XTR112: The output current of the circuit can not keep constant with different RL

Part Number:XTR112

Hello
I have a problem with XTR112, my circuit is according to datasheet XTR112 / FIGURE 1. RTD(PT1000) Input, Current Excitation,input resistant is adjusted in range 700Ω - 1200Ω.
When the input resistant is constant the output current is changing with the load resistance ：(R2+R3).

Thanks for help