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

low-noise wideband read-out electronics for do 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/root Hz and 6.2 pA/root Hz with 1/f corners at 0.2 Hz and 13 Hz, respectively. The SQUID can be operated with both de bias and sc bias of up to 250 kHz. In the latter case, a special circuit synchronously detects the sc bias component at the preamplifier output and automatically removes it by tuning the bias voltage, An insystem programmable microcontroller is used to control all functions of the SQUID system via a serial RS-485 interface, It also generates the sc 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/root Hz and 0.9 fT/root 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 phi (0)/mus to 2.3 phi (0)/mus. A short integrator reset time of <1 mus allows one to increase the dynamic range utilizing the periodicity of the SQUID voltage vs. flux characteristic.