Electronic compensation of capacitive micro-machined sensors parasitic modes in force-feedback interface systems

Description

Operating capacitive sensors in force feedback mode has many benefits, such as improved bandwidth, and lower sensitivity to process and temperature variation. To overcome, the non-linearity of the voltage-to-force relation in capacitive feedback, a two-level feedback signal is often used. Therefore, a single-bit Σ-Δ modulator represents a practical way to implement capacitive sensors interface circuits. However, high-Q parasitic modes that exist in high-Q sensors (operating in vacuum) cause a stability problem for the Σ-Δ loop, and hence, limit the applicability of Σ-Δ technique to such sensors. A solution is provided that allows stabilizing the Σ-Δ loop, in the presence of high-Q parasitic modes. The solution is applicable to low or high order Σ-Δ based interfaces for capacitive sensors.

Aspects

The problem of stabilizing Σ-Δ electro-mechanical loops in the presence of high-Q parasitic modes is addressed. In one embodiment, introducing a second order finite impulse response (FIR) filter into the Σ-Δ electro-mechanical loop stabilizes the loop. This solution is supported by both theoretical and empirical results and is much simpler than other proposed techniques. In another embodiment, a method is provided of electronically interfacing with a MEMS sensor using an interface circuit, the MEMS sensor and the interface circuit together forming a sigma-delta modulator loop. In accordance with the method, a potential parasitic resonant mode of the MEMS sensor is identified, the potential parasitic resonant mode having a frequency and a quality factor. A filter is inserted into the sigma-delta modulator loop having characteristics chosen in accordance with at least one of the frequency and the quality factor of the potential parasitic resonant mode. In another embodiment, an interface circuit is provided for electronically interfacing with a MEMS sensor using an interface circuit, the MEMS sensor and the interface circuit together forming a sigma-delta modulator loop having a potential parasitic resonant mode characterized by a frequency and a quality factor. The interface circuit includes a capacitance to voltage converter; a forward loop circuit coupled to the capacitance to voltage converter and comprising a quantizer; a feedback loop coupled to the quantizer and providing a force feedback signal to the MEMS sensor; and an FIR filter inserted into the forward loop circuit and having characteristics chosen in accordance with at least one of the frequency and the quality factor of the parasitic resonant mode. In a further embodiment, a sensor subsystem includes a MEMS sensor; and an interface circuit coupled to the MEMS sensor, the MEMS sensor and the interface circuit together forming a sigma-delta modulator loop having a potential parasitic resonant mode characterized by a frequency and a quality factor. The interface circuit in turn includes a capacitance to voltage converter; a forward loop circuit coupled to the capacitance to voltage converter and comprising a quantizer; a feedback loop coupled to the quantizer and providing a force feedback signal to the MEMS sensor; and an FIR filter inserted into the forward loop circuit and having characteristics chosen in accordance with at least one of the frequency and the quality factor of the parasitic resonant mode.

Patenting Status

Patent Granted

Status

Patented

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