WO2010056236A1

WO2010056236A1 - Charge amplifiers with dc stabilization

Info

Publication number: WO2010056236A1
Application number: PCT/US2008/083202
Authority: WO
Inventors: Deyou Fang; Dejan Mijuskovic
Original assignee: Freescale Semiconductor Inc.
Priority date: 2008-11-12
Filing date: 2008-11-12
Publication date: 2010-05-20
Also published as: CN102210098B; CN102210098A

Abstract

An integrated charge amplifier circuit [100] for use with a MEMS gyroscope [20] or other device provides an improved low cut-off frequency with a DC stabilization circuit [130] while allowing large leakage current at the sensing node. The charge amplifier circuit includes a first amplifier [110] having a first amplifier input [112] and a first amplifier output [114]. A first capacitor [162] is coupled in parallel to the first amplifier, and the DC stabilization circuit is coupled in parallel to the first amplifier and in parallel to the first capacitor. The DC stabilization circuit includes an integrator [140] having an integrator input coupled to the first amplifier output, and an integrator output. An attenuator [150] having an attenuator input and output is coupled to the integrator output, and a first resistor [132] is coupled between the attenuator output, and the first amplifier input.

Description

CHARGE AMPLIFIERS WITH DC STABILIZATION

TECHNICAL FIELD

[0001] Embodiments of the subject matter described herein relate generally to charge amplifiers such as those used with capacitive accelerometers, gyroscopes and other devices utilizing capacitive sensing mechanisms. More particularly, various embodiments relate to charge amplifier circuits with a DC stabilization feed-back loop.

BACKGROUND

[0002] "Charge amplifiers" are any devices, circuits or systems capable of receiving an input charge and driving an output voltage based on the input charge. In most applications the input charge is not a suitable signal for use in other circuits such as measurement, processing, logic or information circuits. Conventional charge amplifiers, therefore drive an output voltage to be used as a signal by other circuits based on the input charge. Charge amplifier circuits and systems are used in many different applications. Vehicle airbag triggering devices, for example, use capacitive accelerometers that commonly use charge amplifier systems to convert the charge output from the accelerometer to an output voltage. The output voltage is used to determine the conditions for triggering the airbag. Other applications that use charge amplifiers include Micro-Electro-Mechanical Systems (MEMS) such as MEMS gyroscopes and MEMS accelerometers with charge amplifiers. Digital cameras and game console remotes, for example, may use charge amplifiers with MEMS gyroscopes and accelerometers to measure movement.

[0003] Generally, charge amplifier systems include an operational amplifier with a capacitive feed-back loop. The input to a charge amplifier can experience "leakage currents" that are approximated as current flows between the input and ground (or other reference voltage). In some situations, the leakage current can cause DC offset at the charge amplifier output, which can undesirably cause the amplifier to drive into saturation. In a conventional charge amplifier, a resistor is typically used as a DC feed-back path in parallel to a capacitive feed-back loop to provide for the leakage current and to stabilize the DC offset value of the charge amplifier. Large resistors are typically implemented in conventional charge amplifiers as discrete components separate from the charge amplifiers, which are typically implemented on a chip. Implementing the resistor as a discrete component is a disadvantage in costs and size.

[0004] Also, the large resistor is generally used in conventional amplifiers to prevent AC from feeding back through the DC stabilization path and thereby introducing an unwanted signal at the input of the charge amplifier. The DC stabilization path therefore has a cut-off frequency determined at least in part by the resistance of the DC path. [0005] In a conventional charge amplifier, the cut-off frequency in a DC feedback loop is dependent on the value of the resistor in the DC feedback loop as well as the value of the capacitor in the capacitive feedback loop. As the values of the resistor and capacitor increase, the cut-off frequency becomes lower. In terms of transfer functions, the resistor and capacitor in the feedback paths determine a pole frequency in the frequency response of the charge amplifier; these paths therefore influence the cut-off frequency in the DC feedback loop. As the size of the resistor becomes large, it can be increasingly difficult to supply leakage current through the resistor implemented on a chip. Further, a large capacitance in the capacitive feedback path can undesirably cause a decrease in the signal level at the output of the charge amplifier, resulting in a low signal-to-noise ratio (SNR) at the charge amplifier output.

[0006] One conventional charge amplifier uses an active low-pass filter in the DC feedback loop to prevent the unwanted AC signal in the DC stabilization path. The resistor supplying the leakage current is implemented on chip, but this still requires a relatively large capacitor and a relatively large resistor to achieve a desirably low cut-off frequency. Therefore the conventional charge amplifier can provide a relatively low output signal level when implemented to have a low cut-off frequency.

