Integrated Readout and Drive Circuits for a Microelectromechanical Gyroscope Tao Yin1, Huanming Wu1, HaigangYang1*, Qisong Wu1, Jiwei Jiao2 1
2
Institute of Electronics, Chinese Academy of Sciences, Beijing, 100190, China Shanghai Institute of Microsystem and Information Technology, CAS, Shanghai, 200050, China * Email:
[email protected]
Abstract An ASIC is implemented for readout and drive of a bulk micromachined gyroscope utilizing electromagnetic actuation and sensing. A fully differential structure is used in the readout channel and drive loop to reduce the common-mode noise and interference. A low-noise front-end amplifier is designed to sense the rotation induced voltage signal. A switched-capacitor BPF with adjustable center frequency is proposed to track the resonance frequency and suppress the noise and harmonic components out of the signal bands. An AGC-based driving loop with PLL for demodulation clock generation is also designed and integrated on chip. The interface circuits are designed in a 0.35μm 2P4M CMOS process. The simulation results show that the readout circuit achieves an input referred noise voltage of 16.5nV/√Hz and dynamic range of 80dB over the 100Hz bandwidth from a single 5V supply.
magnetic field perpendicularly and produce the induction voltage, which is directly proportional to the input angular rate and can be measured by a readout circuit. Because the induced voltage change at the sensing mode is only on microvolt scale, it poses a challenge to the low-noise readout circuits design. Apart from the readout channel, a drive loop is used to maintain the oscillation of the resonator with a constant amplitude and frequency, and is also vital for high performance gyroscopes.
1. Introduction High performance MEMS gyroscopes are needed in a number of applications thanks to their advantages in low power consumption, miniature dimensions and low cost. Shock survivability is a common and vital aspect for harsh environment applications of inertial sensors. Capacitive gyroscope sensors are widely used for high sensitivity applications; however, these sensors are susceptible to damage at high linear accelerations and shocks. Gyroscopes with electromagnetic actuation and sensing can be highly shock tolerant if they are appropriately designed[1]. In this paper, an ASIC is designed to interface a high-g survival MEMS electromagnetic gyroscope with a resonance frequency of about 10kHz. The circuit is based on the structure shown in Fig.1. The gyroscope structure was further designed and improved for high shock tolerance. Under a magnetic field in Z axis direction, Ampere force generated by AC current running through the driving wires drives the oscillating frames to vibrate along X axis direction, known as the driving mode of the microgyroscope. When an angular rate around Z axis direction is applied, two detecting masses will move along Y axis direction due to a Coriolis force, also known as the sensing mode. Under the Coriolis force, the wires in detecting masses cut the uniform
2. System Architecture
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Figure 1. Schematic diagram of the sensor element
Figure 2. Block diagram of the system Fig. 2 shows the block diagram of the implemented system. In the readout channel, low noise front-end amplifiers are designed to amplify the induced voltage from the sense mode. Thereafter, the signals are high-pass filtered to suppress the DC offset and amplified with programmable-gain amplifiers (PGA). Then a switched-capacitor BPF filters the signal to suppress the noise and harmonic components out of the signal bands, whose center frequency is adjustable to track the resonance frequency of gyroscope. The signal is then demodulated by a switch set type demodulator and low-pass filtered to generate the rate and quadrature
signal in readout channel. In driving loop, the signal after BPF is amplified by an automatic gain control (AGC) circuit to drive the primary resonator of gyroscope. An on-chip phase-locked loop (PLL) is adopted to provide carefully phased signals (drv0o and drv90o) for demodulation. 3. Circuit Blocks 3.1 Sensor Readout The gyroscope produces only about 10μV induced voltage at 1o/s angular input, so the noise performance of sensor readout channel is vital for gyroscope’s resolution. The front-end amplifier is at the forefront of the readout channel, which dominants the noise contributions and determines the resolution of the readout circuit. The front-end amplifier in this paper is based on resistive feedback scheme and provides a gain of 34 dB as shown in Fig.3, in which a folded-cascode operational trans-conductance amplifier (OTA) is used with dedicated noise optimization. The equivalent input noise voltage of the low-noise front end amplifier is given by (1), which includes the noise contribution of OTA and resistor R1,2. 2 2 Vnoise ,in = Vn ,amp ⋅ (
and harmonics out of the signal band. In order to integrate passive devices on the same chip, a switched-capacitor scheme is selected as shown in Fig.5. To be compatible with the wide distribution of gyroscope resonance frequency induced by MEMS process variation, a relaxation oscillator with adjustable frequency is proposed to generator clock fc for the BPF, in which an off-chip resistor Rset is used to control the filter center frequency (f0) as (2). A three-opamp, two-stage instrument amplifier (IA) is adopted to provide differential to single-end conversion and additional amplification after demodulation. An integrated 2nd-order Sallen-Key low-pass filter with off-chip capacitors sets the cut-off frequency of system to 100 Hz. f C 2 C3 1 C2C3 (2) ∝ ⋅ f0 ≈ c 2π C ACB 2π Rset Cm C ACB
R1 R + 1) 2 + 4k BTR1 ( 1 + 1) (1) R2 R2
Where, Vn,amp is input-referred voltage noise of OTA, kB is Boltzmann’s constant. To improve the noise performance of the front-end amplifier, the inputreferred noise of OTA and the value of resistor R1 should be reduced as much as possible. Limited by the power consumption and source resistor(Rs) of gyroscope, 8nV/√Hz input-referred noise of OTA is designed and 1kΩ of R1 is selected in this design. To drive the resistor R2 of 50kΩ, buffers are added at the output of OTA in Fig3(a).
