Impedance of differential pairs next to chassis ground

[bharris]

We need to route differential pairs on a two layer flex circuit which has to be located next to metal which is connected to chassis ground. There is thus a signal ground plane below the traces and a chassis ground plane above the traces. The traces are differential embedded microstrips with 660Mb/s LVDS signals and are about 50mm long.

Local signal ground and chassis ground cannot be connected together - their DC connection is at a power supply which is quite far away and there is no way to connect them locally with AC coupling capacitors..

As there is no return path in chassis ground, should the impedance be calculated as if the traces are microstrips or as striplines? Or is there another way to calculate impedance?

Thanks

 

[FvM]

For differential impedance, it doesn't matter if the chassis ground is connected to local ground, connected elsewhere or just floating.