generating a -10v...+10v control signal from an arduino

That site is very useful. The result with those numbers is not very good, so needing something more like
Fpwm=10Khz, Fc=100Hz, or 100:1 to get low ripple.

5K and 10uF -> F3db = 3.18 Hz, which is quite low, but probably ok if all you're doing is changing slowly.