High impedance opamp attenuator
Normally it's no problem at all if the output signal you want to feed into the low pass filter has some non-zero impedance. It's just important to know what impedance it has.

Therefor you can use a voltage divider after a voltage follower (upper subcircuit).  

It's Thévenin equivalent (lower subcircuit) is a voltage source \$v_{th} = v_{in} \frac{R2}{R1+R2}\$ with series resistance \$R_{th} = R_1 || R_2\$.






simulate this circuit &ndash; Schematic created using CircuitLab