selection of proper begining and number of samples so that peak amplitude will not change

[delhi]

i want to take the FFT of the signal and want to note down its peak amplitude but everytime when i change the begining of sample, peak amplitude changes. but it should not, as the starting and the end of the signal is a dc line which will not contibute anything to the FFT mathematically and the peak amplitude remains same. i am attaching the block diagram. please check this and tell me where i am doing the mistake. i have submitted my problem many times but not getting any reply.please help me..

[DFGray]

Please post your block diagram so we can respond.  However, I do have a couple of questions that may help you out.

1. Are you absolutely sure your data peak amplitude is not changing?  If you are acquiring data using a digitizer, the amplitude will almost certainly change at each iteration due to noise.  How much is your value changing? 10%? 1%? 0.1%?  Is this in the noise spec of the digitizer?
2. In the ideal world, an FFT would not change when you changed the length of DC components at the end.  However, FFTs are calculated using finite precision, so roundoff errors do occur.  How much they effect you depends on the algorithm.  Once again, how much variation are you seeing?

Post your code, sample data (if any), and your signal variation and we will try to help more.