Linearity Measurements on Critical Noise Thermometer Components

A method for measuring the nonlinearity of noise thermometer components was developed that reduces the demands on the test signal generator. The method was applied to characterize the preamplifier and analog-to-digital converter (ADC) of PTB's new noise thermometer. For the complete signal path, gain nonlinearities below about one part in 105 were experimentally demonstrated at input rms noise levels between $71\ \mu \mathrm{V}$ and $284\ \mu \mathrm{V}$. It was further demonstrated that dither can reduce the ADC nonlinearity by at least one order of magnitude.