opamp input capacitance ???

Dose anyone know how the close feedback affect the input impedance (capacitance) of the operational amplifier. Got a sensor circuit which is modeled as two capacitors, and these is a DC block cap between the sensor and opamp. An existed input capacitance from opamp would cause insertion loss. But when I checked the loss from simulation, I found two different results , the input cap seems be smaller by close loop operation. Because the inertion loss seems smaller. I tried to put an equivalent cap (about 3pF from Cgs of opamp input) only instead of an opamp. Then found the loss is much bigger than an opamp close loop operation for the sensor. Doesn't the input cap of the input pairs of opamp could simply modeled its value as Cgs ? Or it's close loop which reduce the capacitance ? Anyone knows why ???