Calculating 20A Measurement Using CT, ADC0804, 89C51, 2.56V Ref, 0.5Ω Shunt, LCD Output

Dear Sir,

I want to calculate 20 Ampere through current transformer and then need to show on LCD through 89c51 please advise me on my calculation if any changes required.

I am using ADC (0804) for signal conditioning and

set ref volts 2.56

Analog input ranges from 0 to 2.56v

step size 10mv.
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If i calculate shunt resistance required to get max 2.56 v across current transformer terminals , i take 20/5A with turn ratio 1:4

So Ro= 2.56v/5A = 0.51

if i connect two 1 ohm resistor in parallel to divide the resistance of both resistor and get the 0.5ohm value and directly connect these terminals to the ADC and from ADC to 89c51 lets calculate digital data output from ADC through formula

D0=2.56/0.01= 256 decimal number output from 89c51
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lets calculate a factor inside the 89c51 through programming...

256/12.8=20A
which can be display on LCD....

Please guide me if any changes in the calculation required.