hal , version 1-20 Feb 2008

Transcription

1 Author manuscript, published in "DTIP 27, Stresa, lago Maggiore : Italy (27)" Stresa, Italy, 2527 April 27 NOISE AND THERMAL STAILITY OF IRATING MIROGYROMETERS PREAMPLIFIERS R. Levy 1, A.Dupret 1, H. Mathias 1, JP. Gilles 1, Fabien Parrain 1, runo Eisenbeis 2, S. Megherbi 1 1 ONERA, 29 avenue de la division Leclerc 2 IEF, bat. 22, université Paris sud 9232 hâtillon 9145 Orsay hal257677, version 1 2 Feb 28 ASTRAT The pre is a critical component of gyrometer s electronics. Indeed the resolution of the sensor is limited by its signal to noise ratio, and the gyrometer s thermal stability is limited by its gain drift. In this paper, five different kinds of pres are presented and compared. Finally, the design of an integrated pre is shown in order to increase the gain stability while reducing its noise and size. 1. INTRODUTION During the last decades, vibrating MEMS have been widely developed worldwide and have met many applications such as inertial sensors: accelerometers [1,2] and gyrometers [3,4], biosensors [5], or force microscopy[6]. The mechanical parameter to sense (amplitude or freuency variation) is converted into charges by capacitive sensing or piezoelectricity. These small charges are then sensed with pres. For the gyrometer, the goal of the first detection stage is to sense the amplitude of the oriolis induced charges on the detection electrodes. This voltage is then converted into a dc voltage proportional to the angular velocity with analog or digital electronics. As the detected charges are very small compared to the parasitic charges induced by parasitic mechanical and capacitive couplings [7], the pre is a critical component. Indeed its signal to noise ratio limits the resolution of the sensor, and the thermal stability of the gain limits the thermal stability of the sensor s output. A few kinds of pres have already been studied like the differential charge [8,9] and the switched capacitor charge [1,11]. ut these architectures have not been compared, and their thermal stability has not been taken into account. In this paper, we will compare five different pres in terms of noise and thermal stability. The benefits and drawbacks of each kind of circuit are discussed. The best suited circuit for the gyro is integrated in.35µm imos technology, and specific design and layout is performed to ensure low noise and thermal stability 2. SPEIFIATIONS FOR THE GYRO S PREAMPLIFIER 2.1. Description of the gyro s associated electronics The physical phenomenon used for vibrating gyros is the oriolis force induced by rotation. The shape of the gyro, either a ring [12], a tuning fork [13] or a disk [14], allows two orthogonal modes of vibration; the drive mode along the x axis which is excited at resonance, and the detection mode along the y axis induced by the oriolis force due to a rotation along the z axis. The amplitude of the detection mode is proportional to the angular rate velocity. F k y ρ y The drive mode is excited by the voltage x at its resonance freuency ω x, and the charges on the electrodes of the detection mode y and y include the oriolis charges, the charges induced by capacitive coupling, and those induced by mechanical coupling: X X cos( ω xt ϕ) (e. 5) y ε ( Ω).sin( ω sin( ω M cos( ω (e. 6) y ε ( Ω).sin( ω sin( ω M cos( ω (e. 7) oriolis signal k x Ω x m o ρ x x Figure 1: mass and stiffness model of a vibrating microgyro. Ω is the angular velocity and Fx the drive force. apacitive coupling Mechanical coupling EDA Publishing/DTIP 27 ISN:

