Problems in coupling a dc-SQUID to the external world

We constructed two identical planar dc-SQUIDs coupled to different input coils on a single silicon chip. The inductances of the coils differ by two orders of magnitude. The ultimate noise performances do not depend significantly on the input circuit if, as in our case, the coupling constant is of the order of 0.5. In a flux locked loop operating mode the energy sensitivity of the more tightly coupled device is sligtly higher than for the less tightly coupled one. We coupled the device to a second-order gradiometer used for biomagnetic measurements obtaining our best performances to date in such a configuration.