Mark Jurras wrote:
>
> Do anyone know the reason the values of standard 1% Tolerance Resistors were
>  chosen the
> way they are? I need to choose some resistors to offset my A/D and was
 thinking
>  about
> getting excel to crunch the numbers. The values seem to increase at
>  approximately 2.5%
> the previous value but no quite.

There are 96 resistance values per decade for the 1% Tolerance Resistors. The
values are 10.0, 10.2, 10.5, 10.7, etc

If you call the 10.0 ohm resistor R0, the 10.2 R1, etc, then this simple formula
will tell you the value for RN:

RN = 10 ^ (1 + N/96)

For example, the 15th resistor has a value of 10 ^ (1 + 15/96) = 14.330126 ohms,
or to 3 sig figs, 14.3 ohms. The step in resistance values can be calculated
by taking the ratio of RN and RN+1:

RN+1 / RN = 10 ^ ( 1 + (N+1)/96 - 1 - N/96)
          = 10 ^(1/96)
          = 1.024275

which as you noted is "approximately 2.5% but not quite".


> Do anyone know of any programs that select resistor values for voltage
 dividers
>  and op-amp
> gains other than brute force guess and select?

Yeah, I threw together a BASIC program some time back that allowed you to enter
in the resistance ratio and it would calculate the two 1% resistors that would
closest meet it. I'd post it, but it ain't at work. I'll dig it up and send it
tomorrow --- unless someone else posts their program.

Scott