A double might not have sufficient capacity to store 30 digits of precision. It only has the capacity for around 15 decimal digits. Please read the article about doubles on wikipedia if you want to understand better.
Here is an illustration of their limitation:
a(1) = 10^-40;
a(2) = 1234567890.12345678901234567890;
fid = fopen('test.txt','w');
fprintf(fid,'%30.30f \n',a); %floating point with a width and precision of 30 numbers
fclose(fid);
This might be an interesting read as well.
