CMOS ASIC for MHz Silicon BAW Gyroscope
CMOS ASIC for MHz Silicon BAW Gyroscope Jalpa Shah, Houri Johari, Ajit Sharma and Farrokh Ayazi School of Electrical and Computer Engineering Georgia Institute of Technology, Atlanta, GA 30332 [email protected] , [email protected] Abstract — The paper reports a CMOS Interface IC designed for electrostatic actuation and read-out a 6MHz Silicon Bulk Acoustic Wave (BAW) gyroscope. The supporting electronics for a high quality factor (Q>100,000) BAW gyroscope include: an electro-mechanical drive oscillator loop to excite the gyroscope, a low noise transimpedance front-end that offers gain of 126dBȍ and 3-dB bandwidth of 26MHz; and a Coriolis signal demodulator with minimum capacitance resolution of 0.014aF/¥Hz. The interface ASIC, a first for a high frequency BAW gyroscope, is fabricated in a 0.6µm 2P3M CMOS process, occupies 2.25mm2 and consumes 10.9mW. The gyroscope system achieves a noise floor of 0.37o/¥hr/¥Hz and rate sensitivity of 205µV/ o/sec.
I. INTRODUCTION Silicon angular rate sensors have become an important part of electronic motion control systems in consumer products like gaming devices, image stabilizing modules in digital cameras, automotive roll stability control systems and inertial navigation units[1,2]. A vast majority of these gyroscopes are micromachined low-frequency vibratory gyroscopes operating in their flexural resonance modes [1].
constraints on the design of the sustaining electronics since the bandwidth (BW) requirements on the front-end are higher and parasitic capacitance effects are more significant. In this paper, we have demonstrated a 6MHz MEMS silicon BAW gyroscope that includes drive resonant loop, low noise sense amplifier and synchronous demodulator. II. SYSTEM BLOCK DIAGRAM Fig. 1 shows block diagram of the implemented high frequency Si BAW disk gyroscope system. The gyroscope is designed to operate in the secondary degenerative elliptic modes that are spatially 30o apart. The disk gyroscope requires two basic blocks: 1) the drive loop to actuate the disk at the resonance frequency and 2) Sense amplifier with demodulator to extract the rotation rate [5].
The key parameter in determining the resolution of the sensor is the Brownian noise of the device itself. The stateof-the art flexural mode microgyroscopes operate at low frequencies (Ȧo = 3-30kHz) and rely on higher structural mass and drive amplitude (qdrive) to reduce the Brownian noise of the micromechanical sensor [3]: Ω z ( Brownian ) ∝
1 q drive
4k B T ω 0 MQ Effect − Sense
(1) Figure 1. System Block Diagram
Further, the need for vacuum packaging to obtain high Q in these devices precludes widespread use of such sensors in low cost consumer applications. To avoid using large proof masses and still achieve higher resolution, the operating frequency of the gyroscope can be increased by 2 to 3 orders of magnitude (6MHz) [4]. If the sensor is excited in the bulk acoustic wave mode, it will result in high Q (>100,000) which further reduces the noise floor. This is achieved in moderate vacuum and even at atmospheric pressure [4], thus alleviating costly packaging requirements. The low polarization voltage (6MHz). In this architecture, the optimal RF of 20kȍ yielded in-min of 2.3pA/¥Hz (Fig. 5), which corresponds to minimum detectable change in the capacitance of 0.014aF/¥Hz at 6MHz with 10V DC polarization voltage. Although the noise is lower with RF of 50kȍ, the larger signal phase shift in this case may result in an incorrect rate response when demodulated, and is hence avoided.
C. Rate Read-out Electronics Since the motional current is in the range of nanoamperes, the output voltage of the front-end TIA discussed earlier is amplified using two non-inverting gain stages for read-out. This results in overall trans-impedance gain of about 1Mȍ for the sense amplifier chain. To extract the rate information, the output signal of the sense amplifier is applied as one of the inputs to an amplitude demodulator. Since, the carrier frequency of the AM signal is the drive frequency of the device, the output of the drive loop oscillator is used as the other input to the demodulator. A switching demodulator, shown in Fig. 7 (along with the clock generator and the waveforms), is used for this purpose. To ensure a rail-to-rail drive input, the drive oscillator output is passed through successive inverter stages. The drive amplitude can hence be automatically clamped to the supply voltage, thus obviating the need for an ALC loop to maintain constant drive amplitude and avoid an incorrect read-out of the rotation rate signal. The modulation index of the AM signal is typically less than 1% thus requiring a demodulator with good sensitivity (0.2% for this design).
