Can I put a capacitor across R5 to get a smoothed output to feed an ADC?

 

hi Albert,
Please improve readability your image.
E

 

Why smoothing .. what are you trying to achieve ?
Is it RMS power measurement , then your best off oversampling and doing maths digitally
If it's an alias filter then what low pass values do you need ?

 

Yes. that can smooth the output to (near) DC which will give you the average value of the input sinewave.

 

I have a circuit almost like yours. Here "out" is without a cap, and "out2" has a 0.01uf cap.
Note it takes a while for the cap to charge up.
1Vpk in 5khz.
Make C2 the size you want. It makes reading the voltage easy. Click on the picture to see a bigger version.