Abstract:
A round-robin sensing device is disclosed. The round-robin sensing device comprises a MEMS device, wherein the MEMS device includes first and second sense electrodes. The round-robin sensing device also comprises a multiplexer coupled to the first and second sense electrodes, at least one sense amplifier coupled to the multiplexer, a demodulator coupled to the at least one sense amplifier, and an integrate and dump circuit coupled to the demodulator. Finally, the round-robin sensing device comprises an analog-to-digital converter (ADC) coupled to the de-multiplexer, wherein the multiplexer, the at least one sense amplifier and the demodulator provide a continuous time sense path during amplification that is resettable and wherein the integrate and dump circuit and the ADC provide a discrete time processing path.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
       [0001]    This application claims benefit under 35 USC 119(e) of U.S. Provisional Patent Application No. 61/780,371, filed on Mar. 13, 2013, entitled “ROUND-ROBIN GYROSCOPE,” which is incorporated herein by reference in its entirety. 
     
    
     FIELD OF THE INVENTION 
       [0002]    The present invention relates generally to sensors and more particularly to sensing of multi-axis sensors. 
       BACKGROUND 
       [0003]    MEMS sensing devices such as multi-axis gyroscope are utilized in a variety of applications. Normally each sensing axis has its own sensing circuit, meaning three copies of the same sensing circuit might be required for a three-axis sensor. This is particularly true of continuous-time sensing. The duplication of the sensing circuit leads to higher costs and sizes of the devices and there is a desire to reduce them by sharing the sensing circuit. The present invention addresses such a need. 
       SUMMARY 
       [0004]    A round-robin sensing device is disclosed. The round-robin sensing device comprises a MEMS device, wherein the MEMS device includes first and second sense electrodes. The round-robin sensing device also comprises a multiplexer coupled to the first and second sense electrodes, at least one sense amplifier coupled to the multiplexer, a demodulator coupled to the at least one sense amplifier, and an integrate and dump circuit coupled to the demodulator. Finally, the round-robin sensing device comprises an analog-to-digital converter (ADC) coupled to the de-multiplexer, wherein the multiplexer, the at least one sense amplifier and the demodulator provide a continuous time sense path during amplification that is resettable and wherein the integrate and dump circuit and the ADC provide a discrete time processing path when switching between the first and second sense electrodes. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0005]      FIG. 1   a  is a block diagram of a first embodiment of a round-robin multi-axis gyroscope. 
           [0006]      FIG. 1   b  is a block diagram of a second embodiment of a round-robin multi-axis gyroscope. 
           [0007]      FIG. 2  shows an embodiment of the timing diagram for the control signals of the round-robin gyroscope. 
           [0008]      FIG. 3  is a flow chart of a method of operation of the round-robin gyroscope in accordance with an embodiment. 
           [0009]      FIG. 4  shows an embodiment of a part of the round-robin gyroscope sense path including a MEMS device, a drive system, and a first sense amplifier with an offset cancellation circuit. 
       
    
    
     DETAILED DESCRIPTION 
       [0010]    The present invention relates generally to sensors and more particularly to round-robin sensing devices. The following description is presented to enable one of ordinary skill in the art to make and use the invention and is provided in the context of a patent application and its requirements. Various modifications to the preferred embodiments and the generic principles and features described herein will be readily apparent to those skilled in the art. Thus, the present invention is not intended to be limited to the embodiments shown, but is to be accorded the widest scope consistent with the principles and features described herein. 
         [0011]    In the described embodiment, round-robin method refers to the method where each axis of the gyroscope is sensed for a certain period of time. Round-robin multi-axis gyroscope refers to the gyroscope with multi-axis sensing and one sensing circuit, where each of the axes of the gyroscope is read out by the sensing circuit one at a time. By applying a round-robin method to a sensing device such as a three axis gyroscope the number of sense path components can be substantially reduced. In so doing, the overall chip size and cost is substantially reduced. To describe the features of the present invention in more detail refer now to the following description in conjunction with the accompanying figures. 
