Single-Ended MEMS Sensor Bias Circuit With Differential Noise Rejection
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Solution Overview
Problem
MEMS sensors with single-ended transducers suffer from poor power supply rejection ratio (PSRR) and electromagnetic compatibility (EMC) interference, making it difficult to distinguish between common-mode noise and actual signals.
Innovation Solution
A MEMS sensor design incorporating a differential amplifier and feedback control circuit, where the MEMS transducer is biased with a charge-pump DC voltage, and a replica variable capacitor is used to create a differential signal, effectively canceling common-mode noise by matching the capacitance of the transducer and interface circuit.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a single-ended transducer architecture is used, then device complexity is reduced, but power supply rejection ratio and noise suppression deteriorate
Solution Approach 1:
The single-ended transducer signal path is segmented into two separate paths: one for the actual signal and another for the common-mode noise. A differential amplifier then processes these separated signals, allowing the noise to be rejected while maintaining the simplicity of the single-ended transducer architecture.
Solution Approach 2:
A differential amplifier is introduced as an intermediary component between the single-ended transducer and the output. This intermediary converts the single-ended signal into a differential signal, enabling noise rejection without modifying the transducer itself.
2Ease of manufacture
If a single-ended transducer architecture is used, then ease of manufacture is improved, but electromagnetic compatibility interference worsens
Solution Approach 1:
The differential amplifier serves as an intermediary that filters out electromagnetic interference and common-mode noise from the signal path. This allows the simple single-ended transducer to be manufactured easily while the interference problem is solved at the signal processing stage.
Solution Approach 2:
The differential amplifier configuration provides inherent feedback mechanisms that reject common-mode signals (including EMI) while amplifying differential signals. This feedback-based noise rejection occurs automatically without additional complex filtering components.
3Reliability
If a differential amplifier is introduced to reject common-mode noise, then power supply rejection ratio is improved, but device complexity increases
Solution Approach 1:
The patent uses a replica capacitor that copies the electrical characteristics of the transducer capacitor. This replica is used in the differential amplifier circuit to match impedance and optimize noise rejection, achieving high PSRR without requiring complex custom-designed components.
Solution Approach 2:
The differential amplifier circuit parameters (such as capacitor values and resistor ratios) are carefully adjusted to match the transducer characteristics. By changing these parameters to specific values, the circuit achieves optimal noise rejection with minimal complexity.
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 configuration suppresses amplification of common-mode signals caused by noise and interference, enhancing signal clarity and noise rejection in MEMS sensors.
Implementation Method 1
the variable capacitor of the MEMS microphone changes its capacitance in dependence on a sound pressure impacting on the microphone
Implementation Method 2
The differential amplifier is configured to provide the output signal at the at least one output node of the differential amplifier
Implementation Method 3
The feedback control circuit is configured to provide a feedback signal at the base terminal of the output filter of the bias voltage generator so that the voltage potential at the base terminal of the output filter is changed in the same way as the voltage potential at the output node of the bias voltage generator is changed
Data Source
AI summary
A MEMS sensor (1) comprises a MEMS transducer (10) being coupled to a MEMS interface circuit (20). The MEMS interface circuit (20) comprises a bias voltage generator (100), a differential amplifier (200), a capacitor (300) and a feedback control circuit (400). The bias voltage generator (100) generates a bias voltage (Vbias) for operating the MEMS transducer. The variable capacitor (300) is connected to one of the input nodes (I200a) of the differential amplifier (200). At least one of the output nodes (A200a, A200b) of the differential amplifier is coupled to a base terminal (T110) of an output filter (110) of the bias voltage generator (100). Any disturbing signal from the bias voltage generator (100) is a common-mode signal that is divided equally on the input nodes (I200a, I200b) of the differential amplifier (200) and is therefore rejected.