2 R. Levy 1, A.Dupret 1, H. Mathias 1, JP. Gilles 1, Serge Muller 2, runo Eisenbeis 2, S. Megherbi 1 NOISE AND THERMAL STAILITY OF IRATING MIROGYROMETERS PREAMPLIFIERS hal257677, version 1 2 Feb 28 The gyro needs an oscillator circuit to maintain the drive mode at resonance and a detection circuit to obtain a voltage proportional to the angular velocity Ω. A first stage of pres converts charges y and y into voltages. Then a differential stage follows to cancel the capacitive coupling which is in common mode on the two detection electrodes, and a demodulation stage is used to remove the mechanical coupling signal which is in phase uadrature with the oriolis signal. y x oscillator y Figure 2: The gyrometer's associated electronics 2.2. The pre s specifications A model of the microgyro including its mechanical part, piezoelectricity and the associated electronics has been developed for the gyrometer IG developed at ONERA [15]. Thanks to simulations performed with this model, it appears that the signal to noise ratio of the pre is proportional to the resolution of the gyro, and the gain stability of the pre limits the stability of the gyrometer s output. It is then important to develop the right pre adapted to the vibrating microgyrometer performing low noise and good gain stability. In the next section, five kinds of pres are presented and compared in terms of noise and thermal stability. 3. OMPARISON OF THE PREAMPLIFIERS The five pres are shown on figure 3: the current pre (a) that converts current i into voltage s with a feedback resistor, the charge (b) that converts charge into voltage s with a feedback capacitance, a feedback resistance is used to discharge the capacitance in low freuency and make the circuit stable, SF.Ω harge Substractor Demodulation s to cancel the stage to cancel capacitive pollution the mechanical pollution the voltage (c), the differential charge (d), and the switched capacitor ( S) (e), the switches are used to discharge the feedback capacitance in low freuency. Figure 3: the five kinds of pres: the current (a), the charge (b), the voltage (c), the differential charge (d), the switched capacitor charge (e) 3.1. Noise performance In order to compare the pres noise performances, we have calculated the charge input referred noise in / Hz: For the current : 4 kt 1 in en en R // R ω ω R ω o Where R is the feedback resistance, Ro o and ω are respectively the motional resistance, interelectrodes capacitance, and resonance freuency of the resonator. e n and i n are the voltage and current noise of the operational. For the voltage : R (a) R R (d) Ri 4kT R 1 in ω ω e n (b) R φreset φsense s φsense s e (e) (c) s G e φreset EDA Publishing/DTIP 27 ISN:

3 R. Levy 1, A.Dupret 1, H. Mathias 1, JP. Gilles 1, Serge Muller 2, runo Eisenbeis 2, S. Megherbi 1 NOISE AND THERMAL STAILITY OF IRATING MIROGYROMETERS PREAMPLIFIERS R is a resistance put in parallel with o to discharge o in low freuency to make the circuit stable. For the charge and the differential charge : The capacitance and resistance thermal stabilities are, for SMD components 3 ppm/. In order to increase the differential gain after the subtractor, the two feedback capacitances can be matched to have the same thermal variations. hal257677, version 1 2 Feb 28 4kT 1 in en ( R // Ro ω ω For the S, the largest noise is the kt/ noise due to charge injection of the switches. onsidering the gyrometer IG, o1pf, Ro1,5 MΩ, and ω2.1 5 rad, the OpAmp voltage noise is the largest noise contribution for the pres without switches. As 1 to obtain the same output R ω voltage s, the noises of the charge and current s are eual. The noise of the voltage is smaller Thermal stability The pre gain is set : by the s gain and the interelectrodes capacitance of the resonator for the voltage : G A major drawback of the voltage is its sensitivity to input parasitic capacitances p that makes it unstable over temperature whereas the charge and current s are not sensitive to input parasitic capacitances because their input voltage is put at ground. s G p by the feedback resistance for the current : ( s Rω ) Freuency drifts over temperature make the current pre drift. ) 3.3. Integration The current needs a high value resistance for the feedback resistor R R 1 MΩ. This resistance is too high to be integrated and would take too much space on the chip. For the charge s, the feedback resistor doesn t have to be neither stable nor linear because it is the feedback capacitance that sets the gain. It is then possible to use a transistor as a nonlinear resistance. This way the charge s can be integrated. The voltage can also be integrated. 3.4 ONLUSION The benefits and drawbacks of the five pres are shown on table 1. The charge pres are the most stable pres over temperature. The switch capacitor charge doesn t show a good signal to noise ratio because of charge injection. The differential has the same signal to noise ratio that the charge and has a better common mode rejection due to its differential architecture. Signal /noise ratio Thermal stability integration simplicity harge S charge Differential charge by the feedback capacitors for the charge s (charge, differential charge and S charge ): s urrent oltage We conclude that charge s are the best suited s to achieve gyrometers thermal stability. Table 1: enefits and drawbacks of the five pres EDA Publishing/DTIP 27 ISN:

