DETECTION CIRCUIT USING A DIFFERENTIAL CAPACITIVE SENSOR WITH INPUT-COMMON-MODE CONTROL IN A SENSE INTERFACE

• US 20070163815A1
• Filed: 11/28/2006
• Published: 07/19/2007
• Est. Priority Date: 11/29/2005
• Status: Active Grant

• Current Assignee: STMicroelectronics SRL (STMicroelectronics NV)
• Original Assignee: STMicroelectronics SRL (STMicroelectronics NV)
• Inventors: Lasalandra, Ernesto, Ungaretti, Tommaso
• Granted Patent: US 9,201,090 B2
• Time in Patent Office: Days
• US Class Current: 178/18.60
• CPC Class Codes: G01P 15/125 by capacitive pick-up
• CPC Class Codes: G01P 15/131 with electrostatic counterbalancing means

Abstract

A detection circuit is provided with a differential capacitive sensor and with an interface circuit having a first sense input and a second sense input, electrically connected to the differential capacitive sensor. Provided in the interface circuit are: a sense amplifier connected at input to the first sense input and to the second sense input and supplying an output signal related to a capacitive unbalancing of the differential capacitive sensor; and a common-mode control circuit, connected to the first sense input and to the second sense input and configured to control a common-mode electrical quantity present on the first sense input and on the second sense input. The common-mode control circuit is of a totally passive type and is provided with a capacitive circuit, which is substantially identical to an equivalent electrical circuit of the differential capacitive sensor and is driven with a driving signal in phase opposition with respect to a read signal supplied to the differential capacitive sensor.

Claims

1. A detection circuit, comprising:

a differential capacitive sensor; and

an interface circuit having a first sense input and a second sense input electrically connected to said differential capacitive sensor, said interface circuit including;

a sense amplifier connected at input to said first sense input and said second sense input and configured to supply an output signal related to a capacitive unbalancing of said differential capacitive sensor; and

a totally passive first common-mode control circuit connected to said first sense input and said second sense input, and configured to control a common-mode electrical quantity present on said first sense input and said second sense input, wherein said first common-mode control circuit comprises generating means for generating a balancing electrical quantity such as to balance said common-mode electrical quantity;

said balancing electrical quantity being substantially equal in absolute value and opposite in sign to said common-mode electrical quantity.

2. The detection circuit according to claim 1, wherein said balancing electrical quantity is such as to maintain substantially constant a common-mode voltage present on said first sense input and said second sense input;

said common-mode electrical quantity being a common-mode amount of charge, and said balancing electrical quantity being an amount of charge equal in absolute value and opposite in sign to said common-mode amount of charge.

3. The detection circuit according to claim 1, wherein said differential capacitive sensor has an equivalent electrical circuit;

and said generating means comprises a capacitive circuit having a circuit configuration substantially identical to said equivalent electrical circuit, and having a driving terminal configured to receive a driving signal;

said driving signal having a control amplitude variation opposite to a read amplitude variation of a read signal supplied in use to said differential capacitive sensor.

4. The detection circuit according to claim 3, wherein said capacitive circuit is substantially identical to said equivalent electrical circuit, and said control amplitude variation of said driving signal is equal in absolute value, opposite in sign, and substantially simultaneous to said read amplitude variation of said read signal;

in particular, said driving signal and said read signal being signals in phase opposition.

5. The detection circuit according to claim 3, wherein said equivalent electrical circuit comprises a first and a second sense capacitor connected between a read terminal, configured to receive said read signal, and, respectively, said first sense input and said second sense input, and having a capacitance at rest in common and an unbalancing equal in absolute value and opposite in sign, and equal to said capacitive unbalancing;

said common-mode electrical quantity being generated by said first sense capacitor and said second sense capacitor in response to said read signal.

6. The detection circuit according to claim 5, wherein said differential capacitive sensor comprises a micro-electromechanical sensor having a fixed body and a mobile body, free to move with respect to said fixed body as a function of said quantity to be detected, thus generating said capacitive unbalancing;

said read terminal being electrically connected to said mobile body, and said first sense input and said second sense input being electrically connected to said fixed body.

