Answer 7—The design works by creating an isolated DC-DC converter to drive the sensor, and measuring the current on the primary side of the isolation transformer, which should be proportional to the sensor current. The square wave drive minimizes the ripple current in the capacitor on the secondary side, so it should be insignificant relative to the DC current of the sensor. In any case, it should be a constant offset. There will be some losses in the transformer itself. The magnetizing current will cause a fixed offset, and the resistive losses in the coils and the core will require a scaling factor.

Contributor: David Tweed

Published September 2003