Signal conditioning methods and circuits for a capacitive sensing integrated tire pressure sensor
First Claim
1. A tire pressure monitoring system comprising a switched capacitor circuit, wherein said switched capacitor circuit comprises:
- a. a clock, wherein said clock comprises two non-overlapping clock phases that control a state of analog switches of said switched capacitor circuit, whereas said phases comprise phase I and phase II, whereby said switched capacitor circuit operates at frequencies of said clock;
b. tire pressure sensor MEMS capacitors, wherein said MEMS capacitors comprise at least one pair of sense capacitors, whereby said sense capacitor pair is measured differentially;
c. a capacitance-to-voltage converter, wherein said capacitance to voltage converter is connected to said MEMS sense capacitor;
d. a sigma-delta converter, wherein said sigma-delta converter comprises a comparator having a first digital output state and a second digital output state, wherein said first output state comprises a sum of reference voltages and said second output state comprises a difference of said reference voltages, whereas when in said first output state a first capacitor of said MEMS is charged to said first output state on said phase II and a second capacitor of said MEMS is charged to said second output state on said phase I and when in said second output state said first capacitor is charge to said second output state on said phase II and said second capacitor is charged to said first output state on said phase I, whereby an average of said output states is determined and provided to said capacitance-to-voltage converter, whereas an average value of said capacitance-to-voltage converter output is driven to a zero value and a digital output is provided of said average output states that is equal to a difference between said MEMS capacitors divided by their sum multiplied by a ratio of said reference voltages.
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Abstract
A tire pressure monitoring system is provided that includes a switched capacitor circuit having a clock with two non-overlapping clock phases that control a state of analog switches of the switched capacitor circuit. The system uses tire pressure sensor MEMS capacitors that are measured differentially. A capacitance-to-voltage converter is connected to the MEMS sense capacitor, and a sigma-delta converter having a comparator with a first digital output state and a second digital output state is used. The first output state is a sum of reference voltages and the second output state is a difference of the reference voltages. An average value of the capacitance-to-voltage converter output is driven to a zero value and a digital output is provided of the average output states that is equal to a difference between the MEMS capacitors divided by their sum multiplied by a ratio of the reference voltages.
63 Citations
9 Claims
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1. A tire pressure monitoring system comprising a switched capacitor circuit, wherein said switched capacitor circuit comprises:
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a. a clock, wherein said clock comprises two non-overlapping clock phases that control a state of analog switches of said switched capacitor circuit, whereas said phases comprise phase I and phase II, whereby said switched capacitor circuit operates at frequencies of said clock; b. tire pressure sensor MEMS capacitors, wherein said MEMS capacitors comprise at least one pair of sense capacitors, whereby said sense capacitor pair is measured differentially; c. a capacitance-to-voltage converter, wherein said capacitance to voltage converter is connected to said MEMS sense capacitor; d. a sigma-delta converter, wherein said sigma-delta converter comprises a comparator having a first digital output state and a second digital output state, wherein said first output state comprises a sum of reference voltages and said second output state comprises a difference of said reference voltages, whereas when in said first output state a first capacitor of said MEMS is charged to said first output state on said phase II and a second capacitor of said MEMS is charged to said second output state on said phase I and when in said second output state said first capacitor is charge to said second output state on said phase II and said second capacitor is charged to said first output state on said phase I, whereby an average of said output states is determined and provided to said capacitance-to-voltage converter, whereas an average value of said capacitance-to-voltage converter output is driven to a zero value and a digital output is provided of said average output states that is equal to a difference between said MEMS capacitors divided by their sum multiplied by a ratio of said reference voltages. - View Dependent Claims (2, 3, 4, 5, 6)
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7. A tire pressure monitoring system comprising a switched capacitor circuit, wherein said switched capacitor circuit comprises:
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a. a clock, wherein said clock comprises two non-overlapping clock phases that control a state of analog switches of said switched capacitor circuit, whereas said phases comprise phase I and phase II, whereby said switched capacitor circuit operates at frequencies of said clock; b. MEMS capacitors, wherein said MEMS capacitors comprise at least one pair of sense capacitors, whereby said sense capacitor pair is measured differentially; c. an analog to digital converter; d. a multiplexer comprising a plurality of multiplexer inputs and outputs, wherein a portion of said multiplexer inputs are connected to sense terminals of said MEMS capacitors; e. a pair of test capacitors connected in parallel with said MEMS capacitors to said multiplexer inputs; f. a difference amplifier wherein inputs of said differential amplifier is connected to outputs of said multiplexer, whereas outputs of said differential amplifier is connected to inputs of said analog to digital converter; g. a common mode correction amplifier connected to said multiplexer outputs and said multiplexer outputs are connected to said difference amplifier input; and h. a sum amplifier, wherein an input terminal of said sum amplifier is connected to an output of said multiplexer and an output terminal of said sum amplifier is connected to a reference input of said analog to digital converter. - View Dependent Claims (8, 9)
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Specification