Low-Complexity Autonomous RC Time Constant Calibration for Accuracy Improvement of CMOS Integrated Analog Frequency Filters

This paper presents a reduced complexity method for calibrating the RC time constant in integrated circuits. It generates an output DC voltage proportional to the calibrated RC time constant τ, which can be used for trimming analog functions such as the frequency filters or to assess process accuracy. Generating the output voltage proportional to the RC time constant is advantageous over prior solutions that usually generate pulsating voltages, as it simplifies handling the output by the calibration algorithm. Voltage output also enables analog-to-digital conversion and the realization of calibrated voltage-controlled tunable frequency filters. The autonomous operation facilitates real-time calibration during circuit operation, accounting for parameter deviations including temperature variation, DC-bias dependency, or aging. The generation of the voltage proportional to the RC time constant is achieved through synchronous rectification and averaging of the exponentially decaying transient voltage produced by the calibrated RC network. Besides the accurate driving frequency, the implementation requires only the calibrated RC elements, a switch, and an averaging output low-pass filter. The paper provides theoretical description of the method and presents an example of a calibrated 50kHz first-order anti-aliasing low-pass filter implemented in a 0.35μm BCD process.