Ratio-Metric Capacitance Converter for Common-Mode Noise Cancellation
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Solution Overview
Problem
Capacitance sensing systems face challenges with noise interference from power supply voltages, clock frequencies, and reference voltage variations, leading to decreased signal-to-noise ratios (SNRs) in high-sensitivity and fast-scanning applications, limiting their performance.
Innovation Solution
The implementation of a ratio-metric capacitance-to-digital converter (CDC) that uses a sensor cell and a reference cell with modulation capacitors to generate a bitstream representing the ratio between sensor and reference capacitances, canceling common mode noise and being independent of clock frequencies and voltage sources.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional capacitance sensing systems are used, then capacitance measurement is achieved, but noise interference from power supply voltages, clock frequencies, and reference voltage variations decreases signal-to-noise ratio
Solution Approach 1:
The patent converts the harmful common-mode noise (power supply variations, clock frequency variations, reference voltage variations) into a beneficial cancellation mechanism by using ratio-metric measurement. The system measures both sensor capacitance and reference capacitance using the same noisy clock and voltage sources, causing the noise to appear identically in both measurements and cancel out when the ratio is calculated. This transforms the harmful noise into a common reference that eliminates itself in the differential measurement.
Solution Approach 2:
The patent introduces a reference capacitance cell as an intermediary element that experiences the same noise conditions as the sensor cell. By measuring the reference capacitance under identical noisy conditions and using it to normalize the sensor measurement, the system creates a noise-cancellation pathway. The reference cell acts as a mediator that carries the same noise signature, allowing the ratio-metric calculation to eliminate the noise while preserving the actual capacitance signal.
2Reliability
If conventional capacitance-to-digital converters are used, then capacitance conversion is achieved, but performance is limited by dependence on clock frequencies and voltage sources
Solution Approach 1:
The patent fundamentally changes the measurement parameter from absolute capacitance value to capacitance ratio. Instead of measuring capacitance directly against a fixed reference, the system measures the ratio between sensor capacitance and reference capacitance, both of which are subject to the same clock and voltage variations. This parameter transformation makes the measurement invariant to frequency and voltage changes, as these variations affect both numerator and denominator equally and cancel out in the ratio calculation.
Solution Approach 2:
The patent creates a measurement system that universally handles various operating conditions (different clock frequencies, different voltage levels, different process conditions) through a single ratio-metric approach. The same circuit architecture and measurement methodology work across all these conditions without requiring calibration or adjustment, because the ratio measurement inherently compensates for variations in any parameter that affects both sensor and reference equally.
3Measurement precision
If capacitance sensing systems operate in harsh conditions, then sensing capability is maintained, but noise interference increases and limits performance
Solution Approach 1:
The patent converts the harmful environmental noise and variations into a beneficial common-mode signal that cancels in ratio-metric measurement. By designing the system so that both sensor and reference capacitances are measured under identical environmental conditions (same clock, same voltage supplies, same temperature effects), the environmental variations become common to both measurements and eliminate themselves in the ratio calculation, leaving only the differential signal of interest.
Data Source
AI summary
Apparatuses and methods of capacitance-to-digital code conversion are described. One apparatus includes a bridge circuit and a modulator front-end circuit. The bridge circuit includes a first terminal to couple to a reference cell and a second terminal to couple to a sensor cell. The modulator front-end circuit includes a comparator coupled to the bridge circuit, a first modulation capacitor coupled to a first input of the comparator, and a second modulation capacitor coupled to a second input of the comparator. The modulator front-end circuit provides a digital bitstream. A duty cycle of the digital bitstream is representative of a ratio between a capacitance of the sensor cell and a reference capacitance of the reference cell.