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微电阻四线量测法.pdf

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  • 常见问题
    • 除了在第 3.2 节介绍的所有低电压测量中要考虑的问题之外 , 在低电阻的测 量中更容易引进附加的误差源,其中包括引线电阻、非欧姆接触以及器件的 加热问题这一节将要介绍这些误差源以及将其消除或者降至最小的各种方 法此外还要介绍其它一些测量中要考虑的因素,包括干电路测试和电感性 器件的测试等 3.3.1Lead Resistance and Four-Wire Method 引线电阻和四线方法引线电阻和四线方法 Resistance measurements are often made using the two-wire method shown in Figure 3-14. The test current is forced through the test leads and the resistance (R) being measured. The meter then measures the voltage across the resistance through the same set of test leads and computes the resistance value accordingly. 电阻的测量常常使用图 3-14 所示的两线方法来进行 。

      我们迫使测试电 流流过测试引线和被测电阻(R)然后仪表通过同一套测试引线来测量电 阻两端的电压,并计算出相应的电阻数值 The main problem with the two-wire method as applied to low resis tance measurements is that the total lead resistance (RLEAD) is added to the measurement. Since the test current (I) causes a small but significant volt age drop across the lead resistances, the voltage (VM) measured by the meter won’t be exactly the same as the voltage (VR) directly across the test resistance (R), and considerable error can result. Typical lead resistances lie in the range of 1m. to 10mΩ, so it’s very difficult to obtain accurate two- wire resistance measurements when the resistance under test is lower than 10. to 100. (depending on lead resistance). 两线测量方法用于低阻测试时的主要问题是测量结果中增加了引线的 总电阻(RLEAD )。

      由于测试电流(I)在引线电阻上产生了一个小的、但是 很重要的电压降,所以仪表测量的电压(VM)就不会和被测电阻(R)上的 电压完全相同,于是产生了相当的误差典型的引线电阻在 1mΩ 到 10mΩ 的范围内,所以当被测电阻小于 10Ω 到 100Ω 时,就很难用两线测量方法来 获得准确的测量结果(取决于引线电阻的数值) Due to the limitations of the two-wire method, the four-wire (Kelvin) connection method shown in Figure 3-15 is generally preferred for low resistance measurements. These measurements can be made using a DMM, micro-ohmmeter, or a separate current source and voltmeter. With this configuration, the test current (I) is forced through the test resistance (R) through one set of test leads, while the voltage (VM) across the DUT is measured through a second set of leads called sense leads. Although some small current may flow through the sense leads, it is usually negligible and can generally be ignored for all practical purposes. The voltage drop across the sense leads is negligible, so the voltage measured by the meter (VM) is essentially the same as the voltage (VR) across the resistance (R). Consequently, the resistance value can be determined much more accurately than with the two-wire method. Note that the voltage-sensing leads should be connected as close to the resistor under test as possible to avoid including the resistance of the test leads in the measurement. 由于两线方法的局限性,所以对低阻测量来说,人们一般都喜欢采用 如图 3-15 所示的四线连接方法(开尔文法)。

      这种测量工作可以使用数字 多用表、微欧姆计或者分离的电流源和电压表来进行在这种配置下,迫使测试电流(I)经过一套测试引线流过被测电阻(R);而被测电阻上的电压 (VM)则是通过称为取样(Sense)引线的第二套引线来测量的虽然在取 样引线中有小的电流流过,但是这些电流在所有实际测量工作中都是可以忽略的 由于取样引线上的电压降是可以忽略的 , 所以仪表测量出的电压 (VM) 和电阻(R)上的电压实际上是相同的这样,就能以比两线方法高得多的 准确度来确定电阻的数值注意,应当把电压取样引线连到尽可能接近被测电阻的地方,以避免在测量中计入测试引线的电阻 3.3.2 Thermoelectric EMFs and Offset Compensation Methods 热电热电 动势和偏置补偿的方法动势和偏置补偿的方法 Thermoelectric voltages, as described in Section 3.2.1, can seriously affect low resistance measurement accuracy. The current-reversal method, the delta method, and the offset-compensated ohms method are three common ways to overcome these unwanted offsets. 如第 3.2.1 节所述,热电动势电压能够严重地影响低电阻测量的准确 度。

      电流反向法、Delta(德尔塔)法和偏置补偿欧姆法是消除这些不希望 的偏置量的三种最常用的方法 Current-Reversal Method 电流反向法电流反向法 Thermoelectric EMFs can be canceled by making two measurements with currents of opposite polarity, as shown in Figure 3-16. In this diagram, a voltmeter with a separate bipolar current source is used. With the positive current applied as in Figure 3-16a, the measured voltage is: 使用极性相反的电流进行两次测量能够抵消热电动势,如图 3-16 所 示在该图中,使用了一个电压表和一个分离的双极性电流源如图 3-16a 所示加入正极性电流时,测得的电压为: VM+ = VEMF + IR Reversing the current polarity as shown in Figure 3-16b yields the following voltage measurement: 如图 3-16b 所示,将电流的极性反向,则测量出的电压如下: VM- = VEMF - IR The two measurements can be combined to cancel thermoelectric EMFs: 将两次测量结合起来就能够抵消热电动势: The measured resistance is computed in the usual manner: 用通常的方法计算出被测电阻: Note that the thermoelectric voltage (VEMF) is completely canceled out by this method of resistance calculation. 注意,用这种方法计算电阻时,完全抵消了热电动势电压(VEMF)。

      For the current-reversal method to be effective, it’s important to use a low noise voltmeter with a response speed that is fast compared with the thermal time constant of the circuit under test. If the response speed is too slow, any changes in the circuit temperature during the measurement cycle will cause changes in the thermoelectric EMFs that won’t be completely canceled, and some error will result. 为了使这种电流反向法有效,很重要一点是要使用响应速度比被测电 路的热时间常数更快的低噪声电压表如果电压表的响应速度太慢的话,那 么测量期间被测电路温度的变化将会引起热电动势发生变化。

      这样就不能完全抵消热电动势,从而产生某些误差 Delta Method Delta 法法 When the thermoelectric voltages are constant with respect to the measurement cycle, the current-reversal method will successfully compensate for these offsets. However, if changing thermoelectric voltages are causing inac。

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