Hi zhenyu,
You are right! The ground for the operational amplifier is assumed to be the same as the electrical reference of the entire system. The operational amplifier output is dependent on the voltage difference between its inverting and non-inverting terminals, as shown in the output voltage calculation equation. Both the input voltages and the output voltage of the amplifier are already measured with respect to the electrical reference to the system in the model.
I assume you need the dashed line to represent ground at any of the input terminals. But, the inputs may or may not be connected to the ground. Connecting one of the inputs to ground is equivalent to connecting it with a constant 0 voltage. So, I do not see any problem here unless you need to specify your customized voltage reference.
Hope this answers your query.
