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 (Ccm\$C_{cm}\$) on the input differential line, right next to the input termination resistor.
![enter image description here][1] [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 Fcm = 1/(2piR1Ccm), acts to change my 3MHz low pass differential response. If the differential filter is Fdiff = 1/(2pi(R1+R1)(Cdiff+Ccm/2)), I cannot get common mode filtering as far below the differential as I'd like, as Ccm acts to swamp Cdiff in the Fdiff equation.
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?
Thanks. Ed
