As Kalpak mentions , you haven't told us how the external ram is connected also you haven't told us what language you are using.

In the case of assembler it might be something like:

mov dptr,#table ;dptr-> ram
mov a,ADC ;get value to store
movx @dptr,a ;store to external xram

In 'c' it might be something like:

xdata char adc_buffer[1000]; //xdata is compiler dependant - read the compiler documentation


adc_buffer[0] = ADC; //store adc value in xram


Analog Devices have plenty of sample code for their micros, have you not investigated there? Also, the aduc812 external memory is no different to most other 8051s, so read up on the examples available here.

Remember, a precise question will most likely get a precise answer.