RFID tag antenna design

Hi everyone,
I am working on RFID tag design and i want to choose the chip so i have chosen Alien higgs 4 , they mention in the datasheet the parallel capacitance input is equal to 0.89 PF and the resistance parallel input is 1800 K ohm but when i have read some papers they mention the Higgs 4 chips impedance is equal to (18-181j ) , so if anyone can help how to get the impedance of the chip from datasheet ? especially for Alein higgs 4 or Allein Higgs 3 .

thanks

I think the (18-181j) ohm is the series impedance at one particularly frequency point.
So draw a circuit with a 1800 K ohm resistor and a 0.89 PF capacitor in parallel. Then calculate the impedance form this circuit at your frequency. At 900 MHz the impedance is (21 - 196j) ohm

thanks flanello , so at 915 Mhz the impedance is (20.97-193.2j) ohm , right?

Yes, that is correct.

Alternatively, the chip input impedance is calculated by ADS software. Thus, one has to simulate its chip first in ADS in order to obtain the approximated chip input impedance.