Figure 5. Measured input referred current noise for different VGC(RF)
Fig. 6 shows the linearity of the TIA configured as a voltage amplfiier. SNR of 96dB was achieved for the frontend TIA at 6MHz with RF of 20kȍ for sensor bandwidth of 60Hz.
Figure 7. Schematic of a switching quad demodulator,clock generator, and measured demodulated response
V.
INTERFACED MEASURED RESULTS
A. Drive Resonator Oscillator The gyroscope is actuated and the vibration is maintained at the resonance frequency. The TIA with the inverting buffer is used to lock into oscillations satisfying the Barkhausen’s criteria of unity gain and zero degree phase shift. Fig. 8 shows drive oscillation waveform and spectrum signal for the gyroscope with the motional impedance of 500kȍ.
Figure 6. Measured SNR plot of the TIA with RF =20kȍ at 6MHz
B. Drive Loop Sustaining Amplifier The sustaining amplifier for a high frequency BAW disk gyroscope would require high gain with sufficient bandwidth (>10MHz) in order to lock into oscillations with motional impedances of about 200k-1Mȍ. Hence, the same front-end amplifier is used in the drive loop with gain stages at the output. An inverting buffer is used at the output of the amplifier to maintain a loop phase shift of zero and to drive an off-chip load.
B. Rotation Rate After the gyroscope is excited at the resonance frequency, a rotation rate response test is performed by subjecting the system to a rotation on the Z-axis. To maximize sensitivity to rotation, the drive and sense modes are matched by electrostatic tuning. As expected sense signal is 90o out of phase with the drive signal (shown in Fig. 9). A low pass filter with 100Hz cut-off frequency is used at the
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output of the demodulator to filter the high frequency components.
our knowledge, this work is the first reported ASIC for an integrated BAW gyroscope system. TABLE I.
SUMMARY OF MEASURED RESULTS
Gyroscope Specification Resonance Frequency 5.9MHz Matched Mode Quality Factor 180,000 Drive Motional Impedance 500kȍ Disk Diameter 800µm Thickness 40µm Capacitive Gap Size 200nm Circuit Specifications Transimpedance Gain 126dBȍ 3-dB Bandwidth 26MHz Total Power Consumption 10.9mW Die Area 2.25mm2 System Specification Rate Sensitivity Gyro+IC 205µV/o/sec Minimum Detectable ǻC (at 6MHz) 0.014aF/¥Hz Electronic Noise Floor 0.36o/¥hr/¥Hz Brownian/Mechanical Noise Floor 0.087 o/¥hr/¥Hz
Figure 8. Closed loop drive oscillation waveform and spectrum
Figure 9. Matched mode response, zero rate response of drive and sense mode, demodulated waveform and sensitivity of gyroscope
The scale factor of the gyroscope interfaced with the sense amplifier was characterized by subjecting the system to varying rotation rates. The sensitivity of the system was measured to be 205µV/o/sec. The demodulated response is shown in Fig. 9 along with the scale factor. Table 1 summarizes the gyroscope and circuit specifications. Fig. 10 shows the micrograph of the interfaced IC fabricated in a 0.6µm standard CMOS process. VI.
CONCLUSIONS
A high frequency angular rate sensor system comprising of a micromachined Si-BAW gyroscope interfaced with a 0.6µm CMOS ASIC that achieves sensitivity of 205µV/o/sec with noise floor of (Total of Brownian noise floor and Electronic noise floor) 0.37o/¥hr/¥Hz has been reported. The supporting electronics including a drive loop oscillator, a low noise transimpedance front-end for sensing and a Coriolis rate demodulator have been described. The front-end sense amplifier offers input referred current noise of 2.3pA/¥Hz at 6MHz with minimum detectable capacitance change of 0.014aF/¥Hz. The total power consumption is 10.9mW. The system is a first prototype toward developing low cost, small form factor rate sensors targeting consumer applications. To
Figure 10. Die micrograph of interface IC (1.5mm X 1.5mm)
REFERENCES [1]
N. Yazdi, F. Ayazi and K. Najafi, “Micromachined inertial sensors” Proc. IEEE, pp. 1640–1659, Aug. 1998.
[2]
J. Geen et. al. “ Sigle-Chip Surface micromachined integrated gyroscope with 50º/h Allan deviation” IEEE J. Solid State Circuits, vol. 37, no. 12, pp. 1860-1866, Dec. 2002.
[3] A. Sharma, M. Zaman, and F. Ayazi, “A 104-dB dynamic range transimpedance-based CMOS ASIC for tuning fork microgyroscopes,” IEEE J. Solid-State Circuits, vol. 42, no. 8, pp. 1790-1802, Aug. 2007. [4]
H. Johari, J. Shah and F. Ayazi, "High frequency XYZ-axis singledisk silicon gyroscope," Proc.. IEEE MEMS pp 856-859, Jan 2008.
