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　　计算机编程确定应用的最好值。

　　如果进行模拟电路设计，偶尔需要使用非标准值电阻产生特别的增益、比例或衰减因数。通过并行连接两个1%误差的标准值电阻，可以创造特殊阻值的电阻。由于轻易预测哪个电阻对降到需要值附近是不可能的，计算机编程可以通过计算所有标准1%电阻的组合，帮助确定应用的最好值。

　　Visual Basic编译、执行文件Rratio2.exe在所需比率、衰减因数或同向运放增益的特定范围里，检查所有标准1%的电阻值（图1）。单击下载程序。由程序窗口按钮选择计算模式。作为一个选择，可以选择是否编程显示所有可能值或唯一接近目标的值。

　　使用标准1%的电阻值，第二个程序RPar2a.exe，点击，检查和显示所有合适的组合产生所需的并联电阻值。程序通常计算若干并联组合值，降到所需值的0.1%以内。比较起来，单个1%电阻的标称值也许与所需值有最大1.45%的不同。注意到，对这两个程序，计算电阻值依靠成对电阻的误差。

　　英文原文：

　　Programs calculate 1% and ratio-resistor pairs

　　Computer programs determine the best values for an application.

　　Carl Rutschow, Upland, CA; Edited by Brad Thompson and Fran Granville -- EDN, 12/16/2005

　　If you perform analog-circuit design, you'll occasionally need to use a resistor with a nonstandard value to 
produce a particular gain, ratio, or attenuation factor. You can create resistors of unusual values by connecting two standard-value resistors of 1% tolerance in parallel. Because it is impossible to readily predict which resistor pairs will fall closest to the desired value, a computer program can help by calculating all combinations of standard 1% resistors to determine the best values for your application.

　　The Visual Basic, compiled, executable file Rratio2.exe checks all standard 1%-resistor values in a given range for a desired ratio, attenuation factor, or noninverting operational-amplifier gain (Figure 1). Click here to download the program. You select the calculation mode via the program's window buttons. As an option, you can choose whether the program displays all possible values or only the values closest to the target value.

　　Using standard 1%-resistor values, a second program, RPar2a.exe, click here, checks and displays all appropriate combinations that generate a desired parallel resistor's value. The program generally calculates several parallel combination values that fall well within 0.1% of the desired value. By comparison, a single 1% resistor's nominal value may differ by as much as 1.45% from the desired value. Note that, for both programs, the calculated resistance values depend on the paired resistors' tolerances.