On the proper way to power a firewire device

[ionomolo]

I have just bought an apogee duet for my laptop. The problem is that it has a mini-firewire instead of a normal, powered bus.

Now i'm drawing the power from my charger in that way:

DA0 => DA0
DA1 => DA1
DB0 => DB0
DB1 => DB1

Inner shield => Inner shield
Outer shield => Outer

P+ => AC adapter power
PG => AC adapter ground

I belive this to be fine because the power charger ground is related to data signals by the circuitry at the guts of the laptop, but sice i plan to add an external charger for the duet to allow better mobility and also add some snake oil i will need to relate the power ground to the signal voltages.

Should the power ground be tied to the external shield at the power taking point?

Thanks