Figure 4. (a) High-pass filter (b) OTA with input common voltage monitor
Figure 5. (a) SC band-pass filter with adjustable center frequency (b) Frequency adjustable oscillator
Figure 3. (a) Low-noise front-end amplifier (b) Fully differentail OTA scheme An active-RC high-pass filter is used in front of PGA to cancel the DC offset as shown in Fig.4. The −3dB frequency is set to about 300Hz and the feedback resistors are implemented using MOSFETs (M1-9), which operate in the linear region[2]. A PGA with resistive feedback scheme is used to further amplifier the signal. The gain control range is from 14dB to 30dB, which is realized by control of the feedback resistor value. After the PGA, a BPF is used to suppress the noise
3.2 Driving Loop An AGC driving circuit is designed to drive the gyroscope vibrating at resonance frequency with stable frequency and amplitude, as shown in Fig.2. The same structure of LNA, PGA, HPF and BPF as the readout channel is used in the drive loop. A switch-based rectifier and a 1st order low-pass filter detect the vibrating amplitude. A Gm-C based proportional- integrator (PI) controller is used to control the vibrating amplitude to a desired value, as shown in Fig.6(a). An off-chip RC network of R1 and C in the controller promises sufficient tunable range. A VGA is proposed based on three-opamp, two-stage IA structure, as shown in Fig.6(b). The transistor MG operating at linear region and works as a linear voltage controlled resistor, which facilitates the VGA work as a PGA and brings good linearity, low
output DC offset and large output range. A folded-cascode amplifier with class-AB output stage is designed as drive buffer to excite the gyroscope and provide drive current as large as 80mA.
Figure 6. (a) gm-C based PI controller (b) IAstructure-based VGA without buffer 4. Simulation Results The interface circuit is designed in a standard 0.35μm 2-poly/4-metal CMOS process. The readout channel and drive loop consumes 8.5mA and 12mA static current, respectively. The simulation frequency response of the SC BPF is shown in Fig.7(a) with different Rset values to set the center frequency to be 5k, 10k and 15kHz, respectively . The response of the readout channel versus input sinusoidal voltage magnitude is plotted in Fig.7(b) at different gain settings. The overall output linearity is better than 1‰ in the range of 0.5–4.5V.
Figure 7. (a) Frequency response of the freq. adjustable SC BPF (b) Transfer function of the readout channel
Figure 8. Noise characteristics of readout channel The readout circuit shows an output noise down to about 20μV/√Hz from 1 to 100Hz with 61.5dB gain setting as shown in Fig.8. The input-referred voltage noise is 165nVrms in 100Hz bandwidth. The output voltage range of the readout channel is about +/-2V, so dynamic range is about 80dB. The output noise also shows the LPF cutoff frequency of 100Hz. Fig.9 shows the transient simulation response under a 10o/s 100Hz sinusoidal angle rate input. The output at MEMS sense mode and demodulator is also given. The scale factor of MEMS gyroscope system is about 8.2mV/(o/s) with 61.5dB gain setting of readout channel. A summary of
simulated performance for the readout chip and whole gyroscope system is given in Table.1.
Figure 9. Transient response to 10 o/s 100Hz sine angle rate input Table 1. Summary of the chip and system performance Interface chip: Gain 50~85dB (adjustable) Resolution 16.5nV/√Hz @61.5dB gain Linear output range 0.5 ~ 4.5V Dynamic range 80dB @100Hz BW & 61.5dB Non-linearity 1‰ @0.5 ~4.5V output Supply Voltage 5V Current consumption 8.5mA (readout)+12mA(drive) Gyroscope system: @61.5dB gain setting Scale factor 8.2mV/(o/s) Full scale angular rate +/- 240o/s Bandwidth 100Hz 5. Summary An ASIC implementation for readout and drive of an electromagnetic MEMS gyroscope has been presented. Both the system level implementation and details on the subsystems circuit have been described. Besides several off-chip passive components, the readout and drive circuits are integrated on a single chip. The readout circuit achieves an input noise voltage of 16.5nV/√Hz and dynamic range of 80dB over the 100Hz bandwidth from a single 5V supply. The simulation results show that the system achieves a good performance and prove the feasibility of the chosen architecture. Acknowledgments The authors gratefully acknowledge National Natural Science Foundation of China (NSFC) for financial support (project 61106025). References [1] Azgin, K., Y. Temiz, et al.. "A novel in-operation high g-survivable MEMS gyroscope." The 7th IEEE conference on sensors, pp.111-114 (2007). [2] Saukoski, M., L. Aaltonen, et al.. "Interface and control electronics for a bulk micromachined capacitive gyroscope." Sensors and Actuators A147, pp.189-193 (2008).
