In this paper, a position based impedance controller (i.e. admittance controller) is designed by utilizing measurements of a force/torque sensor, which is mounted at the robot's base. In contrast to conventional force/torque sensing at the end-effector, placing the sensor at the base allows to implement a compliant behavior of the robot not only with respect to forces acting on the end-effector but also with respect to forces acting on the robot's structure. The resulting control problem is first analyzed in detail for the simplified one-degree-of-freedom case in terms of stability and passivity. Then, an extension to the Cartesian admittance control of a robot manipulator is discussed. Furthermore, it is shown how the steady state properties of the underlying position controller can be taken into account in the design of the outer admittance controller. Finally, a simulation study of the Cartesian admittance controller applied to a three-degrees-of-freedom manipulator is presented.