Capacitance-to-Frequency Circuit for Linear Sensor Readout
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Solution Overview
Problem
Conventional capacitance sensors using charge transfer techniques face challenges in linearizing the exponential relationship between voltage on the summing capacitor and charge transfer cycles, making it difficult to accurately calculate capacitance as a function of voltage potential over time or cycles.
Innovation Solution
A capacitance to frequency converter is introduced, which converts the measurement of capacitance into the measurement of frequency of a feedback pulse signal, providing a substantially linear relationship between frequency and capacitance, allowing for precise capacitance determination through frequency measurement.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If charge transfer technique is used to measure capacitance, then sensitivity to RF fields and RF noise is reduced, but the relationship between voltage on summing capacitor and charge transfer cycles becomes exponential and difficult to linearize
Solution Approach 1:
The patent changes the measurement parameter from voltage (which has an exponential relationship with time) to frequency (which has a linear relationship with time). By measuring the frequency of oscillation rather than the voltage level, the system achieves linearization without requiring complex linearization circuits or calculations, while maintaining the noise immunity benefits of the charge transfer technique
Solution Approach 2:
The patent replaces the voltage measurement approach with a frequency measurement approach. Instead of measuring the voltage on the summing capacitor and performing linearization, the system measures the frequency of oscillation, which naturally provides a linear relationship with capacitance, substituting a more suitable measurement domain
2Measurement precision
If voltage on summing capacitor is measured after predetermined time or cycles, then capacitance can be determined, but exponential relationship requires linearization
Solution Approach 1:
The patent changes the measured parameter from voltage to frequency. By measuring the frequency of oscillation in the RC circuit rather than the voltage level, the system obtains a linear relationship with capacitance value, eliminating the need for linearization while maintaining measurement precision
3Measurement precision
If conventional voltage measurement method is used, then capacitance can be calculated from voltage potential, but exponential voltage rise makes calculation difficult
Solution Approach 1:
The patent changes from measuring voltage potential to measuring oscillation frequency. The frequency of oscillation in an RC circuit is directly related to the time constant and has a linear relationship with capacitance, making the calculation straightforward and eliminating the complexity of dealing with exponential voltage rise
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This solution enables accurate and efficient capacitance measurement by converting capacitance to frequency, overcoming the exponential voltage rise issue and providing a linear correlation, thus improving the reliability and precision of capacitance sensing.
Implementation Method 1
Capacitance sensor 100 uses a charge transfer technique. Referring to FIG. 1A, the charge transfer technique charges a sensing capacitor Cx in one phase (switch SW1 closed, switch SW2 open) and discharges the sensing capacitor Cx into a summing capacitor Csum in a second phase (SW1 open, SW2 closed).
Implementation Method 2
Analog comparator 210 has non-inverting and inverting inputs. The non-inverting input is coupled to node N1 and the inverting input is coupled to a reference voltage Vref. When the voltage on summing capacitor Csum reaches reference voltage Vref, the output of analog comparator 210, MOD_EN, transitions from a low state to a high state.
Data Source
AI summary
A capacitance to frequency converter includes a switching capacitor circuit, a charge dissipation circuit, a comparator, and a signal generator. The switching capacitor circuit charges a sensing capacitor and transfers charge from the sensing capacitor to a circuit node of the charge dissipation circuit. The comparator is coupled to the charge dissipation circuit to compare a potential at the circuit node to a reference voltage. The signal generator is coupled to an output of the comparator and to the charge dissipation circuit. The signal generator is responsive to the output of the comparator to generate a signal fed back to control the charge dissipation circuit. A frequency of the signal is proportional to a capacitance of the sensing capacitor.