Ultra-low-drift and very fast dc SQUID readout electronics

High-slew rate and thermo-stable readout electronics for dc superconducting quantum interference devices (SQUIDs) is presented. This electronics represents an optimum combination of the most important parameters for the SQUID-system user. In the new flux-locked loop (FLL) - unit design we have decreased a voltage white noise level down to the 0.32 nV/Hz1/2 , with a flicker noise corner frequency of ∼0.1 Hz. By making use of electro-thermal feedback in the first stage amplifier, and multi-loop conventional feedback throughout the whole amplifier we have achieved a very low thermal drift <10 nV/K for T = 20 - 65 deg. C. FLL gain stability is improved down to ∼1% for the temperature range 10 - 60 deg. C and for 5 V (peak-peak) ± 30% of the power supplier voltage range. The maximum gain bandwidth product of the FLL-unit was increased 10 times up to ∼5 GHz. In turn, we could reach more than 10 Mφ0/s slew rate while using a conventional LTS SQUID (connected to the FLL by a 1.2 meterlength twisted pair), with a maximum voltage of the field-to-flux transfer function of ≥60 μV (peak-peak). At the same time, we reduced by 40% the number of chips used in the FLL unit