Abstract:
In a circuit for use with a micromachined device having a movable mass that forms an inner electrode of a differential capacitor, oppositely phased square waves are applied to two outer electrodes of the differential capacitor. A reset voltage is applied to the inner electrode synchronously with the square waves to stabilize and control the potential on the inner electrode. The signal on the inner electrode is demodulated by sampling during a first half of the square wave and a second half of the square wave between applications of the reset pulse to obtain a voltage that does not contain noise due to the reset switch.

Description:
FIELD OF THE INVENTION 
     This invention relates to circuitry for use with a micromachined device. 
     BACKGROUND OF THE INVENTION 
     Sensors and actuators, such as accelerometers for use in automobiles, can be formed as micromachined silicon structures. Examples of a micromachined accelerometer and of a process for fabricating such an accelerometer are provided in U.S. Pat. Nos. 5,345,824 and 5,326,726, which are incorporated herein by reference. As described in these patents, a mass is suspended in a plane over a surface and is movable along a sensitive axis. The mass has fingers that extend away from the mass in a direction transverse to the sensitive axis. Each of these fingers is between a pair of stationary fingers to form pairs of capacitors that make up a differential capacitor. 
     In some prior devices, equal and oppositely phased 1 MHz sine waves or square waves are applied to the stationary fingers. When the movable mass is not moved by an external force, it is centered between the stationary fingers and no signal appears on the mass; but when an external force is applied along the sensitive axis to cause the mass to move relative to the stationary fingers, the capacitances of the two capacitors change and the mass provides a signal indicating the acceleration. The signal is provided to a buffer, a synchronous switching demodulator, and an output amplifier. 
     For the movable mass to provide a signal proportional to the magnitude of the acceleration, it must not be tied to ground or to another constant voltage. But if the mass is left entirely floating, stray charges can accumulate on the mass and change its potential until the potential is high enough with respect to the underlying surface to cause it to be electrostatically pulled down to the surface. In the incorporated patent, to maintain the potential of the movable beam, the mass is connected into a feedback loop through a large resistor, such as 3 Mohms. While such a connection maintains the potential of the beam, the resistor adds noise that can adversely affect resolution. 
     SUMMARY OF THE INVENTION 
     The present invention includes a circuit and method for a device having one or more differential capacitors including a movable electrode between fixed electrodes. The fixed electrodes receive equal and opposite periodic signals of period T. The movable electrode receives a DC voltage through a reset switch that is activated synchronously with the periodic signals. Activation of the reset switch resets the DC voltage on the movable beam. 
     In preferred embodiments, the periodic signals are square waves of equal frequency and approximately equal amplitude, but 180° out of phase; the reset switch includes a transistor; and the DC voltage is the arithmetic average of the maximum and minimum voltages of the square waves. The reset switch is closed for a short duration relative to the period of the square wave, and has a period of T/2 or nT, with n a natural number. In each case, the pulse is preferably closed shortly after the square waves transition. The reset switch thus stabilizes the potential of the movable electrode. 
     The signal from the movable beam may be amplified and provided to a discrete time demodulator that demodulates the amplified signal and provides an output signal. The demodulator is controlled so that it samples during different half waves between activations of the reset switch to obtain a magnitude that indicates the amount of displacement of the mass, while subtracting noise that occurs when the switch opens. The demodulator thus samples so that noise sampled by opening the reset switch is subtracted out. 
     Other features and advantages will become apparent from the following detailed description, the drawings, and the claims. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS 
     FIG. 1 is a plan view of a portion of a known micromachined sensor. 
     FIG. 2 is a partial block, partial schematic circuit diagram of an embodiment of the present invention. 
     FIG. 3 is a timing diagram, illustrating the voltages at various nodes of the circuit of FIG. 2. 
     FIG. 4 is a timing diagram, illustrating the effects of noise on the voltages at various points of the circuit of FIG. 2. 
     FIG. 5 is a timing diagram illustrating reset and sampling timing in an alternative embodiment. 
    
    
     DETAILED DESCRIPTION 
     Referring to FIG. 1, the present invention is preferably used with a surface micromachined accelerometer as generally described in the incorporated patents, but can be used with differential capacitors and with micromachined differential capacitors generally. In a known example of such a device, a mass 18 is suspended in an x-y plane above and parallel to an underlying substrate 22, and is tethered to be movable along a sensitive x-axis in response to an external force applied along that axis. Mass 18 has a central beam 20 elongated along the x-axis and a plurality of movable fingers 24 extending away from either side of center beam 20 along the y-axis. Each finger 24 is centered between a first stationary finger 26 and a second stationary finger 28. 
     Referring also to FIG. 2, each group of fingers 24, 26, and 28 forms a differential capacitor 40 with stationary fingers 26 and 28 serving as outer electrodes 36 and 38, and finger 24 serving as a movable inner electrode 34. Electrodes 34 and 36 form a first capacitor 42, and electrodes 34 and 38 form a second capacitor 44. Movement of mass 18 in a positive direction along the x-axis increases the capacitance of second capacitor 44 and decreases the capacitance of first capacitor 42, while movement in a negative direction along the x-axis decreases the capacitance of second capacitor 44 and increases the capacitance of first capacitor 42. 
     Referring to FIGS. 2 and 3, electrodes 36 and 38 are coupled to a signal generator 56 that provides square waves 52 and 54 to respective electrodes 36 and 38. Square waves 52 and 54 each have a period T and have equal amplitude, but each is 180 out of phase relative to the other. The square waves preferably alternate between a reference voltage V REF  and ground (GND). 