         [0012]      FIG. 1   a  is the block diagram of a first embodiment of round-robin multi-axis gyroscope  100   a  comprising a sensing device and sensing circuit operating with a continuous-time sensing framework. Although a gyroscope is described herein one of ordinary skill in the art readily recognizes that round-robin sensing device could be a variety of sensing devices. The round-robin multi-axis gyroscope  100   a  includes a DC voltage source  110 , a gyroscope MEMS device  120 , a drive system  105 , a multiplexer  115 , a first sense amplifier  130 , a second sense amplifier  140 , a mixer  150 , an integrate and dump circuit  160 , an analog-to-digital converter (ADC)  180 , and a de-multiplexer  190 . The gyroscope MEMS device  120  includes a proof mass  125 , a MEMS sense electrode for the first axis  121 , a MEMS sense electrode for the second axis  122 , a MEMS drive electrode  123 , and a MEMS drive-sense electrode  124 . 
         [0013]    The first sense amplifier  130  includes an operational amplifier  132 , a reset switch  133 , a feedback capacitor  134 , and a feed-through capacitor  135  for quadrature cancellation. The second sense amplifier  140  includes an input capacitor  141 , an operational amplifier  142 , a reset switch  143 , and a feedback capacitor  144 . The integrate and dump circuit  160  includes an input de-multiplexer  163 , an output multiplexer  164 , two integrators  161  and  162 , and each of them includes a resistor  171 / 176 , a reset switch  173 / 178 , a feedback capacitor  174 / 179 , and an operational amplifier  172 / 177 . 
         [0014]    MEMS proof mass  125  is biased at a high voltage by DC voltage source  110 . The drive system  105  receives input from MEMS drive-sense electrode  124  and drives gyroscope MEMS device  120  to oscillation at drive frequency through drive electrode  123 . Drive signal Vd from drive  105  feeds first sense amplifier  130  for quadrature cancellation through feed-through capacitor  135 . A clock signal LO is generated and sent to mixer  150  for sense signal demodulation. 
         [0015]    Round-robin multi-axis gyroscope  100   a  has two sensing periods. During the first sensing period, multiplexer  115  is configured to pick up the sense signal for the first axis and quadrature signal from sense electrode  121 ; during the second sensing period, multiplexer  115  is configured to pick up the sense signal for the second axis and quadrature signal from sense electrode  122 . The sense signals are amplitude modulated signals with carrier frequency equal to drive frequency. Quadrature signal is a sinusoidal signal with substantially fixed amplitude running at drive frequency. The phase of quadrature signal is in quadrature relationship with that of the sense signals. Quadrature signal is mostly cancelled by the drive signal Vd through the feed-through capacitor  135 , and what remains is called residual quadrature signal. The amplitude of quadrature signal may vary from axis to axis, and feed-through capacitor  135  may be configured differently during each sensing period for maximum quadrature cancellation. The sense signal and residual quadrature signal pass through first and second sense amplifiers  130  and  140  and are demodulated by mixer  150 . Clock signal LO runs at drive frequency and is substantially in-phase with the sense signal to maximize the gain of mixer  150  for the sense signal, and residual quadrature is substantially removed given its phase relationship to the sense signal and LO. The output of mixer  150  is processed by integrate and dump circuit  160  and converted to digital signal by ADC  180 . The ADC output OUT0 then passes through a de-multiplexer  190 . The sense signal for the first axis is de-multiplexed to OUT1 and the second axis is de-multiplexed to OUT2. 
         [0016]      FIG. 2  shows an embodiment of a timing diagram for the control signals of the round-robin multi-axis gyroscope. The operating period for each axis is one period of the sense signal. As drawn, quadrature signal is neglected. When signal axis is set to 0, the first axis is selected and sense electrode  121  is connected to first sense amplifier  130  through multiplexer  115 ; when signal axis is set to 1, the second axis is selected and sense electrode  122  is connected to the first sense amplifier  130  through multiplexer  115 . When signal rst1 is 1, reset switch  133  is enabled to reset first sense amplifier  130 . When signal rst2 is 1, reset switch  143  is enabled to reset second sense amplifier  140 . Second sense amplifier  140  is kept in reset for some duration after first sense amplifier  130  comes out of reset, and by this way the sampled offset and noise from first sense amplifier  130  when it comes out of reset is absorbed by input capacitor  141  of the second sense amplifier and will not propagate to the output. 
