  Johnson noise thermometers (JNT) measure the equilibrium electrical noise,
proportional to thermodynamic temperature, of a sensing resistor. In the
correlation method, the same resistor is connected to two amplifiers and a
correlation of their outputs is performed, in order to reject amplifiers'
noise. Such rejection is not perfect: the residual correlation gives a
systematic error in the JNT reading. In order to put an upper limit, or to
achieve a correction, for such error, a careful electrical modelling of the
amplifiers and connections must be performed. Standard numerical simulation
tools are inadequate for such modelling. In literature, evaluations have been
performed by the painstaking solving of analytical modelling. We propose an
evaluation procedure for the JNT error due to residual correlations which
blends analytical and numerical approaches, with the benefits of both: a
rigorous and accurate circuit noise modelling, and a fast and flexible
evaluation with an user-friendly commercial tool. The method is applied to a
simple but very effective ultralow-noise amplifier employed in a working JNT.