[5]
W. A. Clark and R. T. Howe, “Surface micromachined Z axis vibratory rate gyroscope,” in Proc. Solid State Sensors and Actuator Workshop 1996, pp. 283–287, Jan 2006.
[6]
C.T-C. Nguyen and R.T. Howe, “An integrated high-Q CMOS micromechanical resonator-oscillator,” IEEE J. Solid-State Circuits, vol. 34, no. 4, pp. 440-455, April 1999.
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I. INTRODUCTION Silicon angular rate sensors have become an important part of electronic motion control systems in consumer products like gaming devices, image stabilizing modules in digital cameras, automotive roll stability control systems and inertial navigation units[1,2]. A vast majority of these gyroscopes are micromachined low-frequency vibratory gyroscopes operating in their flexural resonance modes [1].
constraints on the design of the sustaining electronics since the bandwidth (BW) requirements on the front-end are higher and parasitic capacitance effects are more significant. In this paper, we have demonstrated a 6MHz MEMS silicon BAW gyroscope that includes drive resonant loop, low noise sense amplifier and synchronous demodulator. II. SYSTEM BLOCK DIAGRAM Fig. 1 shows block diagram of the implemented high frequency Si BAW disk gyroscope system. The gyroscope is designed to operate in the secondary degenerative elliptic modes that are spatially 30o apart. The disk gyroscope requires two basic blocks: 1) the drive loop to actuate the disk at the resonance frequency and 2) Sense amplifier with demodulator to extract the rotation rate [5].
The key parameter in determining the resolution of the sensor is the Brownian noise of the device itself. The stateof-the art flexural mode microgyroscopes operate at low frequencies (Ȧo = 3-30kHz) and rely on higher structural mass and drive amplitude (qdrive) to reduce the Brownian noise of the micromechanical sensor [3]: Ω z ( Brownian ) ∝
1 q drive
4k B T ω 0 MQ Effect − Sense
(1) Figure 1. System Block Diagram
Further, the need for vacuum packaging to obtain high Q in these devices precludes widespread use of such sensors in low cost consumer applications. To avoid using large proof masses and still achieve higher resolution, the operating frequency of the gyroscope can be increased by 2 to 3 orders of magnitude (6MHz) [4]. If the sensor is excited in the bulk acoustic wave mode, it will result in high Q (>100,000) which further reduces the noise floor. This is achieved in moderate vacuum and even at atmospheric pressure [4], thus alleviating costly packaging requirements. The low polarization voltage (6MHz). In this architecture, the optimal RF of 20kȍ yielded in-min of 2.3pA/¥Hz (Fig. 5), which corresponds to minimum detectable change in the capacitance of 0.014aF/¥Hz at 6MHz with 10V DC polarization voltage. Although the noise is lower with RF of 50kȍ, the larger signal phase shift in this case may result in an incorrect rate response when demodulated, and is hence avoided.
C. Rate Read-out Electronics Since the motional current is in the range of nanoamperes, the output voltage of the front-end TIA discussed earlier is amplified using two non-inverting gain stages for read-out. This results in overall trans-impedance gain of about 1Mȍ for the sense amplifier chain. To extract the rate information, the output signal of the sense amplifier is applied as one of the inputs to an amplitude demodulator. Since, the carrier frequency of the AM signal is the drive frequency of the device, the output of the drive loop oscillator is used as the other input to the demodulator. A switching demodulator, shown in Fig. 7 (along with the clock generator and the waveforms), is used for this purpose. To ensure a rail-to-rail drive input, the drive oscillator output is passed through successive inverter stages. The drive amplitude can hence be automatically clamped to the supply voltage, thus obviating the need for an ALC loop to maintain constant drive amplitude and avoid an incorrect read-out of the rotation rate signal. The modulation index of the AM signal is typically less than 1% thus requiring a demodulator with good sensitivity (0.2% for this design).
Figure 5. Measured input referred current noise for different VGC(RF)
Fig. 6 shows the linearity of the TIA configured as a voltage amplfiier. SNR of 96dB was achieved for the frontend TIA at 6MHz with RF of 20kȍ for sensor bandwidth of 60Hz.
Figure 7. Schematic of a switching quad demodulator,clock generator, and measured demodulated response
V.
INTERFACED MEASURED RESULTS
A. Drive Resonator Oscillator The gyroscope is actuated and the vibration is maintained at the resonance frequency. The TIA with the inverting buffer is used to lock into oscillations satisfying the Barkhausen’s criteria of unity gain and zero degree phase shift. Fig. 8 shows drive oscillation waveform and spectrum signal for the gyroscope with the motional impedance of 500kȍ.