         [0017]    When signal axis is set to 0, de-multiplexer  163  is configured to pass input signal V3 to first integrator  161 . Reset switch  173  is enabled when signal intg is set to 0 and disabled when signal intg is set to 1, and input signal V3 will be integrated onto feedback capacitor  174 . Signal LO transitions from 0 to 1 substantially at the zero crossing of the sense signal for the first axis, and the transition also aligns substantially to the middle of the integration period. By doing so, (a) frequency components near DC which includes the sampled offset and noise from the first and second sense amplifiers  130 / 140  when the second sense amplifier comes out of reset, (b) frequency components near even harmonics of the drive frequency, and (c) residual quadrature signal, are averaged and nullified by the first integrator  161 . Multiplexer  164  is configured to pass output signal V5 from second integrator  162  to the ADC. Reset switch  178  is disabled over the whole period so that second integrator  162  holds the integrated value from the previous period. De-multiplexer  190  is configured to pass ADC output OUT0 to OUT2. 
         [0018]    When signal axis is set to 1, de-multiplexer  163  is configured to pass input signal V3 to second integrator  162 . Reset switch  178  is enabled when signal intg is set to 0 and disabled when signal intg is set to 1, and input signal V3 will be integrated onto the feedback capacitor  179 . Signal LO transitions from 0 to 1 substantially at the zero crossing of the sense signal for the first axis, and the transition also aligns substantially to the middle of the integration period. By doing so, (a) frequency components near DC which includes the sampled offset and noise from the first and second sense amplifiers  130 / 140  when the second sense amplifier comes out of reset, (b) frequency components near even harmonics of the drive frequency, and (c) residual quadrature signal, are averaged and nullified by first integrator  162 . Multiplexer  164  is configured to pass output signal V4 from first integrator  161  to the ADC. Reset switch  173  is disabled over the whole period to hold the integrated value from the previous period. De-multiplexer  190  is configured to pass ADC output OUT0 to OUT1. 
         [0019]    The control signals keep repeating and the sense signals from the first and second axes are processed in a round-robin fashion. The integration window dictated by signal intg and signal LO may shift in time from axis to axis for individual phase alignment. Although the integration window for each axis is less than the period of processing the signal for each axis and may shift in time, integrate and dump circuit  161  does hold the value for the whole period after each integration process. This enables the ADC to have a consistent sampling operation going from axis to axis. 
         [0020]    Although  FIG. 2  indicates that the operating period of each axis is one period of the sense signal, one can extend the idea and instead do half a period of the sense signal, two periods of the sense signal, etc. 
         [0021]    Although the embodiments describe 2-axis operation, other embodiments can provide 3-axis operation or more without departing from the scope and spirit as set forth. 
         [0022]    Although the embodiments describe MEMS signal multiplexing by using multiplexer  115 , signal multiplexing can be achieved in different ways without departing from the scope and spirit as set forth.  FIG. 1   b  shows an embodiment where first sense amplifier  130  is duplicated so that multiplexer  115  is not necessary. Sense amplifier  130   a  connects to MEMS electrode  121  and sense amplifier  130   b  connected to electrode  122 . The outputs of the sense amplifiers are then multiplexed at V1 by turning only one first sense amplifier on at a time. This embodiment may be advantageous by reducing the number and size of switches at the amplifier inputs. 
         [0023]    The second sense amplifier  140  is optional. If it is removed, the sampled offset and noise from the first sense amplifier  130  will still be nullified by the mixer. 
         [0024]    Although the embodiments describe the integrate and dump circuit  160  having two separate input resistors  171 / 176  after de-multiplexer  163 , one may instead have one resistor before the de-multiplexer  163  while still maintaining the same functionality. 
         [0025]    The round-robin gyroscope can be easily operated as a 1-axis gyroscope when multiplexer  115  is configured to only pick up signal from MEMS electrode  121  or  122  to first amplifier  130  and de-multiplexer  190  is configured to pass signal OUT0 only to OUT1 or OUT2. 
         [0026]      FIG. 3  shows a flow chart of a method of operation of the round-robin gyroscope in accordance with an embodiment. As is seen, a MEMS device is biased at a fixed voltage via step  301 . First and second sense signals are provided via step  302 . With reference to  FIG. 1   a , the first sense signal may refer to sense signal from first axis through MEMS electrode  121  and the second sense signal may refer to the sense signal from second axis through MEMS electrode  122 . Then a sense amplifier is reset, via step  304 . Thereafter, the first sense signal for amplification is selected by first sense amplifier, via step  306 . The selecting is concurrent with resetting the sense amplifier. 
         [0027]    Thereafter, amplification of the first sense signal is provided, via step  308 , upon which commencing a first sampled error is added to the first sense signal at the sense amplifier output. The first sampled error is contributed by the instantaneous first sense signal, quadrature signal, harmonic signals, MEMS noise, and circuit noise. Then the first sense signal and first sampled error are demodulated, via step  310 . The demodulated first sense signal and demodulated first sampled error are averaged so that the demodulated first sampled error is substantially cancelled, via step  312 . The averaged signal is digitized, via step  314 . 
