Automatic balancing of linear AC bridge circuits for capacitive sensor elements

Conventional bridge circuits are balanced by means of mechanical changes of trimming capacitances and resistances. It is a well-known fact that this can be very difficult in many situations, e.g., due to electrical interference between the bridge circuit and the human body or some mechanical part of the screwdriver used. There may also be physical constraints prohibiting mechanical changes of some bridge elements. To avoid these problems, the balancing procedure should be automated. In this article an automatic method for bridge circuit balancing, based on a digital control signal, is presented. Ordinary AC-bridge circuits are nonlinear with respect to changes of the capacitive sensor element. Using some standard operational amplifiers it is easy, however, to obtain a linear bridge circuit output. A theoretical model of the linear bridge circuit is derived. The excellent performance of the linear bridge circuit predicted by theory is verified by practical experiments