[0007] Accordingly, it is desirable to provide a charge amplifier circuit with a DC stabilization path that has a low cut-off frequency and that also provides suitable current for the leakage current while providing a suitable output signal level and obtaining a high signal- to-noise ratio at the charge amplifier output. Other desirable features and characteristics will become apparent from the subsequent detailed description and the appended claims, taken in conjunction with the accompanying drawings and the foregoing technical field and background.

BRIEF DESCRIPTION OF THE DRAWINGS

[0008] A more complete understanding of the subject matter may be derived by referring to the detailed description and claims when considered in conjunction with the following figures, wherein like reference numbers refer to similar elements throughout the figures. [0009] FIG. 1 is a block diagram showing an exemplary MEMS sensor system including a charge amplifier;

[0010] FIG. 2 is a block diagram showing an exemplary DC stabilization path in a charge amplifier;

[0011] FIG. 3 is a circuit diagram of an exemplary charge amplifier circuit; and [0012] FIG. 4 is a flow chart of an exemplary method of operating a charge amplifier in a device;

DETAILED DESCRIPTION

[0013] The following detailed description is merely illustrative in nature and is not intended to limit the embodiments of the subject matter or the application and uses of such embodiments.

[0014] According to various embodiments, systems, circuits, and methods are described for improving the characteristics of a DC stabilization path in a charge amplifier circuit. [0015] Unlike conventional charge amplifiers, an exemplary embodiment provides an integrator in the DC stabilization path of a charge amplifier to create a second order DC stabilization circuit that may further include a low pole in the frequency response analysis as discussed in further detail below. The DC stabilization path of many embodiments provides a lower cut-off frequency through the DC stabilization path than conventional charge amplifiers implemented on chip. Many embodiment also providing leakage current and a suitable output signal level. In the exemplary embodiment, a DC stabilization path in a charge amplifier includes a first resistance to supply leakage current and a second resistance implemented in an integrator to provide the low cut-off frequency. The value of the first resistor supplying leakage current can be smaller than in conventional charge amplifiers, thereby allowing for implementation on chip in many embodiments. The second resistor does not need to supply leakage current in most embodiments, and can therefore be selected together with a capacitor in the integrator according to the desired cut-off frequency. Unlike the components in feedback paths of conventional charge amplifiers, the components implemented in the integrator of many embodiments do not undesirably affect the output signal level of the charge amplifier. Therefore, in many embodiments, a charge amplifier is implemented on chip including a resistor in the DC stabilization path to supply leakage current, and an integrator implemented in the DC stabilization path to provide a desirably low cut-off frequency in the DC stabilization path. Further, in addition to the above achievements, most embodiments provide suitable signal levels at the output of the charge amplifier.

[0016] Details of several exemplary embodiments will now be presented with particular reference to the drawing figures.

[0017] FIG. 1 is a block diagram of a sensor system 10 with feedback control. The sensor system of the exemplary embodiment has a MEMS sensor 20 with a MEMS input 22 and a MEMS output 24. A feedback loop 12 may include a charge amplifier 100 and an automatic gain control 30. In an exemplary embodiment the charge amplifier includes a first amplifier 110 as shown in FIG. 2, including a first amplifier output 114. [0018] Sensor system 10 is any electronic system that senses an electronic phenomenon and has an output based on that phenomenon. In the example embodiment of FIG. 1, a sensor system 10 has a feedback loop 12. A MEMS gyroscope, for example, may have a feedback loop 12 to maintain an oscillating mass in the MEMS gyroscope. In alternative embodiments, sensor system 10 may not have a feedback loop 12. Photo detectors, CCD imaging devices, fiber optic receivers, and accelerometers, for example, may be used in a sensor system 10 without feedback loop 12. For the purpose of discussion, an exemplary sensor system 10 including a MEMS gyroscope will now be discussed, although other systems 10 could be equivalently applied.

[0019] In exemplary applications where charge amplifiers are used, for example, with certain MEMS gyroscopes or accelerometers 20, a charge amplifier 100 has an input charge Q with a carrier frequency. The carrier frequency is any suitable frequency, and in an exemplary embodiment is at a frequency selected from within a range of about 2 kHz to about 20 kHz. In one embodiment the MEMS gyroscope has a carrier frequency of about 12 kHz. For a charge amplifier operating with a carrier signal input in this range, as well as other ranges, it has been identified that lowering a low pass cut-off frequency in the DC stabilization path is desirable.

