Programs calculate 1% and ratio-resistor pairs
Carl Rutschow, Upland, CA; Edited by Brad Thompson and Fran Granville - December 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.