I'm using a fully differential analogue chain, using 20Hz HP, 3MHz LP and x2 Gain. My signal of interest is fully differential with a 200KHz fast modulation and a 100Hz sawtooth ramp. The high frequency chirp but may change within the range of 50KHz to 1MHz. I've therefore ensured a flat pass band (0dB) from 100Hz to 1MHz.

The differential structure will naturally attenuate CM signals, however the industrial environment the system will be working in will contain a number of high amplitude common mode signals. 
I've therefore decided to use common mode filtering capacitors (\$C_{cm}\$) on the input differential line, right next to the input termination resistor. 

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  [1]: https://i.sstatic.net/d2yMJ.gif

What I need to do, is provide further CM **only** filtering of 50Hz, 100Hz 1KHz etc but not interfere with my differential signal path.

Unfortunately, adding suitable common mode capacitors $$F_{cm} = \frac{1}{2\pi R_1C_{cm}}$$ acts to change my 3MHz low pass differential response. If the differential filter is $$F_{diff} = \frac{1}{2\pi(R_1+R_1)(C_{diff}+C_{cm}/2)}$$ I cannot get common mode filtering as far below the differential as I'd like, as \$C_{cm}\$ acts to swamp \$C_{diff}\$ in the \$F_{diff}\$ equation.

How should I go about common mode rejection of lets say 50Hz mains hum, while keeping my differential mode 100Hz to 1MHz pass band?