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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.

enter image description here 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?

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  • \$\begingroup\$ I'm confused, aren't you going to take the difference (V(+) - V(-)) somewhere in software? That should get rid of the common mode. Or are you expecting lots of CM signal and are worried about dynamic range issues? \$\endgroup\$ – George Herold Jan 7 '15 at 17:12
  • \$\begingroup\$ The ADC is differential so yes CM issues should be removed, however the system is going to operate in a high voltage, high current industrial environment, so yes to the dynamic range issues. \$\endgroup\$ – user2286899 Jan 8 '15 at 12:06
  • \$\begingroup\$ OK you've got a very wide bandwidth, so all I can think of are notch filters at 50,100, 150 Hz? That or better shielding. I can't think of a way to filter common mode, and leave the differential, without taking the difference of the signal. \$\endgroup\$ – George Herold Jan 8 '15 at 14:31
  • \$\begingroup\$ Mind that the capacitors have bad accuracy, meaning you screw up the CMRR by the two caps to GND. Only use one between the line to ensure symmetry \$\endgroup\$ – Gregory Kornblum May 23 '17 at 11:46
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In the circuit you showed one would generally make the pole for the differential signal smaller (~20 times) than the common mode pole. The reason for this is due to component tolerances. If the common mode filter for the one leg has a different pole frequency then this would affect the differential signal. This is because the filtering is no longer identical on both sides.

This could cause worse interference than not filtering, since the differential OP-AMP and ADC should have at least 50dB CMMR at low frequencies (if not higher).

If the differential pole is lower, then when the common mode filters start "interfering" there is no longer a differential signal of interest. This will prevent high frequency CM noise interfering with your measurements. This is important as CMMR drops off with frequency.

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