A differential difference amplifier with automatic gain selection as readout interface for CMOS stress sensors in orthodontic brackets

This paper presents two types of CMOS stress sensors and the associated readout interface. Utilizing a differential difference amplifier (DDA) in combination with a digitally assisted gain selection, the gain is maximized during each readout while the amplified sensor signal is kept within the linear output range of the DDA. The readout consumes 290 μA with a measured resolution of 22 μV at maximum gain. For the presented sensors an in-plane shear stress or differences of normal stresses as low as 24 kPa can be resolved at a full scale signal of 100 MPa, while non-mechanical contributions are suppressed. Sensors and readout are fabricated in a 0.35 μm CMOS process operating at 3.3 V.