     If no external force causes mass 18 to move along the x-axis, no net signal should appear on electrode 34. If, in response to an external force, mass 18 is displaced so that fingers 24 are closer to stationary fingers 26 than to stationary fingers 28, the capacitance of capacitor 42 increases and the capacitance of capacitor 44 decreases. As a result, the signal on inner electrode 34 and on a node 60 is a square wave that follows square wave 52 with a peak-to-peak voltage that is related to the displacement of mass 18. Similarly, if mass 18 is displaced so that fingers 24 are closer to stationary fingers 28 than to stationary fingers 26, the signal on inner electrode 34 on node 60 is a square wave that follows square wave 54. 
     Node 60 is coupled to a first port 63 of a switch 64, which can be implemented as a transistor. A second port 65 of switch 64 is connected to a reset voltage source 67 which receives a reference voltage V REF  from generator 56 and provides a DC reset voltage V RESET  that preferably is the arithmetic average of the minimum and maximum voltages of square waves 52 and 54, i.e., for square waves alternating between V REF  and GND, V RESET  =V REF  /2. 
     Square waves 52, 54 and pulse signal 57, which periodically closes reset switch 64, are shown in FIG. 3. Rows 1 and 2 show square waves 52, 54, each of which alternates between about V REF  and GND with a period T, and row 3 shows an embodiment of pulse signal 57. When signal 57 is high, reset switch 64 closes to provide V RESET  to node 60 and thus to inner electrode 34. The pulses of signal 57 are short relative to T, are synchronous with square waves 52 and 54, and occur shortly after the square waves transition. 
     In this exemplary embodiment, the pulses are provided with a period T, although they can have a period of T/2 or nT. If square wave 52 goes high at t=0, a reset pulse occurs at 0&lt;t1&lt;T/2. Assuming no noise, the amplitude of the signal at node 60 (row 4) alternates between V RESET  and V RESET  +V D , with V D  being a voltage indicating the displacement of electrode 34. When there is no displacement, the signal at node 60 is a constant V RESET . The beam&#39;s potential is thus periodically returned to V RESET  by operation of reset switch 64. 
     Referring again to FIG. 2, node 60 is also coupled to an AC amplifier 62 that provides an output signal 70 to a discrete time demodulator 68. Demodulator 68 translates the carrier-based signal to baseband by sampling the amplified output, preferably once during each half cycle of square waves 52 and 54, regardless of the period of reset pulse signal 57. The output of demodulator 68 may be coupled to a low-pass filter 80, which substantially blocks signals at and above the frequency of square waves 52, 54. 
     Referring also to FIG. 3, when there is a square wave signal at node 60, amplifier 62 amplifies this signal to increase the magnitude so that the signal alternates between voltages V1 and V2 as shown as signal 70 in row 5. Signal 70 is affected by noise, particularly kT/C noise, due to the opening of reset switch 64. 
     Referring to FIGS. 3 and 4, by sampling each half cycle, noise signals that are the same during the half cycles are effectively subtracted out. Rows 6 and 7 of FIG. 3 show demodulator sampling signals 58 and 59 for enabling sampling by demodulator 68. As shown in row 6, first demodulator signal 58 has a period T and its pulses occur at time t2 after each reset pulse, i.e., t1&lt;t2&lt;T/2. The delay between times t1 and t2 is sufficient to permit the signals to stabilize. As shown in row 7, second signal 59 preferably has a period T, and provides a pulse at time t3. Signal 59 preferably also has a phase difference of T/2 relative to signal 58, i.e., T/2&lt;t3&lt;T, and t3=t2+T/2. 
     Referring to FIG. 4, when reset switch 64 is closed, the signal at node 60 is forced to V RESET . When reset switch 64 is opened, the signal at node 60 jumps to a new level (V RESET  +V A ), with V A  representing voltage added by a charge step and kT/C noise. This voltage level is read by demodulator 68 at time t2. The square waves then transition at time T/2, and the signal at node 60 increases to V RESET  +V D  +V A , with V D  being the displacement voltage and with V A  unaffected by the transition at T/2. When demodulator 68 takes its second reading at time t3, the resulting difference is V D , which is what the signal would have been in the absence of such noise. If during the next cycle, the effect of the noise is V B , a voltage different from V A , the readings remain accurate because V B  is again constant for the two demodulator readings, and again the demodulator output is V D . 
     To more precisely maintain the DC potential, the reset switch could be pulsed synchronously with signals 52, 54 every half-cycle, i.e., with a period of T/2. Such switching would not enable the rejection of noise, such as kT/C noise, however, because the errors added by these effects would change between the time that the first half wave is sampled and the time that the second half wave is sampled. 
     Referring to FIG. 5, the reset switch also could alternatively be pulsed every two or more cycles or, more generally, at nT. Here, n is a natural number that is sufficiently small so that the potential of the movable beam cannot deviate too much from the desired potential. As shown in FIG. 5, with n=2, resetting is done at times t1, t1+2T, t1+4T, etc., and more generally at t1, t1+nT, t1+2nT, etc. Regardless of the period of the reset switch, however, the activation should be synchronous with the periodic signals. 
     As also indicated in FIG. 5, while the demodulator preferably samples every half cycle, it could sample less frequently if the reset switch closes every nT, with n&gt;1. 
     To measure the signal, however, samples are taken during any is first half cycle and any second half cycle, although these samples need not be during the same cycle. As shown in FIG. 5, a first sample is taken at time t2a and a second sample is taken at time t3a, with t3a preferably at t2a+3T/2. Accordingly, to eliminate noise due to reset switch resistance, the demodulator should sample at a high value and at a low value of the square wave between closings of the reset switch. 
     While there have been shown and described examples of the present invention, it will be readily apparent to those skilled in the art that various changes and modifications may be made therein without departing from the scope of the invention as defined by the appended claims. While the signal generator has been shown as a single element, it can include devices working together.