Thermal Noise    (see for example: http://fr.mathworks.com/help/physmod/elec/ref/resistor.html)

    2*k*T/R is the double-sided thermal noise power distribution
    (the single-sided equivalent is 4*k*T/R).

===================================================================
i= v/R + iN

If the sampling frequency is fs, then the thermal noise current iN is given by:

iN = sqrt(2*k*T/R) * rand_gaussian(1.0) * sqrt(fs)

(current source in parallel with the resistor)

====================================================================
v = R*i + vN

If the sampling frequency is fs, then the thermal noise voltage vN is given by:

vN = sqrt(2*k*T*R) * rand_gaussian(1.0) * sqrt(fs)

(voltage source in serie with the resistor)