Physio-control Manufacturing Corporation

Randall J Makela

Douglas Keith Medema

Daniel Yerkovich

A method and apparatus for controlling the common mode impedance misbalance of an isolated single-ended circuit for all common mode paths, thereby allowing the balancing of the common mode impedances which reduces common mode effects while maintaining the advantages of the single-ended amplifier including circuit simplicity and the reference input connected to circuit ground. In one embodiment, two solid shields enclose the circuit as completely as possible with the inner shield connected to circuit ground which is also the reference for all other inputs to the circuit. A discrete capacitor is connected between the outer shield and each of the non-reference inputs. When the shield is complete, i.e., solid, almost solid with minimal holes or a fine mesh, the value of the discrete capacitor is selected to match the parasitic capacitance formed between the outer shield and the inner shield. In another embodiment, the shield may be incomplete, i.e., a grid, coarse mesh or a solid shield only enclosing a portion of the circuit; this shield is connected to the non-reference input of the circuit. In this case, the exposures of the incomplete outer shield and the electronic circuit ground plane (or inner shield) to an external noise source are matched to balance the effect of their parasitic capacitances. Also, in the case of an incomplete shield, a discrete capacitor may be connected between the outer shield and the non-reference input of the circuit to enable balancing the impedances to common mode currents.

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Kirkland, WA, United States of America

Randall J Makela