# Choosing resistors in series to make standard values

I am working on designing a precision analog filter for a university electronics project.

The resistance values I am using (to meet the filter specifications) are not E96 values, so I am combining smaller value resistors in series to make the non-standard resistor. Is there any guidance for choosing these resistor values?

For example, to create an 11.9 kΩ resistor, I could use a 100 Ω resistor and an 11.8 kΩ resistor; or I could use a 7.15 kΩ resistor in series with a 4.75 kΩ resistor.

Intuitively I feel having the resistors a similar size may be better, but I am not sure if that is correct, or why it may be the case.

Also, would this change significantly based on the precision of my resistors? For example if I moved from 1% tolerance resistors to 0.5% or 0.1% values?

• qsl.net/in3otd/parallr.html – Jeroen3 May 19 '20 at 13:05
• I was actually using that site @Jeroen3 However, using the example of 11.9, the site provides 6 combinations which could make up a 11.9 value. Is there any way of choosing which combination is best? – JAS May 19 '20 at 13:09
• While this may be of some theoretical interest, it has no practical value. The tolerance of your resistor combination is going to be no better than that of the individual resistors, so you might as well just pick the closest standard value to begin with. – Dave Tweed May 19 '20 at 13:11
• Besides which, if you're designing filters, can you get 1% (or 0.5% or 0.1%) capacitors? If you need filters that are THAT precise, you really should be doing DSP. – Dave Tweed May 19 '20 at 13:17
• @DaveTweed I was planning to use 1% capacitors, but appreciate that may not be feasible. Appreciate the comment – JAS May 19 '20 at 13:33

For example, an 11.8k$$\\Omega\$$ resistor with a 1% tolerance can actually have a resistance that is +/- 118$$\\Omega\$$ different from the marked value. Adding a 100$$\\Omega\$$, 1% resistor in series will give you a nominal resistor of 11.9k$$\\Omega\$$ +/-119$$\\Omega\$$. So, the actual resistance value could be as low as 11.781k$$\\Omega\$$.
If you really need 11.900k$$\\Omega\$$ you will need to buy resistors with a much smaller tolerance or measure each one. Oh, and be sure to calculate the accuracy of your ohmmeter.