Integrated Bipolar Circuit for the Readout of a Capacitive Torque Sensor

A capacitive torque sensor has been developed in which a twist angle in an axle between two angular displacement sensors spaced 10 cm apart results in a phase modulation of an excitation voltage. A bipolar readout circuit has been designed for the readout of this sensor. An important restriction for the design is that the phase error introduced by the circuit must be as small as possible. The static phase error is compensated, however, the variable phase error due to, for example, changes in the operating frequency must be as small as possible. The circuit consists of a charge amplifier, a differential amplifier, a second-order bandpass filter and an output buffer. The most important subcircuit is a operational transconductance amplifier (OTA) in which a fixed transconductance and a programmable current gain have been cascaded to give a programmable OTA. This concept has been used for realizing OTA's with a very low transconductance (g
_am
= 6.3 μA/V) as well as for OTA's with a high transconductance (g
_am
=35 mA/V).