Improved direct-coupled dc SQUID read-out electronics with automatic bias voltage tuning

A low-noise wideband read-out electronics for dc superconducting quantum interference devices (SQUIDs) is presented. The preamplifier which is directly connected to the SQUID has white voltage and current noise levels of 0.4 nV/√Hz and 6.2 pA/√Hz with 1/f corners at 0.2 Hz and 13 Hz, respectively. The SQUID can be operated with both dc bias and ac bias of up to 250 kHz. In the latter case, a special circuit synchronously detects the ac bias component at the preamplifier output and automatically removes it by tuning the bias voltage. An in-system programmable microcontroller is used to control all functions of the SQUID system via a serial RS-485 interface. It also generates the ac bias clock and sets the SQUID working point via low-noise D/A converters. The read-out electronics has been used to operate low-noise SQUID magnetometers. Noise levels down to 35 fT/√Hz and 0.9 fT/√Hz with 1/f corners at about 2 Hz have been achieved with thin-film SQUID magnetometers operated at 77 K and 4.2 K, respectively. With static bias a high bandwidth of up to 6 MHz was achieved without affecting the noise level. With 100 kHz bias reversal the rms noise increased by about 3% when increasing the system bandwidth from 100 kHz to 1.4 MHz. The system slew rate was 0.4 Φ₀/μs to 2.3 Φ₀/μs. A short integrator reset time of <1 μs allows one to increase the dynamic range utilizing the periodicity of the SQUID voltage vs. flux characteristic