Integrated SQUID-gradiometer system for magneto-cardiography without magnetic shielding

First-order planar superconducting quantum interface device (SQUID) gradiometers were fabricated on the basis of the Nb/AlO_X/Nb technology developed at IPHT Jena. The gradiometers have two pickup loops each with quadratic dimensions of 2 cm and a baseline of 4 cm, integrated on chip. The intrinsic noise corresponds to a field resolution in one loop better than 3 fT/√Hz. Because of the high gradiometer balance (>10⁵) the main signals masking a cardiogram are magnetic field gradients caused by nearby sources (laboratory equipment). To suppress the disturbances a second-order gradiometer was realized electronically. In addition, three washer-SQUID´s arranged as a three-axis magnetometer were used to improve the disturbance suppression. All SQUID´s were operated with directly coupled low-noise high-speed electronics controlled by a computer via a RS232 port. They have an input voltage noise of about 0.33 nV/√Hz, a 1/f corner frequency of 0.1 Hz and bandwidth of 5 MHz. The SQUID signals were digitized using a commercially available 16-bit ADC and digitally filtered. The system shows stable operation in the laboratory without magnetic shielding. A human cardiogram could be recorded in real time with a signal-to-noise ratio better than 20. First results of the measurement of foetal cardiogram in a magnetically shielded room are discussed.