Figure 6. Measured SNR plot of the TIA with RF =20kȍ at 6MHz
B. Drive Loop Sustaining Amplifier The sustaining amplifier for a high frequency BAW disk gyroscope would require high gain with sufficient bandwidth (>10MHz) in order to lock into oscillations with motional impedances of about 200k-1Mȍ. Hence, the same front-end amplifier is used in the drive loop with gain stages at the output. An inverting buffer is used at the output of the amplifier to maintain a loop phase shift of zero and to drive an off-chip load.
B. Rotation Rate After the gyroscope is excited at the resonance frequency, a rotation rate response test is performed by subjecting the system to a rotation on the Z-axis. To maximize sensitivity to rotation, the drive and sense modes are matched by electrostatic tuning. As expected sense signal is 90o out of phase with the drive signal (shown in Fig. 9). A low pass filter with 100Hz cut-off frequency is used at the
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output of the demodulator to filter the high frequency components.
our knowledge, this work is the first reported ASIC for an integrated BAW gyroscope system. TABLE I.
SUMMARY OF MEASURED RESULTS
Gyroscope Specification Resonance Frequency 5.9MHz Matched Mode Quality Factor 180,000 Drive Motional Impedance 500kȍ Disk Diameter 800µm Thickness 40µm Capacitive Gap Size 200nm Circuit Specifications Transimpedance Gain 126dBȍ 3-dB Bandwidth 26MHz Total Power Consumption 10.9mW Die Area 2.25mm2 System Specification Rate Sensitivity Gyro+IC 205µV/o/sec Minimum Detectable ǻC (at 6MHz) 0.014aF/¥Hz Electronic Noise Floor 0.36o/¥hr/¥Hz Brownian/Mechanical Noise Floor 0.087 o/¥hr/¥Hz
Figure 8. Closed loop drive oscillation waveform and spectrum
Figure 9. Matched mode response, zero rate response of drive and sense mode, demodulated waveform and sensitivity of gyroscope
The scale factor of the gyroscope interfaced with the sense amplifier was characterized by subjecting the system to varying rotation rates. The sensitivity of the system was measured to be 205µV/o/sec. The demodulated response is shown in Fig. 9 along with the scale factor. Table 1 summarizes the gyroscope and circuit specifications. Fig. 10 shows the micrograph of the interfaced IC fabricated in a 0.6µm standard CMOS process. VI.
CONCLUSIONS
A high frequency angular rate sensor system comprising of a micromachined Si-BAW gyroscope interfaced with a 0.6µm CMOS ASIC that achieves sensitivity of 205µV/o/sec with noise floor of (Total of Brownian noise floor and Electronic noise floor) 0.37o/¥hr/¥Hz has been reported. The supporting electronics including a drive loop oscillator, a low noise transimpedance front-end for sensing and a Coriolis rate demodulator have been described. The front-end sense amplifier offers input referred current noise of 2.3pA/¥Hz at 6MHz with minimum detectable capacitance change of 0.014aF/¥Hz. The total power consumption is 10.9mW. The system is a first prototype toward developing low cost, small form factor rate sensors targeting consumer applications. To
Figure 10. Die micrograph of interface IC (1.5mm X 1.5mm)
REFERENCES [1]
N. Yazdi, F. Ayazi and K. Najafi, “Micromachined inertial sensors” Proc. IEEE, pp. 1640–1659, Aug. 1998.
[2]
J. Geen et. al. “ Sigle-Chip Surface micromachined integrated gyroscope with 50º/h Allan deviation” IEEE J. Solid State Circuits, vol. 37, no. 12, pp. 1860-1866, Dec. 2002.
[3] A. Sharma, M. Zaman, and F. Ayazi, “A 104-dB dynamic range transimpedance-based CMOS ASIC for tuning fork microgyroscopes,” IEEE J. Solid-State Circuits, vol. 42, no. 8, pp. 1790-1802, Aug. 2007. [4]
H. Johari, J. Shah and F. Ayazi, "High frequency XYZ-axis singledisk silicon gyroscope," Proc.. IEEE MEMS pp 856-859, Jan 2008.
[5]
W. A. Clark and R. T. Howe, “Surface micromachined Z axis vibratory rate gyroscope,” in Proc. Solid State Sensors and Actuator Workshop 1996, pp. 283–287, Jan 2006.
[6]
C.T-C. Nguyen and R.T. Howe, “An integrated high-Q CMOS micromechanical resonator-oscillator,” IEEE J. Solid-State Circuits, vol. 34, no. 4, pp. 440-455, April 1999.
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