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.
You can hook up the ADuM7440 this way. Just make sure that Vix doesn’t exceed 0.5V or fall below -5.5V. The data sheet says -0.5V for the absolute minimum voltage for Vin, but it assumes a 0V ground. It’s warning you not to have Vix below their ground - 0.5V. Basically you have 5VDC across the grounds, which is well within the 148Vrms working voltage for this part.