[0020] An exemplary MEMS sensor system 10 with a MEMS gyroscope will now be discussed in connection with one embodiment. In one embodiment, MEMS sensor 20 is a MEMS gyroscope with a MEMS output 24. A MEMS gyroscope includes an oscillating mass oscillating at a frequency, for example about 12 kHz. The MEMS gyroscope measures the change in the rotation of the oscillating mass in the form of a charge "Q". MEMS output 24, in many embodiments, is a charge Q that is not typically suitable for direct use in logic circuits and other circuits. MEMS output 24 is therefore coupled to charge amplifier 100 to be converted to a voltage signal that is useful in other circuits. In the exemplary embodiment, charge amplifier 100 drives a voltage signal at the frequency of input charge Q, for example about 12 kHz, at a first amplifier output 114. Charge variations in the input carrier signal generally represent rotations of the gyroscope, although charge variations may also represent imperfections in the many embodiments. Charge Q, including variations in the signal, can be converted to a suitable voltage signal by charge amplifier 100. [0021] With charge Q from MEMS gyroscope 20 converted to a suitable voltage signal in exemplary MEMS sensor system 10, the voltage signal can be used in feedback loop 12 to drive the MEMS gyroscope oscillator. In most embodiments the voltage signal is used as an input to other circuits such as a logic or control circuit. Amplifier output 114 of charge amplifier 100 can be electrically coupled to an automatic gain control 30 as part of feedback loop 12. The voltage signal from charge amplifier 100 is therefore amplified to an appropriate level by automatic gain control 30. Automatic gain control 30 is coupled to MEMS input 22 to drive the oscillator at a desired frequency, for example about 12 kHz. The variations in the voltage signal used for feedback may be used to correct for the imperfections in the MEMS gyroscope. The variations may also be filtered or otherwise modified and then used to drive the oscillator in the MEMS gyroscope. An exemplary embodiment of charge amplifier 100 is discussed below as shown in FIG. 2. [0022] FIG. 2 shows a charge amplifier including a second feedback loop 130. A DC stabilization circuit may be part or all of second feedback loop 130 of the exemplary embodiment. In alternative embodiments other components or devices may be included in second feedback loop 130. Charge amplifier 100 receives a signal in the form of charge Q and may have a first amplifier 110 with a first amplifier input 112. Charge amplifier 100 suitably includes first amplifier output 114, which is the node for the output of the charge amplifier V_out. Charge amplifier 100 may have a first capacitor 162 in a first feedback loop 160. Second feedback loop 130 in the exemplary embodiment includes an integrator 140, an attenuator 150 and a first resistor 132. First amplifier 110 may be a differential amplifier with a negative differential input 115 and a positive differential input 117. Positive differential input 117 may be connected to a first reference voltage V_ren 119, such as the analog ground of a circuit. Other reference voltages may be used for first reference voltage Vr_en 119, including for example, a negative side of the output from a MEMS gyroscope, or a common ground. As previously discussed, leakage current at the first amplifier input 112 may be represented as a current flowing to electrical ground, and is shown by way of illustration as W 116 in FIG. 2.

[0023] Second feedback loop 130 in the exemplary embodiment of FIG. 2 may also be called a DC stabilization path. Second feedback loop 130 or DC stabilization path of an exemplary embodiment includes a DC stabilization circuit coupled between first amplifier output 114 and first amplifier input 112, and in parallel to first feedback loop 160. Second feedback loop 130 includes an integrator 140 and an attenuator 150, as appropriate. [0024] Integrator 140 in DC stabilization circuit or second feedback loop 130 may be implemented in any manner. In one embodiment, a resistor and capacitor implemented with integrator 140 provide a low pole in the frequency response analysis for the low pass cut-off frequency. Integrator 140 may also be implemented to include a feed-forward capacitor that provides a stabilizing zero in the frequency response analysis for the DC stabilization circuit. [0025] Integrator 140 is any device or component that accumulates an electrical input voltage. Integrator 140 suitably includes an integrator input 142 and an integrator output 144. In one exemplary embodiment, integrator 140 produces an integrated voltage at integrator output 144.

[0026] Integrator output 144 in the exemplary embodiment may be coupled to an attenuator input 152. Attenuator 150 is any device or component that attenuates an electrical voltage signal. Attenuator 150 of the exemplary embodiment can attenuate the integrated voltage, and suitably includes an attenuator output 154. Attenuator output 154 may be coupled to first resistor 132.

[0027] In one exemplary embodiment, as discussed previously, MEMS gyroscope 20 may operate at a carrier frequency, for example about 12 kHz. An exemplary charge amplifier 100 produces an output voltage signal V_out, modulated at the carrier frequency. With second feedback loop 130 coupled between amplifier output 114 and amplifier input 112, the output signal at the carrier frequency can interfere with the input signal if transmitted through feedback loop 130. Therefore, in many embodiments, it is desirable for the second feedback loop 130 to attenuate higher frequencies. Second feedback loop 130 in an exemplary embodiment, for example, has a suitable cut-off frequency in the range of about 100 Hz to about 1 kHz.

