negative ground with ADuM7440

I would like to hook up an ADuM7440 to convert 4 signals from NEGCMOS [Logic ‘1’ = 0V, Logic ‘0’ = -5V] to normal +3.3V LVCMOS [Logic ‘1’ = +3.3V, Logic ‘0’ = 0V] on the other side.

  NEGCMOS side:
   VDD1 -  GND
   GND1 -   -5V

LVCMOS side:

   VDD2 - +3.3V
   GND2 - GND

Is
this is acceptable?  The datasheet seems to imply that only positive rails are acceptable, even though the 1kV of digital isolation should be sufficient. 

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