AN ELECTRONIC SECOND-ORDER GRADIOMETER FOR BIOMAGNETIC APPLICATIONS IN CLINICAL SHIELDED ROOMS

A modular 55-channel magnetometer for biomagnetic studies in clinical shielded rooms is described. A total of 76 SQUID magnetometers in three levels are used to electronically form first-order or second-order gradiometer configurations. The magnetometers are completely integrated on 7.2 x 7.2 mm(2) chips. APF (Additional Positive Feedback) is used to increase the gradient of the V-phi characteristics to dV/d phi = 1 mV/phi(0), and a novel scheme called BCF (Bias Current Feedback) reduces the bias current sensitivity to partial derivative Phi/partial derivative I-b = 0.02 phi(0)/mu A. This allows to couple the SQUIDs directly to simplified feedback electronics without helium temperature impedance matching circuits or flux modulation techniques. The magnetometer noise level is typically 1.4 fT/root Hz at 1 kHz and 3.2 fT root Hz at 1 Hz, measured inside a Pb/Cryoperm shield. Large preamplifier offset voltage fluctuations of +/- 5 mu V increase the typical 1 kHz noise level only slightly to 1.7 fT/root Hz. A 7-channel prototype module has been tested successfully inside a clinical shielded room (Vakuumschmelze type AK 3.1:1 A typical first-order gradiometer noise level of 2.2 fT/root Hz at 1 kHz and 7 fT/root Hz at 2 Hz has been achieved, including the noise contribution of the home-made dewar and of the background field in the shielded room.