[0028] In one exemplary embodiment second feedback loop 130 is a second order DC stabilization circuit in that second feedback loop 130 may implement a circuit that has a second order (or higher) transfer function with two (or more) poles in the frequency response. In the exemplary embodiment the lower pole determines the cut-off frequency for the DC stabilization path, and is related to a first set of components in the DC stabilization path. A second set of components may determine the gain of the charge amplifier 100. The lower pole in the DC stabilization circuit may be decoupled from the second set of components to provide a relatively low cut-off frequency determined by the first set of components, and the gain function of charge amplifier 100 can be determined by a separate set of components. In one exemplary embodiment of FIG. 2, a second feedback loop 130 is a second order DC stabilization circuit with integrator 140. In other embodiments second feedback loop 130 may be a circuit with a higher order transfer function and more poles in the frequency response. By using a second (or higher) order filter, the separation between the two resistors used in the DC feedback path can be maintained or improved. Further, the higher order filter typically provides more effective filtering at frequencies above the cutoff frequency, thereby improving filter effectiveness as well.

[0029] Charge amplifier 100 in the exemplary embodiment of FIG. 2 may include an integrator 140. Integrator 140 may be configured to provide a low cut-off frequency in the DC feedback path that is substantially independent of the values of first resistor 132 and first capacitor 162. In terms of transfer functions, integrator 140 in an exemplary embodiment is configured to provide the lowest frequency pole and to therefore determine the low frequency response of the charge amplifier. First resistor 132 may then be selected at a suitable value and implemented on chip while supplying leakage current Ii_eak 116. Similarly, first capacitor 162 may be selected at a value for a suitable output signal level. In the exemplary embodiment, first resistor 132 and first capacitor 162 provide a high pole in the frequency response analysis of exemplary charge amplifier 100. Further details of various implementations of integrator 140 are discussed below in connection with FIG. 3. [0030] FIG. 3 shows a more detailed view of an exemplary embodiment of charge amplifier 100. In the exemplary embodiment of FIG. 3, second feedback loop 130 includes a low pass filter 120 coupled between first amplifier output 114 and integrator 140. Details of an exemplary embodiment implementing integrator 140 are shown including a second amplifier 141, a second resistor 134, an integrator feedback loop 149 with an integrator feedback capacitor 136, an integrator feed-forward capacitive loop 146 with a feed-forward capacitor 138 and a third amplifier 148 with a gain Ai.

[0031] First amplifier 110 and second amplifier 141 may be any amplifier used to amplify a voltage. First amplifier 110 and second amplifier 141 are shown in the exemplary embodiments as relatively high gain operational amplifiers. Amplifiers 110 and 141 are differential amplifiers in many embodiments, but may also be implemented as fully differential amplifiers or other types of amplifiers in other embodiments. [0032] Second amplifier 141 is shown in the exemplary embodiment of FIG. 3 as a differential amplifier including a negative differential input 143, a positive differential input 145, and a second amplifier output 147. Positive differential input 145 may be connected to an integrator reference voltage V_ref2 139. Integrator reference voltage V_reG 139 in the exemplary embodiment is an electrical ground reference separate from first reference voltage Vrefi 1 19. In alternative embodiments, integrator reference voltage V_reπ 139 may be other reference voltages, such as, a common ground with first reference voltage V_reπ 1 19, or a positive or negative voltage value or signal.

[0033] As discussed in connection with FIG. 2, integrator 140 of one exemplary embodiment may be implemented to produce a low pole in the frequency response analysis. Integrator 140 may be implemented, for example, with a relatively high value second resistor 134 and a relatively high value integrator feedback capacitor 136. In one exemplary embodiment, a feed-forward loop 146 is implemented in integrator 140 to establish a stabilizing zero in the frequency response, and therefore allow the exemplary embodiment to operate in a stable manner. Other methods of stabilizing a DC feedback loop in a charge amplifier may be used in other embodiments.

