An Improved Closed-Loop Switched Capacitor Capacitance-to-Frequency Converter and Its Evaluation

This paper presents a new capacitance-to-frequency converter (CFC) with high linearity and accuracy. In the CFC, the capacitance-to-frequency conversion is achieved using a negative feedback mechanism. In the feedback, the output frequency controls the charge injected by a switched-capacitor (SC) network such that the difference error is automatically minimized. The feedback mechanism ensures that the final output is insensitive to the nonlinearity errors of the units employed to realize the forward path, namely, a capacitance-to-voltage converter and a voltage-to-frequency converter. A simple error correction mechanism employed ensures that the digital output obtained is insensitive to the number of nonideal circuit parameters. The proposed converter uses a dc reference voltage for the excitation and helps to achieve high accuracy. A prototype of the improved CFC has been developed and tested for sensor capacitance C-x in the range of 5-25 pF. A prototype of the conventional open-loop CFC was also developed, and the linearity has been evaluated. The proposed closed-loop approach has lower (0.1%) nonlinearity error compared to the conventional CFC (0.8%). This improvement is achieved without the use of any error-compensating schemes, and hence the complexity level of the system remains the same as that of any typical CFC. The response time of the CFC designed is about 400 mu s for a 5-25 pF step change in C-x. The results obtained from the prototype CFC confirmed its usefulness.