Sigma-Delta Microsystems for readout and servo control

Several microsystem realizations are presented in this paper, which embed MEMS within a sigma-delta analog to digital (A/D) converter. A first category performs the readout and A/D conversion of capacitive sensors,. while a second category, in addition to the readout and A/D conversion, provides advanced control techniques such as Proportional-Integral (PI) or Proportional-Integral-Derivative (PID) servo control to improve linearity and stability. A design methodology for this latter category is presented. For the readout-only microsystems category, the examples of pressure and acceleration readout are presented with SOI-based micromachined MEMS. Worst-case A/D conversion resolutions of 12 to 13 bits on 100Hz bandwidths are reported, with typical power consumptions for the A/D parts of around 500muW. The pressure sensor microsystem includes full on-chip digital filtering and an RF powering and data transmission module for implanted applications. For the readout and servo control category, a porous silicon needle integrating a heating resistor and thermopiles is used to perform thermal conductivity measurements. Average A/D conversion resolution of 13 bits for 100Hz bandwidth is obtained and worst-case temperature regulation accuracy is 0.08degreesC. Power consumption of the circuit remains around 500muW. Detailed measurement results underline the valididy of the considered approaches for the implementation of very compact, high-resolution, improved-stability and low-power microsystems.