[0034] In one embodiment the low-pass filter 120 has a negligible effect of the frequency response of the DC stabilization loop. Using conventional electrical engineering principals, it can be readily shown that the open loop transfer function of the DC stabilization loop in the circuit shown in FIG. 3 at the frequency range around the unit-gain frequency is as follows:

Where "s" is the complex frequency, Ri, R₂, Ci, C₂, and C3, are component values of corresponding first resistor 132 (Ri), second resistor 134 (R₂), first capacitor 162 (Ci), feedback capacitor 136 (C₂), and feed-forward capacitor 138 (C₃). Ai and A₂ of the above transfer function are gain values of corresponding feed-forward amplifier 148 (Al) and attenuator 150 (1/A₂). For one embodiment such as the DC loop shown in FIG. 3, A₂ has a negative sign because the loop is a negative feedback loop. Generally, the open loop transfer function predicts the stability of the system in terms of frequency response. In considering the exemplary circuit shown in FIG. 3 with the above transfer function, the RiCi term provides a pole in the frequency analysis, and the A₂R₂C₂ term provides another pole. Depending on the frequency, range of operation, and the location of the poles, a zero may be useful to provide stability, such as a zero provided in the above equation by the AiR₂C₃ term. In the exemplary circuit, integrator feed-forward loop 146 including feed-forward amplifier 148 and feed-forward capacitor 138 determines the location of the zero in the frequency analysis. In an alternative embodiment, feed-forward loop 146 may be implemented without third amplifier 148. When an exemplary embodiment is implemented with feed-forward amplifier 148, however, feed-forward capacitor 138 may be implemented with a smaller value to achieve an equivalent zero in the frequency analysis.

[0035] In the exemplary embodiment of FIG. 3, first resistor 132 may be implemented as a smaller resistor than second resistor 134. As second resistor 134 and feedback capacitor 136 provide the lowest frequency pole in the exemplary embodiment and establish the low cut-off frequency, a larger resistor value for second resistor 134 and a larger capacitor value for second capacitor 136 may result in a lower pole. Larger resistors, however, are generally more difficult to implement on chip when the resistor needs to supply a significant amount of current, such as may be suitable to supply leakage current Ii_eak 116. In the exemplary embodiment, current for Ii_eak 116, however, does not need to flow through second resistor 134. As a result, a larger resistance for second resistor 134 may be accomplished on chip. Further, in many embodiments a suitably sized resistor for first resistor 132 may be implemented on chip, for example using one or more controlled long-length MOSFETs, with first resistor 132 supplying leakage current Ii_eak 116. For one exemplary technique using controlled long-length MOSFETs, please see the reference: Geen et al., "Single-Chip Surface Micromachined Integrated Gyroscope With 50° /h Allan Deviation," IEEE Journal of Solid- State Circuits vol. 37 No. 12 Dec. '02 Pages: 1860 - 1866. Other techniques may be used in other embodiments to implement first resistor 132.

[0036] One specific exemplary embodiment will now be discussed to clarify exemplary features of various embodiments. In one exemplary embodiment, second resistor 134 may be implemented on chip using one or more controlled long-length MOSFETs, for example, as a resistor of about 50MΩ or less. Integrator feedback capacitor 136 may be implemented as a capacitor of about lOpF. Attenuator 150 may be implemented with an inverse gain of 1/A₂, where A₂ is selected within a range of about 10 to 20. In this exemplary embodiment, the A₂R₂C₂ term provides a low pole around 10 Hz to 30 Hz in the frequency analysis. First resistor 132 may be implemented on chip as a resistor of about 20MΩ to 40MΩ, and first capacitor 162 may be a capacitor of about IpF. In the exemplary embodiment, the RiCi term therefore provides a higher frequency pole. The lowest frequency pole to achieve suitable cut-off frequency characteristics may be provided by the components of integrator 140. Further, first resistor 132 may supply leakage current Ii_eak 116, for example about 2nA of current, while implemented on chip. The size of first capacitor 162 determines the gain of the charge amplifier, and therefore the magnitude of output signal V_out- With a smaller first capacitor 162, such as a IpF capacitor in an exemplary embodiment, a larger output signal may be obtained. For example, with a 12 kHz input charge from an exemplary MEMS gyroscope, about a 1 Vpp signal can be achieved at first amplifier output 114 as V_out. In the exemplary embodiment shown in FIG. 3, low pass filter 120 is shown coupled between first amplifier output 114 and integrator 140. Second feedback loop 130 may be implemented without low pass filter 120. Low pass filter 120 may, however, attenuate the signal at the carrier frequency of V_out to reduce the swing requirement for implemented components of integrator 140 such as second resistor 134.

