27.3 A 3-axis open-loop gyroscope with demodulation phase error correction

Abstract

Consumer-electronic (CE) gyroscopes have recently enjoyed broad deployment in high-volume applications, largely due to intuitive user interfaces in smart phones and video game controllers. For their continued expansion into more demanding CE applications, a further reduction of their noise, offset drift, and power dissipation, especially in the emerging always-on category, is mandatory. To be viable, solutions to these conflicting requirements must overcome the challenges of low cost and ever-shrinking package size. This paper describes one such solution with special emphasis on offset drift reduction. The system presented here discards the standard practice of electrically cancelling the quadrature error, and instead combines information derived from continuously monitoring the quadrature error together with a single-point temperature calibration to reduce offset drift. This paper presents the architecture and circuits used to realize a 3-axis open-loop gyroscope with a one-sigma TCO of 0.0065°/s/K. Figure 27.3.1 illustrates a fundamental mechanism of offset drift in an open-loop vibratory gyroscope.A suspended mass is driven into a steady state oscillation in a drive axis. Under rotation, some of the kinetic energy in the drive oscillation couples onto a sense axis. Manufacturing imperfections also cause some of the drive oscillation to appear directly in the sense axis independent of rotation, a stress-sensitive coupling referred to as the quadrature error. Due to their modulation by the drive velocity and displacement respectively, the desired angular-rate and the unwanted quadrature-error signals are orthogonal, which, in principle, allows their discrimination via synchronous demodulation. However, the phase delay through the sense axis introduces a skew between the modulated signals and the demodulation clock, which allows a portion of the quadrature error to appear as an angular-rate signal at the output. For high yield, the magnitude of the acceptable worst-case quadrature error can exceed the full-scale angular rate by over 20dB, simultaneously placing a heavy burden on the dynamic range of the readout circuitry and very tight bounds on the phase error. To ease the dynamic range burden on the readout circuitry, state-of-theart CE gyroscopes electrically cancel the quadrature error at the input of the sense channel capacitance-to-voltage (C/V) converter [1, 2]. However, besides the noise penalty that the cancelation signal injection incurs, the inevitable phase mismatch between the quadrature error and the cancellation signal can even exacerbate the offset drift problem.The sensing element used in this work operates far below the sense resonance frequency, where the phase delay through the sense axis is inversely proportional to the quality factor of the sense resonance. While the quality factor varies with process and temperature, its temperature dependence when normalized to its value at a reference temperature is known and quite stable over process. This implies that the phase delay is φ≅φ(T 0 )G(T 0 ,...

A Real-Time Circuit Phase Delay Correction System for MEMS Vibratory Gyroscopes

“…In order to verify these characteristics of the MEMS gyroscope, an open-loop frequency sweep test on the front-end analog circuit was carried out, as shown in [2]. The excitation signal with a fixed amplitude (100 mV) and a linearly increasing frequency (13,408 Hz; 0.05 Hz) was generated by FPGA programming, and the amplitude and phase response under each excitation were automatically collected. As shown in Figure 4a, when the frequency of excitation signal was 13,401.45 Hz, the amplitude response of the gyroscope reached its maximum value, and the phase delay between the excitation signal and the output signal was 108.286 • in this case.…”

“…exists [12]. In this case, the quadrature error is not −90 • out of phase with the angular rate signal, introducing a skew between the quadrature and in-phase channel and hence causing in-phase and quadrature (IQ) coupling [13,14].…”

An Interface ASIC for MEMS Vibratory Gyroscopes With a Power of 1.6 mW, 92 dB DR and 0.007°/s/<inline-formula> <tex-math notation="LaTeX">$\sqrt {\rm {Hz}} $ </tex-math> </inline-formula> Noise Floor Over a 40 Hz Band

“…The current flowing through this resistor is integrated on the capacitor C int . Therefore, the resistive compensation generates a current through R comp = R int , driven by (6) in order to balance the current I FB .…”

“…Such portable devices are mostly battery powered. Hence, the interface ASIC must offer a low power consumption to achieve long operation times [6]. Satisfying the low power consumption and high performance requirements, which are given by the automotive and nowadays the more and more dominating consumer market with its large quantities, another requirement is set.…”

“…In contrast to [12], where for the drive and sense loop DT -modulation is used or the open-loop readout presented in [13], the interface ASIC in this work requires no external HV supply. Interfaces requiring no external HV supply but utilizing an open-loop readout scheme are reported in [6], [14]. To decrease the sensitivity of the system to ambient vibrations, the resonance frequencies of vibratory gyroscopes are increased from the previous one-digit frequencies up to 30-100 kHz [15].…”