ADSP BF 548 FFT and aliasing effect

Hi everyone,

I am using ADSP BF 548 evaluation board in my project, i have to perform FFT for the acquired samples from ADC (AD7767), our sampling rate is 8200 sample/sec, we are performing 4096 point FFT for every 500msec, our requirement for input is 7Vpp and 0-3.5KHZ, if we give input of 1VPP and 1KHZ, we are getting FFT spectrum correctly, but when we change the frequency to 5khz keeping the same voltage, still we are getting FFT spectrum without any decrease in amplitude. here we are facing aliasing effect i.e if we give frequency above 4 khz also amplitude is not reducing and frequency is rolling over i.e it is starting from 0 hz again, how to overcome aliasing effect?is there any inbuit DSP function to reduce aliasing effect?

Best regards

Amar TR 

Software Engineer

SLN Technologies.