[0037] FIG. 4 shows an exemplary method 400 of operating charge amplifier 100. Method 400 may be implemented in hardware, software, firmware, or any combination thereof, including any combination of digital and analog circuitry. In various embodiments, method 400 may be implemented with the circuitry and components as discussed above in connection with FIGS. 2 and 3, although alternative embodiments may implement some or all of the steps of method 400 using other devices, circuitry, code, and/or logic. [0038] Generally speaking, method 400 involves the broad steps of receiving an input charge (step 410), driving an output voltage (step 420), filtering the output voltage (step 430), integrating the output voltage (step 440), attenuating the integrated output voltage (step 450), and feeding back the attenuated voltage through a resistor (step 460). MEMS sensor 20 is shown in FIG. 4 to further facilitate discussion of exemplary method 400. [0039] Beginning with step 410, charge amplifier 100 receives an input charge in any manner. As discussed above, input charge Q may be from a MEMS sensor 20, or from other components or devices. In an exemplary embodiment input charge Q is a signal with a carrier frequency as discussed in exemplary embodiments above. Alternatively, input charge Q may be a single charge, or intermittent charges of varying positive and/or negative values. In an exemplary embodiment, input charge Q is received at a first amplifier input 112 as shown in FIG. 2.

[0040] In step 420 driving an output voltage V_out is performed by any component and/or device designed to drive output voltage V_out- The inputs to the component or device driving output voltage V_out may include the input charge received in step 410 and feedback voltage from step 460. Other inputs may be used as inputs to drive output voltage V_out- Output voltage V₀Ut in the exemplary method 400 is used to drive the MEMS sensor 20, which may be an oscillating MEMS gyroscope. In alternative embodiments, output voltage V_out may be used to drive a separate device or display, and/or to provide an input signal to a separate system. The output voltage may be further modified as appropriate. Output voltage V_out in the exemplary method 400 is also used in a feedback loop such as a DC stabilization loop including the steps of filtering the output voltage 430, integrating the output voltage 440, attenuating the integrated output voltage 450 and feeding back the attenuated voltage through a resistor 460.

[0041] In step 430 output voltage V_out is filtered in any electrical manner. In an exemplary embodiment of method 400, step 430 is implemented with a low pass filter that attenuates higher frequency components of V_out- In alternate embodiments of method 400, step 430 may be performed in conjunction with other steps. Integrating the output voltage step 440, for example, may be combined with filtering the output voltage step 430. [0042] In step 440, integrating is performed by any component or device capable of integrating a voltage and providing an integrated output voltage. In an exemplary embodiment, integrator 140 of FIG. 2 may be an analog integrator implemented as part of a circuit. In an exemplary embodiment the integrated voltage is substantially a DC voltage without significant AC components. Alternatively, the integrated voltage may have substantial AC and/or DC components. In one embodiment as discussed above, integrator 140 is implemented including integrator feed-forward capacitive path 146, shown in FIG. 3. [0043] In step 450, attenuation is performed by any component or device capable of attenuating a voltage. In an exemplary embodiment, attenuator 150 is coupled to the output of integrator 140 as shown in FIG. 2. After attenuating the integrated output voltage 450, the attenuated voltage may be a level suitable for feeding back through a resistor. In alternative embodiments, step 450 of attenuating may be performed as part of another step. Step 440 of integrating the output voltage, for example, may be performed with an integrator that also attenuates the voltage, and/or provides an output voltage suitable for feeding back through a resistor without further attenuation.

[0044] In step 460, feeding back is performed by any method of feeding back a voltage in a charge amplifier. In an example embodiment, the attenuated integrated output voltage from step 450 may be fed back through a resistor as a feedback input to drive the output voltage in step 420. The resistor may be a resistor implemented as part of an integrated circuit on chip such as first resistor 132 in the exemplary embodiment of FIG. 2. In alternative embodiments, the resistor may be other types or forms of resistors, and may be implemented in various ways. Step 460 may include other functions as part of a feedback loop before feeding the voltage as an input to drive the output voltage in step 420. [0045] Various systems, techniques and structures have therefore been described to improve the characteristics of charge amplifiers including the cut-off frequency in the DC stabilization path. In exemplary embodiments, an integrator in the DC feedback path of the charge amplifier lowers the cut-off frequency thereby improving stability in the charge amplifier. Further, in exemplary embodiments, leakage current can be supplied with a resistor implemented on chip as part of an integrated circuit, thereby reducing the overall cost of implementing a charge amplifier.

[0046] Systems, devices, and methods may therefore be configured in accordance with many different exemplary embodiments. In one embodiment a charge amplifier circuit is provided comprising a first amplifier [110] having a first amplifier input [112] and a first amplifier output [114], a first capacitor [162] coupled in parallel to the first amplifier, and a DC stabilization circuit [130] coupled in parallel to the first amplifier and coupled in parallel to the first capacitor. The DC stabilization circuit comprises an integrator [140] comprising an integrator input [142] coupled to the first amplifier output, and an integrator output [144], an attenuator [150] comprising an attenuator input [152] coupled to the integrator output, and an attenuator output [154], and a first resistor [132] coupled between the attenuator output, and the first amplifier input.

