An active charge cancellation system for switched-capacitor sensor interface circuits

Interface circuits determine the performance of microsensors in diverse automotive, medical, and process-control applications. Microsensor systems for acceleration, rotation, pressure, and other signals most typically rely on capacitive displacement measurement in microstructures. Displacement sensitivity requirements are typically 0.1 pm with sensor capacitor values of 100 fF or less. Conventional architectures for capacitive displacement measurement have required bias control with either large value off-chip or large-area integrated resistive circuit components. Switched-capacitor measurement methods may provide a fully integrated solution with compact geometry but have been limited by severe charge injection error signals. The active charge cancellation system (ACCS), reported here, provides the first direct compensation of switched-capacitor charge-injection error at the measurement system input. The ACCS, implemented in a 0.8-μm digital CMOS technology, uses a novel feedback network to directly cancel charge injection and reduce error by over two orders of magnitude without need for component matching