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

 

hi Albert,
Please improve readability your image.
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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.