From my understanding you want to convert negative hex numbers to decimal, The ability to use these functions on negative numbers is not available in MATLAB.
To work around this issue, use the following code:
ndec2hex.m
function [hexstring]=ndec2hex(x,n)
% x : input decimal number
% n : number of bits to perform 2's complements
% hexstring : hex representation of two's complement of x
x=x + (x<0).*2^n;
hexstring=dec2hex(x, ceil(n/4));
nhex2dec.m
function [x]=nhex2dec(hexstring,n)
% hexstring : hex representation of two's complement of xmydec=hex2dec(hexstring);
% x : input decimal number
% n : number of bits to perform 2's complements
x = hex2dec(hexstring);
x = x - (x >= 2.^(n-1)).*2.^n;
