  We have developed a nuclear magnetic resonance spectrometer with a series
tuned input circuit for measurements on samples at millikelvin temperatures
based on an integrated two-stage superconducting quantum interference device
current sensor, with an energy sensitivty e = 26 +/-1 h when operated at 1.4K.
To maximise the sensitivity both the NMR pickup coil and tuning capacitor need
to be cooled, and the tank circuit parameters should be chosen to equalise the
contributions from circulating current noise and voltage noise in the SQUID. A
noise temperature TN = 7 +/-2 mK was measured, at a frequency of 0.884 MHz,
with the circuit parameters close to optimum.