         [0028]      FIG. 4  shows an alternative embodiment of the round-robin gyroscope sense path including the MEMS  120 , drive system  105 , and the first sense amplifier  130  with an offset cancellation circuit  410 . One problem for the first sense amplifier  130  during round-robin operation is that whenever reset switch  133  is opened, an offset can be sampled at the output V1 if the instantaneous input signal to amplifier  130  is not zero. Depending on the timing when reset switch  133  is opened, any of or any combination of sense signal, quadrature signal, and harmonics of the sense/quadrature signals may be sampled. 
         [0029]    The sampled offset can potentially cause two issues: (a) if the rate of reset is not aligned properly to the frequencies of the input signals, the sampled offset could change in time and generate a tone that degrades the signal-to-noise ratio; (b) the sampled offset consumes signal headroom and thereby limits the maximum measurable sense signal. 
         [0030]    Regarding issue (a), it is handled by making the reset rate synchronous to the input signal, i.e. reset rate is at drive frequency or sub-harmonic of the drive frequency. In so doing the sampled offset is fixed over time. 
         [0031]    Regarding issue (b), it is handled by offset cancellation circuit  410 . When amplifier  130  is in reset phase (reset switch  133  is closed), the voltage gain of the amplifier is very small and output voltage V1 is close to reference voltage Vref. Switch  412  is turned on at this time and a voltage (Voff−Vref) appears across trim capacitor  411  with total charge of Coff (Voff−Vref). At the moment reset switch  133  is opened, switch  412  is turned off and switch  413  turned on. 
         [0032]    Vref driving trim capacitor  411  causes the charge on trim capacitor  411  to transfer to amplifier feedback capacitor  134 , leading to a DC voltage offset at output V1 equal to −Coff (Voff−Vref)/Cfb. If the value of Voff and Coff are chosen properly, the DC offset can cancel the sampled offset due to a finite input signal at the moment the amplifier comes out of reset, thereby recovering the loss on signal headroom. Although voltages Voff and Vref are being used in offset cancellation circuit  410  as described above, other implementations are possible, e.g. using Voff1 and Voff2. 
         [0033]    Although offset cancellation circuit  410  described above is for single-ended amplifiers, the technique can be easily extended to differential amplifiers by adopting two sets of offset cancellation network: one for the positive, and the other for the negative side of the amplifier. Two bias voltages, e.g. Voff1 and Voff2, can be used to charge trim capacitors to positive and negative voltages. The offset cancellation polarity inversion can be achieved by dumping the charge on the trim capacitor to the opposite side of the amplifier, and Voff1/Voff2 can be of fixed values. 
         [0034]    Note that Voff, Voff1 and Voff2 can be either static voltages or periodic voltage signals as long as their values are consistent every time the reset switch is opened. Based on this argument, Voff, Voff1, and Voff2 can be tied to the drive signal Vd or its differential version Vd1 and Vd2 directly given their frequency is the same as the amplifier reset rate, and the moment the reset switch is opened is close to the peak of the drive signal Vd/Vd1/Vd2. By implementing this way, a voltage buffer for providing Voff or voltage buffers for Voff1 and Voff2 can be eliminated. 
         [0035]    The described embodiments merge two (for 2-axis operation) or three (for 3-axis operation) sense paths into a single sense path which in turn reduces the circuit area and lowers the power consumption when compared to traditional continuous-time sensing gyroscope architectures. 
         [0036]    Compared to a traditional discrete-time gyroscope sensing architecture, this method pertains to the continuous-time circuit scheme and provides (a) a smaller noise bandwidth; (b) compatibility with the use of fixed high-voltage proof-mass biasing to enable larger sense signal; (c) simpler quadrature cancellation scheme as implemented by a feed-through capacitor. For the same power consumption, the described embodiments have better noise performance over discrete-time gyroscope, or for the same noise performance, lower power consumption. 
         [0037]    Although the present invention has been described in accordance with the embodiments shown, one of ordinary skill in the art will readily recognize that there could be variations to the embodiments and those variations would be within the spirit and scope of the present invention. Accordingly, many modifications may be made by one of ordinary skill in the art without departing from the spirit and scope of the present invention.