Self-testing capacitive pressure transducer and method
First Claim
1. A self-testable micromachined transducer apparatus for measuring at least one externally applied condition, the transducer apparatus including at least one solid-state capacitive pressure sensor, the transducer apparatus comprising:
- a rigid support substrate;
a solid-state micromachined pressure sensor, formed at least in part on the support substrate and having a flexible member and a capacitor adjustable by varying pressure applied to the flexible member; and
an electronic read-out circuit, electrically coupled to the solid-state pressure sensor, the read-out circuit including;
(1) a solid-state operational amplifier for producing an analog output signal corresponding to a charge applied to the adjustable capacitor, the amplifier having first and second differential input terminals, and an output terminal upon which the analog output signal is placed, with the first input terminal connected to the adjustable capacitor,(2) a reference capacitor connected to the first input terminal of the operational amplifier,(3) an integrating capacitor connected to the first input terminal and the output terminal of the operational amplifier,(4) means for providing a reset signal to the read-out circuit, the reset signal having a precharge portion and an integration portion which establishes an integration period during which the analog output signal is produced;
(5) means for applying at least one clock signal which alternates between two states substantially in synchronization with transitions between the precharge portion and the integration portion of the reset signal, to the adjustable capacitor and to the reference capacitor, with the beginning of the integrating portion coinciding with the beginning of the integration period established by the reset signal, whereby an output signal is generated on the output terminal of the operational amplifier during the integrating portion of the reset signal; and
(6) means for selectively extending at least one precharge portion of the reset signal so as to cause the flexible member to deflect to a maximum steady-state deflection immediately prior to an integration portion of the reset signal, wherein the means for selectively extending causes the transducer apparatus to be self-tested by determining whether the value of the analog output signal from the operational amplifier includes an electrostatic deflection component consistent with predetermined expectations about same.
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Accused Products
Abstract
A high-performance switched-capacitor circuit for electronic read-out of a pressure sensor-based ultrasensitive microflow transducer. The microflow transducer uses a differential capacitive pressure sensor to measure flow. Read-out electronics associated with the transducer feature a clocking speed of 100 KHz and drive loads up to 35 pF. The read-out electronics include a high DC gain that nulls out stray input capacitance, which is beneficial for the multichip realization of the microflow transducer disclosed herein. The uncompensated linearity of the overall read-out electronics is 10 bits, and the pressure/flow resolution is 12 bits. An ultrasensitive membrane associated with the pressure sensor does not respond to a pulsed waveform for frequencies above 50 KHz. But for lower frequencies, it deflects in response to the time-average voltage applied across the capacitor plates of the pressure sensor. A self-test mode is provided which employs an extremely long pre-charge pulse. An integrated flowmeter package including transducer and circuitry, and a second circuit for extending the range of the microflowmeter circuitry are also disclosed.
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Citations
12 Claims
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1. A self-testable micromachined transducer apparatus for measuring at least one externally applied condition, the transducer apparatus including at least one solid-state capacitive pressure sensor, the transducer apparatus comprising:
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a rigid support substrate; a solid-state micromachined pressure sensor, formed at least in part on the support substrate and having a flexible member and a capacitor adjustable by varying pressure applied to the flexible member; and an electronic read-out circuit, electrically coupled to the solid-state pressure sensor, the read-out circuit including; (1) a solid-state operational amplifier for producing an analog output signal corresponding to a charge applied to the adjustable capacitor, the amplifier having first and second differential input terminals, and an output terminal upon which the analog output signal is placed, with the first input terminal connected to the adjustable capacitor, (2) a reference capacitor connected to the first input terminal of the operational amplifier, (3) an integrating capacitor connected to the first input terminal and the output terminal of the operational amplifier, (4) means for providing a reset signal to the read-out circuit, the reset signal having a precharge portion and an integration portion which establishes an integration period during which the analog output signal is produced; (5) means for applying at least one clock signal which alternates between two states substantially in synchronization with transitions between the precharge portion and the integration portion of the reset signal, to the adjustable capacitor and to the reference capacitor, with the beginning of the integrating portion coinciding with the beginning of the integration period established by the reset signal, whereby an output signal is generated on the output terminal of the operational amplifier during the integrating portion of the reset signal; and (6) means for selectively extending at least one precharge portion of the reset signal so as to cause the flexible member to deflect to a maximum steady-state deflection immediately prior to an integration portion of the reset signal, wherein the means for selectively extending causes the transducer apparatus to be self-tested by determining whether the value of the analog output signal from the operational amplifier includes an electrostatic deflection component consistent with predetermined expectations about same. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8, 9)
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10. A self-testable electronic system located in immediate proximity to a miniature transducer including a solid-state capacitive pressure sensor having an adjustable capacitor with generally parallel electrically conductive plates, the capacitor being responsive to an external force, the electronic system providing electronic read-out signals corresponding to the forces sensed by the adjustable capacitor, the electronic system comprising:
a read-out circuit for reading a parameter of the adjustable capacitor correlatable to the magnitude of an external force applied to the capacitor, the read-out circuit including, (1) a solid-state amplifier for producing an output signal corresponding to a charge applied to the adjustable capacitor, the amplifier having at least a first input terminal and an output terminal upon which the output signal is placed, with the first terminal being adapted for connection to the adjustable capacitor, (2) an integrating capacitor connected to the first input terminal and the output terminal of the operational amplifier, (3) means for providing a reset signal to the read-out circuit to establish the beginning of an integration cycle, said reset signal including a precharge portion and an integration portion, (4) means for applying synchronized clock input signals in synchronization with the reset signal, and (5) means for testing the system by applying a predetermined electrostatically generated force to the adjustable capacitor, said means for testing the system extending at least one precharge portion of the reset signal so as to cause a maximum steady-state electrostatic condition on the adjustable capacitor, wherein the means for testing the system determines whether the value of the output signal from the solid-state amplifier after the steady-state electrostatic condition is placed on the capacitor is consistent with predetermined expectations. - View Dependent Claims (11)
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12. A method of operating a solid state transducer which employs a capacitive pressure sensor and a switched capacitor electronic read-out circuit, the capacitive pressure sensor including at least one adjustable capacitor responsive to at least one variable external condition, the method comprising the steps of:
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(a) obtaining successive read-outs with each such read-out corresponding to a value of the external conditions then sensed by the capacitor, by applying repetitively a waveform having a frequency above a first pre-determined frequency to the adjustable capacitor of the capacitive pressure transducer; (b) generating by use of an electric signal an electrostatic force upon the capacitor of the pressure sensor by selectively holding the waveform in a pre-determined state for a length of time exceeding the normal period of the cycle of the waveform by at least an order of magnitude so as to cause a maximum steady-state condition on the capacitor; and (c) after releasing the waveform from the pre-determined state, immediately obtaining a read-out from the waveform to obtain an output signal including a component due to the electrostatic force generated during the last step.
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Specification