I don't know how the load affect the circuit. the output of buffer is connected to pad.
Added after 54 minutes:
because i need to test the signal from the output of the buffer, actually, this output is connected to the input of the offchip filter, I have tried to disconnect it with the filter in the Spectre simulation , and the output signal of the buffer is good , so i get conclusion, the capacitive input(15pF) of filter is affected the performance of the buffer. But I don't know when i test the signal, how much capacitance be added the output? because i am debugging why i can't detect signals in the oscillograph.
If the capacitive input of filter affected the output of buffer, how should I solve the problem? because on the chip the output of buffer is connected to next stage through 50 ohms resistor. during the full circuit simulation including on chip and offchip component, the signal at the output of buffer is bad, especially up to hith frequency.