Voltage divider for battery monitor ADC - loading effect?
Yes, you definitely need to consider the input resistance and/or bias current requirements of the ADC input.

The usual technique is to keep the Th&eacute;venin resistance of your divider low enough that the offset caused by the ADC input is "low enough" (e.g., less than 1% or 1 LSB), depending on your system requirements.

For example, suppose that your ADC input resistance is 1 M&Omega; and you want to keep the error caused by this to less than 1%. Therefore, the Th&eacute;venin resistance of your divider should be less than 10 K&Omega;.

So, now you have two equations, one for the divide ratio:

$$Ratio = \frac{R1}{R1 + R2}$$

and one for the resistance:

$$R_{TH} = \frac{1}{1/R1 + 1/R2}$$

and two unknowns, R1 and R2. It's straighforward math to plug in your known values for Ratio and RTH and solve for R1 and R2.