[![If you see this you can get some idea][1]][1]

well i also work on a 400V motor driver but I use AMC1300 IC .you want see some details in data sheet . 


[![get know the connections ][2]][2]


here the long story short .
**Vin=Ainp-Ainn**

INP=0V (Exactly) OP=will give a pulse of 50% duty cycle respect to your frequency.

INP=+50mv(Full scale) OP=1 pulse with 89.06% duty cycle (MCU read as "1" )


INP=-50mv(Full Scale) OP=1 pulse with 10.94% duty cycle (MCU read as "0" )

Clipping mode :

Only Zeros(pulse width 10.96%) are transmitted if input is ≤ –64 mV.


Only Ones (Pulse width 89.06%)are transmitted if input is ≥ +64 mV.

so the consolidated answer is 

consider 0V as a center point

If AINP>AINN: Duty cycle is 50% to 89.06% (positive input).

If AINN>AINP: Duty cycle is 50% to 10.94% (negative input).

your assumption is correct. 


  [1]: https://i.sstatic.net/TMmROHXJ.png
  [2]: https://i.sstatic.net/Dde6w3o4.png