Capacitive Sensor Circuit Impedance Transformation
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Solution Overview
Problem
Capacitive sensor circuits face challenges in achieving high input impedance due to low capacitance values, leading to instability and non-linear output issues, particularly when using current mirrors for impedance transformation, which can cause rectifying effects and occupy significant component area.
Innovation Solution
A capacitive sensor circuit design incorporating a dual current mirror configuration with N-type or P-type MOS transistors and a non-linear cancellation unit to provide high input impedance while minimizing component area and rectifying effects, ensuring a stable operating point through symmetrical current-voltage behavior.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Shape
If an input resistor with high impedance is added to the amplifier or buffer input, then the input impedance is improved, but the component area occupied increases significantly
Solution Approach 1:
The patent changes the impedance parameter by using a capacitor in parallel with the input resistor, creating a frequency-dependent impedance network that provides high impedance at DC and low frequencies while maintaining reasonable component area. The capacitor value is specifically chosen to create a pole in the transfer function that boosts the impedance at the frequencies of interest.
2Shape
If a current mirror configuration is used for impedance transformation, then the input impedance is improved, but non-linear output and rectifying effects are introduced
Solution Approach 1:
The patent extracts the problematic non-linear current mirror configuration from the circuit and replaces it with a linear capacitor-based impedance enhancement network. This removes the source of rectifying effects and even-order distortion while preserving the desired high input impedance characteristic.
Solution Approach 2:
The patent introduces a capacitor as an intermediary element between the input resistor and the amplifier input, which mediates the impedance transformation in a linear manner. The capacitor acts as a frequency-selective element that provides impedance boosting without the non-linearities associated with active current mirror circuits.
3Shape
If a current mirror is used to provide impedance transformation, then the input impedance is improved, but the DC operating point of the amplifier or buffer shifts
Solution Approach 1:
The patent removes the DC-coupled current mirror configuration that causes operating point shifts and replaces it with a capacitor-based network that is AC-coupled. The capacitor blocks DC components, allowing the amplifier to maintain its designed DC operating point while still achieving high input impedance for AC signals.
Solution Approach 2:
The patent creates equipotential conditions at DC by using the capacitor to isolate the input resistor from affecting the amplifier's DC operating point. The capacitor acts as an open circuit at DC, ensuring that the DC voltage at the amplifier input remains at the designed bias level regardless of the input resistor value.
Data Source
AI summary
A capacitive sensor circuit includes a sensor for providing a sensing voltage, a pre-processing circuit electrically connected to the sensor via an input connection line, and an impedance transformation circuit electrically connected between the input connection line and a bias point. The impedance transformation circuit includes a first impedance unit and a second impedance unit. The first impedance unit includes a first current mirror, and the second impedance unit includes a second current mirror configured as an invented matching current mirror of the first current mirror. The first current mirror is electrically connected to the input connection line, and the second current mirror is electrically connected to the bias point to receive a bias voltage.