An integrated dc squid gradiometer for biomagnetic application



A first-order off-diagonal gradiometer for biomagnetic measurement is fabricated and tested. The gradiometer consisted of two field pick-up coils, a planar-type DC-SQUID with two superconducting loops connected in parallel, two multiturn input coils, and a modulation-feedback coil. The size of the pick-up coils was 6 × 6 mm2 with a baseline of 8 mm. The overall size of the gradiometer was 15 × 7.5 mm2. The resolution of the magnetic field gradient of the gradiometer increased from 11 to 1.8 pT/m/ Hz corresponding to frequencies from 1 to 600 Hz. The resolution became nearly white in the frequency range above 100 Hz. The intrinsic balance of the gradiometer was better than 1000 ppm for the field perpendicular to its plane. The magnetocardiogram and the alpha rhythmic fields were obtained easily using this gradiometer in a conventional electromagnetically shielded room.