4 R. Levy 1, A.Dupret 1, H. Mathias 1, JP. Gilles 1, Serge Muller 2, runo Eisenbeis 2, S. Megherbi 1 NOISE AND THERMAL STAILITY OF IRATING MIROGYROMETERS PREAMPLIFIERS hal257677, version 1 2 Feb 28 Without taking into account the simplicity of the design, the best suited pre for the microgyrometer IG is the differential charge. Experimental results for the charge are shown in the next section. In order to increase the thermal stability by components matching, an integrated circuit design of the differential charge is presented in section EXPERIMENTAL RESULTS WITH THE HARGE AMPLIFIER In order to validate the noise and thermal stability calculations, experiments are performed with the charge connected to a IG gyro. The output noise measured (figure 4) of 25 n/ Hz at 3kHz is in good agreement with the theoretical calculation. The output amplitude thermal drift (figure 5) of 1m in the temperature range from 4 to 8 is also in good agreement with the theoretical calculation. noise (/Hz) 1,E3 1,E4 1,E5 1,E6 1,E7 harge noise 3,E4; 2,71E8 1,E8 1,E2 1,E3 1,E4 1,E5 Freuency (Hz) Figure 4: charge output noise vs freuency output amplitude drift (v),379,3785,378,3775,377,3765,376,3755,375 output amplitude drift vs temperature Temperature ( ) Figure 5: charge output amplitude drift vs temperature 5. INTEGRATED PREAMPLIFIER DESIGN 5.1. Low noise design A specific design is developed to achieve low noise at the resonator freuency (3kHz for the IG); the input transistors have very big W/L ratios: W/L 22. The input referred noise obtained is: e n 5n/ Hz Low thermal drift design In order to obtain a low thermal drift, the two feedback capacitances that set the differential pre gain are matched. The biasing stage is a bandgap circuit designed to achieve a low thermal sensitivity in the temperature range from 4 to 8. Figure 6: layout of the integrated differential charge : on the left: the bandgap circuit, on the middle the low noise differential OpAmp, on the right: the two matched feedback capacitors. ONLUSION Five kinds of pres for microgyrometers have been presented and compared. The switched capacitor has the largest noise, the current and voltage s show gain drifts over temperature. The differential charge shows a good thermal stability and low noise In order to increase the differential charge thermal stability, its integration is finally presented with special care on noise reduction, and feedback capacitor matching. This integrated circuit has been developed and is being realized. The next step is to test the I with the IG gyro. REFERENES [1] O. Le Traon & al, The IA vibrating beam accelerometer: a new uartz micromachined sensor, Proceedings of the Freuency and Time Forum, vol. 2, pp , 1999 [2] M.Lemkin & al, a threeaxis micromachined accelerometer with a MOS positionsense interface and EDA Publishing/DTIP 27 ISN:

5 R. Levy 1, A.Dupret 1, H. Mathias 1, JP. Gilles 1, Serge Muller 2, runo Eisenbeis 2, S. Megherbi 1 NOISE AND THERMAL STAILITY OF IRATING MIROGYROMETERS PREAMPLIFIERS digital offsettrim electronics, IEEE J.of Solid State ircuits, vol. 34, No 4, pp , 1999 [3] F. Ayazi & al, A HARPSS polysilicon vibrating ring gyroscope, Journal of MEMS, vol.1 No 2, 21. [4] W. Geiger & al, The silicon angular rate sebnsor system DAED, sensors and actuactors 84, pp 28284, 2. [5] IlHan Hwang & al, Selfactuating biosensor using a piezoelectric cantilever and its optimization, Journal of Physics: conference series 34, pp , 26. hal257677, version 1 2 Feb 28 [6] DavidA Mendels & al, Dynamic propertie of AFM cantilevers and the calibration of their spring constants, Journal of Micromechanics and Microengineering, pp , 26. [7] R. Levy & al, A new analog oscillator electronics applied to a piezoelectric vibrating gyro, Proceedings of the Ultrasonics, Ferroelectrics and Freuency ontrol conference, 24 [8] D. Fang & al, A LowPower LowNoise apacitive Sensing Amplifier for Integrated MOSMEMS Inertial Sensors, Proceedings of the ircuits, Signals and Systems conference, 24 [9] M. Suster & al, Lownoise MOS integrated sensing electronics for capacitive MEMS strain sensors, Proceedings of the IEEE ustom Integrated ircuits onference, pp , 24. [1]. aliki Amini & al, Microgravity capacitive silicononinsulator accelerometers, Journal of Micromechanics and Microengineering No 15, pp , 25. [11] N. Yazdi & al, A LowPower Interface ircuit for apacitive Sensors, Proc. SolidState Sensors & Actuators Workshop, pp , [12] F.Ayazi & al, A harpss polysilicon vibrating ring gyroscope, Journal of microelectromechanical systems, vol. 1, n 2, 21. [13] D. Janiaud & al, The IG ibrating Integrated Gyrometer: a new uartz micromachined sensor, Symposium Gyro Technology, 23. [14] W.Geiger & al, The silicon angular rate sensor system DAED, sensors and actuators 84, pp 28284, 2. [15] R.Levy & al, Study and realization of an electronics adapted to the piezoelectric vibrating microgyrometer, PhD from university Paris 11, 21. EDA Publishing/DTIP 27 ISN: