Why "1.5V IO standard" output pin with 50ohm load not 1.5V when Termination is set to OFF

Hi all,

I set a FPGA pin as follow:

IO pin = Output Pins

I/O Standard = 1.5V

1. When i scope physical pin with FPGA pin Termination = (default) series 50 ohm without calibration
   1. with my scope's port coupling set to DC1M : scope measured 1.6V
   2. with my scope's port coupling set to DC50 : scope measured 0.75V
2. When i scope physical pin with FPGA pin Termination = OFF
   1. with my scope's port coupling set to DC1M : scope captured 2.0V (Overshoot included)
   2. with my scope's port coupling set to DC50 : scope measured 1.0

I understand the behaviour of bullet 1a, 1b and i can accept 2a.

What i don't understand is bullet 2b

Since DC50 is just 50ohm load and the series 50ohm is turn OFF on the input pin, i expect the full 1.5V voltage drop on the 50ohm load.

But why am i not measuring 1.5V but instead, i am measuring only 1.0V?

Thanks, JJ