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I'm designing an automotive circuit where I'm using a reference power design from TI. Together with protection circuitry, this design includes a common mode choke.

The same circuit has an antenna input, and this antenna is grounded to the car chassis. This means that the circuit has two ground connections when the antenna is connected.

My understanding of a common mode choke is that it only works as expected when the same amount of current flows in each direction of the choke. (correct me if I'm wrong)

I'm then wondering how this second ground connection through the antenna cable will impact the performance of the common mode choke? Can it still be used?

update: Added schematics. The output of the choke goes to a TI LM53601 switching regulator and a TI TPS7B7701 antenna LDO for supplying phantom power to the antenna. IN GND and GND are separate nets. GND is also the ground connection for the antenna. enter image description here

All the antennas I've investigated have 0 ohm resistance between the fixture that connects the antenna to the car and the shield around the antenna signal (inside the cable). Example of one such antenna is the Antennensysteme 3785.01 as shown below. When tightened to the car roof, the antenna will certainly be grounded to the chassis. enter image description here

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    \$\begingroup\$ That depends on the circuit. Which you have kept secret. \$\endgroup\$ – CL. Oct 19 '17 at 19:13
  • \$\begingroup\$ Where is your schematic? \$\endgroup\$ – winny Oct 19 '17 at 19:35
  • \$\begingroup\$ A diagram would be nice \$\endgroup\$ – Voltage Spike Oct 20 '17 at 16:00
  • \$\begingroup\$ @laptop2d Sorry, had to go on an urgent and unexpected business trip abroad. Please see the added schematics. \$\endgroup\$ – JohnDonut Oct 23 '17 at 6:28
  • \$\begingroup\$ Are you certain the antenna is grounded to the chassis? That doesn't make much sense...surely the antenna shield is grounded, but the center conductor is not, and should be the 'input'. \$\endgroup\$ – evildemonic Oct 24 '17 at 14:48
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A choke increases inductance of the wires running through it. Currents take the path of lowest impedance when faced with two or more paths (the current actually take both paths but usually the impedance differ by several orders of magnitude with a choke or other inductor), so if a high frequency current 'see's the inductance from the choke and a low frequency path, most of the current will flow through the low frequency path. This will reduce the effectiveness of the choke.

schematic

simulate this circuit – Schematic created using CircuitLab

If you need the blocking of the choke and its essential to your design, then you will need to move the ground so they both connect on the other side of the choke.

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