[0047] This charge amplifier circuit may be modified, enhanced, and/or more fully defined in many different ways. For example the DC stabilization circuit may comprise a low pass filter [120] coupled between the first amplifier output and the integrator input. The integrator may comprise a second amplifier [141] having a positive differential input [143] coupled to an integrator reference voltage [139] and a negative differential input [145], and a second amplifier output [147]. The integrator may also have a second resistor [134] coupled between the integrator input and the negative differential input of the second amplifier. The charge amplifier circuit may be produced as an integrated circuit. The DC stabilization circuit may be a second order DC stabilization circuit having a low pole in the frequency response, and wherein the first resistor is decoupled from the low pole. The integrator may comprise a feed-forward capacitor [138] coupled to the integrator input and to the negative differential input in parallel to the second resistor. The integrator may also comprise an integrator feedback capacitor [136] coupled between the negative differential input of the second amplifier and the second amplifier output. The charge amplifier circuit may be produced as an integrated circuit with the first resistor and the second resistor produced as controlled long-length MOSFETs. The integrator may comprise a third amplifier [148] coupled in series between the input of the integrator and the feed-forward capacitor. The third amplifier may have an input coupled to the integrator input and a third amplifier output coupled to the feed-forward capacitor. The first capacitor may have a first capacitance (Ci) and the first resistor may have a first resistance (Ri), and the integrator feedback capacitor may have a second capacitance (C₂), and the second resistor may have a second resistance (R₂) and the feed-forward capacitor may have a third capacitance (C3), and the third amplifier may have a gain (Ai) and the attenuator may have an attenuator inverse gain of (1/A₂), and the first amplifier input may receive a signal having a complex frequency (s), wherein the DC stabilization loop of the charge amplifier circuit has an open loop transfer function (T_open- ϊoop(s)) at a frequency range around a unit-gain frequency as:

The attenuator inverse gain and the second resistance (R₂) and the second capacitance (C₂) may each have values such that the open loop transfer function has an associated pole frequency below 50Hz.

[0048] In still other embodiments, a method of operating a charge amplifier circuit is provided with the amplifier circuit having a circuit input [112] and a circuit output [114]. An output voltage is driven [420] at the circuit output that is based upon an input charge. The output voltage is integrated [440] over time to create an integrated output voltage. The integrated output voltage is attenuated [450] to create an attenuated voltage. The attenuated voltage is fed back [460] to the circuit input though a feedback resistor. [0049] This basic technique may be modified, enhanced, and/or more fully defined in many ways. For example, the output voltage may be filtered [430] using a low-pass filter prior to integrate the output voltage. The step of integrating may be performed using an integrator [140] having a feed-forward path [146] with a feed-forward capacitor [138]. The input charge may be provided by a MEMS sensing device [20]. The MEMS sensing device may be a MEMS gyroscope.

[0050] While at least one exemplary embodiment has been presented in the foregoing detailed description, it should be appreciated that a vast number of variations exist. It should also be appreciated that the exemplary embodiment or embodiments described herein are not intended to limit the scope, applicability, or configuration of the claimed subject matter in any way. Rather, the foregoing detailed description will provide those skilled in the art with a convenient road map for implementing the described embodiment or embodiments. It should be understood that various changes can be made in the function and arrangement of elements without departing from the scope defined by the claims, which includes known equivalents and foreseeable equivalents at the time of filing this patent application. [0051] As used herein, the word "exemplary" means "serving as an example, instance, or illustration", rather than as a "model" that would be exactly duplicated. Any implementation described herein as exemplary is not necessarily to be construed as preferred or advantageous over other implementations. Furthermore, there is no intention to be bound by any expressed or implied theory presented in the preceding technical field, background, brief summary or detailed description.

[0052] As used herein, the term "substantially" is intended to encompass any slight variations due to design or manufacturing imperfections, device or component tolerances, environmental effects and/or other factors. The term "substantially" also allows for variation from a perfect or ideal case due to parasitic effects, noise, and other practical considerations that may be present in an actual implementation. [0053] The foregoing description may refer to elements or nodes or features being "connected" or "coupled" together. As used herein, unless expressly stated otherwise, "connected" means that one element/node/feature is electrically, mechanically, logically or otherwise directly joined to (or directly communicates with) another element/node/feature. Likewise, unless expressly stated otherwise, "coupled" means that one element/node/feature may be mechanically, electrically, logically or otherwise joined to another element/node/feature in either a direct or indirect manner to permit interaction even though the two features may not be directly connected. That is, "coupled" is intended to encompass both direct and indirect joining of elements or other features, including connection with one or more intervening elements.

