FFT for a sampled sine looks wrong even after scaling?

Hello everyone. Before 5 days approximately i have posted a post here to ask why i couldn't take an FFT on MATLAB for a signal has been sampled by an ADC on FPGA board, i got a very nice and helpful replies says that its scaling problem,i scaled it and i could finally see my peaks in the spectrum. But there's still a problem there, which this peak doesn't never change its position when i change the input signal which has a different carrier frequency. The pic below shows a spectrum for 2 different sampled sine waves , the first has 100 KHz carrier frequency and the second is 250 KHz. Their spectrums are looks very close to each other and the peak is approximately about 250KHz for all signals. Does anyone knows the problem comes from where and how it can be solved? I would really appreciate the help. Regards. Mohamed.