guys i solved the concept of a bipolar signal...
according to my knowledge adc 0804 can not convert a digital value of a -ve signal.
so if we are applying a +2V , -2V peak to peak sine ,triangular or square wave, 0804 will only convert data for +ve side.
when -ve signal is coming 0804 is giving 0 value.
what i do , i just store the +ve signal and during -ve wave i plotted the -ve side by inverting the +ve stored signal.
but this program is working only for 20HZ to 300hz signal....