[0054] In addition, certain terminology may also be used in the following description for the purpose of reference only, and thus are not intended to be limiting. For example, the terms "first", "second" and other such numerical terms referring to structures or elements do not imply a sequence or order unless clearly indicated by the context. [0055] For the sake of brevity, conventional techniques related to charge amplifier design, MEMS gyroscopes and accelerometers, automatic gain control, and other functional aspects of the systems (and the individual operating components of the systems) may not be described in detail herein. Furthermore, the connecting lines shown in the various figures contained herein are intended to represent exemplary functional relationships and/or physical couplings between the various elements. It should be noted that many alternative or additional functional relationships or physical connections may be present in an embodiment of the subject matter. Further, the components of the various embodiments may be implemented on a chip using any number of transistors or other non-linear gain elements.

Claims

CLAIMS What is claimed is:

1. A charge amplifier circuit [100] comprising: a first amplifier [110] having a first amplifier input [112] and a first amplifier output [114];

a first capacitor [162] coupled in parallel to the first amplifier;

a DC stabilization circuit [130] coupled in parallel to the first amplifier and coupled in parallel to the first capacitor, the DC stabilization circuit comprising: an integrator [140] comprising an integrator input [142] coupled to the first amplifier output, and an integrator output [144]; an attenuator [150] comprising an attenuator input [152] coupled to the integrator output, and an attenuator output [154]; and a first resistor [132] coupled between the attenuator output, and the first amplifier input.

2. A charge amplifier circuit according to claim 1 wherein the DC stabilization circuit further comprises a low pass filter [120] coupled between the first amplifier output and the integrator input.

3. A charge amplifier circuit according to claim 1 wherein the integrator comprises: a second amplifier [141] having a positive differential input [143] coupled to an integrator reference voltage [139] and a negative differential input [145], and a second amplifier output [147]; a second resistor [134] coupled between the integrator input and the negative differential input of the second amplifier.

4. A charge amplifier circuit according to claim 1 wherein the charge amplifier circuit is produced as an integrated circuit.

5. A charge amplifier according to claim 1 wherein the DC stabilization circuit is a second order

DC stabilization circuit having a low pole in the frequency response, and wherein the first resistor is decoupled from the low pole.

6. A charge amplifier circuit according to claim 3 wherein the integrator further comprises: a feed-forward capacitor [138] coupled to the integrator input and to the negative differential input in parallel to the second resistor; an integrator feedback capacitor [136] coupled between the negative differential input of the second amplifier and the second amplifier output.

7. A charge amplifier circuit according to claim 3 wherein the charge amplifier circuit is produced as an integrated circuit, and wherein the first resistor and the second resistor are controlled long-length MOSFETs.

8. The charge amplifier of claim 6 wherein the integrator comprises a third amplifier [148] coupled in series between the input of the integrator and the feed-forward capacitor, the third amplifier having an input coupled to the integrator input and a third amplifier output coupled to the feed-forward capacitor.

9. The charge amplifier of claim 8 wherein the first capacitor has a first capacitance (Ci) and the first resistor has a first resistance (Ri), and the integrator feedback capacitor has a second capacitance (C₂), and the second resistor has a second resistance (R₂) and the feed-forward capacitor has a third capacitance (C3), and the third amplifier has a gain (Ai) and the attenuator has an attenuator inverse gain of (1/A₂), and the first amplifier input receiving a signal having a complex frequency (s), wherein the DC stabilization loop of the charge amplifier circuit has an open loop transfer function (T_open-io_op(s)) at a frequency range around the unit-gain frequency as:

τ open- jloop ^(s) * d^{+ A}i^{R2c3 g} )

10. A charge amplifier circuit according to claim 9 wherein the attenuator inverse gain and the second resistance (R₂) and the second capacitance (C₂) each have values such that the open loop transfer function has an associated pole frequency below 50Hz.

11. A method of operating a charge amplifier circuit having a circuit input [112] and a circuit output [114], the method comprising: driving an output voltage [420] at the circuit output that is based upon an input charge; integrating the output voltage [440] over time to create an integrated output voltage; attenuating the integrated output voltage [450] to create an attenuated voltage; feeding back the attenuated voltage [460] to the circuit input though a feedback resistor.

12. A method of operating a charge amplifier according to claim 11 further comprising filtering the output voltage [430] using a low-pass filter prior to integrate the output voltage.

13. A method of operating a charge amplifier according to claim 1 1 wherein the step of integrating is performed using an integrator [140] having a feed-forward path [146] with a feed-forward capacitor [138].

14. A method of operating a charge amplifier according to claim 11 wherein the input charge is provided by a MEMS sensing device [20].

15. A method of operating a charge amplifier according to claim 14 wherein the MEMS sensing device is a MEMS gyroscope.