much larger than the source resistance; hence, the source resistance value usually determines the Johnson noise voltage.
在电压测量中，电压源的电阻是与电压表的输入电阻并联的（见图2-1）。输入电阻通常要比源电阻大很多。所以源电阻的数值通常决定了约翰逊噪声电压的大小。
In current measurements, the source resistance and the sensing resistance both contribute noise. The effective resistance is the parallel combination of the source resistance and the feedback (or shunt) sensing resistance. Feedback ammeters with high value sensing resistors in the feedback loop have lower Johnson current noise and thus greater sensitivity than shunt ammeters with lower resistance shunts.
在电流测量中，源电阻和取样电路电阻都会产生噪声。有效电阻为源电阻和反馈（或分流）取样电阻的并联组合。反馈安培计反馈回路中的取样电阻阻值高，其约翰逊电流噪声就比较低，从而比分流电阻值低的安培计的灵敏度更高。
Excess Current Noise 过量电流噪声
The Johnson noise of a resistor is related only to the resistance, the temperature, and the bandwidth. When current passes through a resistor, the noise will increase above the calculated Johnson noise. This increase in noise is sometimes referred to as “excess current noise.” A wirewound resistor is nearly ideal and the noise increase is negligible. Metal film resistors have somewhat greater noise and carbon composition resistors are significantly noisier still. In all cases, this excess noise is directly proportional to the current through the resistor.
电阻器的约翰逊噪声只与电阻值、温度和带宽有关。当电流流过电阻器时，其噪声比计算出的约翰逊噪声还要高。这种增加的噪声常常称为“过量电流噪声（Excess Current Noise）”。线绕电阻器接近理想情况，其增加的噪声可以忽略。而金属膜电阻器的噪声就要大一些，碳合成电阻器的噪声则会更大。在所有这些情况下，此过量电流噪声都直接正比于流过电阻器的电流。
2.6.6 Device Connections 电缆、接头与夹具
Although instrument accuracy is of great importance when making low level measurements, the integrity of device connections is equally important. The complete signal path from connectors, through the cables, and into the test fixture must degrade the measured signal as little as possible. The following paragraphs discuss cable and test fixture requirements and types of connectors generally used when making low level measurements.
在进行低电平测量时，仪器的准确度虽然很重要，设备连接的整体性也同样重要。从连接器，经过电缆，再进入测试夹具的整个信号通道中，被测信号的损失都必须尽可能地低。以下各段将讨论在进行低电平测量时对电缆和测试夹具的要求以及常用的连接器类型。
Cable Requirements 电缆要求
Although DMMs often use unshielded test leads, such connection schemes are generally inadequate for low level measurements made with picoammeters, electrometers, and SMUs. These instruments generally use either coaxial or triaxial cables.
虽然数字多用表常常使用无屏蔽的测试引线，但是这种连接方式在使用皮安计、静电计和SMU等进行低电平测量时一般是不合适的。这些仪器通常使用同轴电缆或三同轴电缆。
A coaxial cable consists of a single conductor surrounded by a shield (Figure 2-54a), while a triaxial cable adds a second shield around the first (Figure 2-54b). With triax cable, the inner shield can be driven at guard potential in order to reduce cable leakage and minimize circuit rise times. The outer shield is usually connected to chassis ground or, in some cases, to the common terminal. In either case, the outer shield must not be allowed to float more than 30Vrms (42.4V peak) above chassis ground for safety considerations. Always use a cable with a tightly woven shield to protect against electrostatic interference.
同轴电缆由屏蔽包围的单芯导体组成（图2-54a），而三同轴电缆在第一层屏蔽之外又加入了第二层屏蔽（图2-54b）。使用三同轴电缆时，为了减少电缆泄漏并尽量降低电路的上升时间，可以将内层屏蔽驱动到保护电位。外层屏蔽通常连到机箱地，有的时候连到公共端。在这两种情况下，考虑到安全因素，外层屏蔽的电位不得比机箱地高出30V有效值（42.4V峰值）。一定要使用编织紧密的屏蔽以避免静电干扰。
Both coaxial and triaxial cables are available in low noise versions, which should be used for low level measurements. Low noise cables have internal graphite coatings to minimize current generated by triboelectric effects. (See Section 2.3.4.) In some cases, ordinary coaxial cable such as RG-58 may be adequate, although both leakage and noise currents will be higher than with low noise cables.
同轴电缆和三同轴电缆都有低噪声的产品型号，在低电平测量中应当使用这种电缆。低噪声电缆的内部有石墨涂敷层，以尽量降低由摩擦电效应产生的电流（见第2.3.4节）。虽然普通同轴电缆的泄漏和噪声电流比低噪声电缆要高，但是在某些情况下，普通的同轴电缆，如 RG-58可能还是适用的。
When measuring high resistance, the insulation resistance of the cable is important. Good quality triaxial cables use polyethylene insulators and have a typical conductor-to-shield insulation resistance of about 1TΩ/ft. Refer to Section 2.2.2 for more information on insulation characteristics.
在测量高电阻时，电缆的绝缘电阻是很重要的。质量良好的三同轴电缆使用聚乙烯绝缘材料，其导体到屏蔽的典型绝缘电阻值大约为1TΩ/英尺。 关于绝缘材料特性的详细信息请参见第2.2.2节。
Parameters like cable resistance, capacitance, and leakage currents change as cable length increases. Thus, it’s important to keep all connecting cables as short as possible. For example, a ten-foot cable with 1TΩ/ft resistance and 100pF/ft capacitance will have an insulation resistance of 100G. and a capacitance of 1000pF.
当电缆的长度增加时，其电缆电阻、电容和泄漏电流等参数也会变化。所以重要之点是使所有的连接电缆尽可能地短。例如，电阻参数为1TΩ/英尺 、电容参数为100pF/英尺的电缆，当其长度为10英尺时，绝缘电阻为100GΩ、电容为1000pF。
Connector Types 连接器类型
Two general types of connectors are used for electrometer, picoammeter, and SMU measurements. The BNC connector shown in Figure 2-55 is a type of coaxial connector. It includes a center conductor and shell or shield connection, while the triax connector shown in Figure 2-56 includes a center conductor, an inner shield, and an outer shield.
静电计、皮安计和SMU的测量工作中使用两种通用类型的连接器。图2-55所示的BNC连接器是一种同轴连接器。它包括中心导体和外壳或屏蔽连接，而图2-56所示的三同轴连接器则包括中心导体、内屏蔽和外屏蔽。
The center conductor of the BNC connector is connected to input HI, while the outer shell is input LO. Note that the shell may be connected directly to chassis ground at the instrument.
BNC连接器的中心导体连到输入HI端，而外壳连到输入LO
