Correlated double sampling in capacitive position sensing circuits for micromachined applications

In micromachined applications, the size of the sense capacitance is typically limited by the technology to a few hundreds femtofarads, thus resulting in a large sampling (kT/C) noise. This paper demonstrates that correlated double sampling (CDS), which has traditionally been used to remove the amplifier offset and attenuate the 1/f noise, can be extended to cancel the kT/C noise and the switch charge injection due to switching operations. A 4 dB noise reduction in the prototype sensing circuit confirms this concept. By optimizing the thermal noise of the amplifier, analytical results show that the kT/C noise cancellation can reduce the thermal noise of the sensing circuit by as much as 20 dB