Capacitive Sensor Read Circuit with Shock-Recovery Switching
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Solution Overview
Problem
Existing MEMS microphones using capacitive sensors face issues with slow recovery times and noise increase due to the discharge of capacitors in response to large input signals, particularly when using high-impedance networks that depend on active components like MOS transistors or diodes, leading to saturation and long recovery times.
Innovation Solution
Incorporating a passive Giga Ohm resistor in series with the high-impedance network and a peak detector to generate a digital shock signal, allowing the high-impedance network to be short-circuited during and after a shock, facilitating faster capacitor charge restoration through the added resistor.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a high-impedance network is used to preserve capacitor charge, then charge preservation is improved, but recovery time increases when discharged by large input signals
Solution Approach 1:
The patent implements a dynamic impedance switching mechanism where the high-impedance network is selectively bypassed during shock conditions. A shock detector monitors the input signal and triggers a switch to connect a low-impedance path in parallel with the high-impedance network when a shock is detected, enabling fast capacitor discharge and recovery. After the shock ends, the switch returns to the high-impedance configuration to resume charge preservation.
Solution Approach 2:
The patent introduces a shock detector and switching circuit as intermediary components between the capacitive sensor and the high-impedance network. This intermediary system detects large input signals and controls the impedance transformation, allowing the circuit to adapt between high-impedance (charge preservation) and low-impedance (fast recovery) modes based on signal conditions.
2Area of stationary object
If active components like MOS transistors or diodes are used to provide high resistance, then area occupation is reduced, but noise levels increase
Solution Approach 1:
The patent extracts the high-resistance function from active components during normal operation and replaces it with a passive resistor during shock recovery. The passive resistor is used specifically for the fast discharge path during shock conditions, eliminating the noise generation associated with active components while maintaining the area efficiency of integrated implementation.
3Reliability
If the high-impedance network impedance decreases during strong input signals, then the capacitor discharges, but the recharging time constant becomes very long
Solution Approach 1:
The patent dynamically changes the discharge path impedance based on signal conditions. During shock conditions, a low-impedance path is activated to rapidly discharge the capacitor. After the shock, the high-impedance path is restored for charge preservation. This dynamic switching prevents the long recharging time constant problem by ensuring the capacitor can be rapidly recharged when needed.
Data Source
AI summary
A read circuit for capacitive sensors such as a MEMS microphones includes a sensor node configured to be coupled to a capacitive sensor to apply a bias voltage to the sensor and sense the capacitance value of the sensor wherein the voltage at the sensor node is indicative of the capacitance value of the capacitive sensor. A switch is provided between the sensor node and the intermediate node. A shock detector coupled to the sensor node and the switch asserts a shock signal to make the switch conductive in response to a shock applied to the capacitive sensor, and de-asserts the shock signal to make the switch non-conductive with a delay after the end of the shock applied to the capacitive sensor.