Amplitude decrease in FFT at higher frequency for multisine I am adding sine signals with different frequencies but same amplitude=100.
Sampling frequency is  50MHz.
I am not getting 100 in frequency domain for frequency 787KHz.
At 1K, 3K I am getting 100 amplitude,
but at 787KHz I am getting 75.9 .
All the signals have 100 V amplitude.
Any reason for this?
What you see is a result of spectral leakage, it can be mitigated by using multitone frequencies that are coherent with the sampling frequency.
There is a nice tutorial about coherent sampling from Renesas.