Reduced nonlinearities in the NIST Johnson noise thermometry system

Improved electronics and synthesized noise waveforms for the NIST quantum-voltage-standard-calibrated Johnson noise thermometer (JNT) have lead to reduced uncertainty in the temperature measurement. Recent measurements show that some of the distortion in the present electronics arises in the differential stage of both the preamplifier and the analog-digital converter (ADC). Distortion from the preamplifier can be reduced by compensating the DC offset of the signal at the inputs to the differential stage. A four-channel system with a new ADC is under construction to achieve the goal of re-determining the Boltzmann constant at a relative uncertainty of 6×10^-6.