7. The detection circuit according to claim 5, wherein said capacitive circuit comprises a first control capacitance connected between said driving terminal and said first sense input;

and a second control capacitance connected between said driving terminal and said second sense terminal;

said first control capacitance and said second control capacitance having a control capacitance value correlated to said capacitance at rest of said first sense capacitor and said second sense capacitor.

8. The detection circuit according to claim 7, wherein said control capacitance value is substantially equal to said capacitance at rest of said first sense capacitor and said second sense capacitor.

9. The detection circuit according to claim 1, further comprising an active second common-mode control circuit provided with a feedback loop;

said feedback loop being configured to generate a feedback voltage of a value such as to contribute to controlling said common-mode electrical quantity.

10. The detection circuit according to claim 9, wherein said feedback loop comprises:

feedback-amplifier means connected at input to said first sense terminal and second sense terminal and having an output configured to generate said feedback voltage; and

first and second feedback capacitances connected between said output and, respectively, said first sense input and said second sense input.

11. The detection circuit according to claim 10, wherein said differential sensor has an equivalent electrical circuit comprising sense-capacitor means having a capacitance at rest;

and said first and second feedback capacitances have a feedback capacitance which is smaller than said capacitance at rest.

12. The detection circuit according to claim 10, wherein said first common-mode control circuit comprises control-capacitor means having a control capacitance value that differs from said capacitance at rest by an implementation difference;

and wherein said first and second feedback capacitances have a feedback capacitance which is a function of said implementation difference.

13. The detection circuit according to claim 1, wherein said differential capacitive sensor and said interface circuit are integrated within a same die of semiconductor material.

14. An interface circuit, for a detection circuit that includes a differential capacitive sensor, the interface circuit comprising:

a first sense input and a second sense input configured for electrical connection to said differential capacitive sensor;

a sense amplifier connected at input to said first sense input and said second sense input and configured to supply an output signal related to a capacitive unbalancing of said differential capacitive sensor; and

a totally passive first common-mode control circuit connected to said first sense input and said second sense input, and configured to control a common-mode electrical quantity present on said first sense input and said second sense input, wherein said first common-mode control circuit comprises generating means for generating a balancing electrical quantity such as to balance said common-mode electrical quantity;

said balancing electrical quantity being substantially equal in absolute value and opposite in sign to said common-mode electrical quantity.

15. The interface circuit according to claim 14, wherein said balancing electrical quantity is such as to maintain substantially constant a common-mode voltage present on said first sense input and said second sense input;

said common-mode electrical quantity being a common-mode amount of charge, and said balancing electrical quantity being an amount of charge equal in absolute value and opposite in sign to said common-mode amount of charge.

16. The interface circuit according to claim 14, wherein said generating means comprises a capacitive circuit having a driving terminal configured to receive a driving signal having a control amplitude variation opposite to a read amplitude variation of a read signal supplied in use to said differential capacitive sensor.

17. The interface circuit according to claim 14, further comprising an active second common-mode control circuit provided with a feedback loop;

said feedback loop being configured to generate a feedback voltage of a value such as to contribute to controlling said common-mode electrical quantity.

18. The interface circuit according to claim 17, wherein said feedback loop comprises:

feedback-amplifier means connected at input to said first sense terminal and second sense terminal and having an output configured to generate said feedback voltage; and

first and second feedback capacitances connected between said output and, respectively, said first sense input and said second sense input.

19. An electronic device comprising:

a processor; and

a detection circuit coupled to the processor and including;

a differential capacitive sensor having an input and differential first and second outputs;

a first signal generator coupled to provide a read signal to the input of the differential capacitive sensor; and

an interface circuit having a first sense input and a second sense input electrically connected to the first and second outputs of said differential capacitive sensor, respectively, said interface circuit including;

a sense amplifier connected at input to said first sense input and said second sense input and configured to supply an output signal related to a capacitive unbalancing of said differential capacitive sensor;

a passive first common-mode control circuit connected to said first sense input and said second sense input, and configured to control a common-mode electrical quantity present on said first sense input and said second sense input, wherein said first common-mode control circuit comprises first and second control capacitances coupled to the first and second sense inputs; and

a second signal generator configured to provide a driving signal to the first and second control capacitances, the driving signal having a control amplitude variation opposite to a read amplitude variation of the read signal.

20. The electronic device according to claim 19, wherein the electronic device is portable and is one of a group consisting of:

a mobile phone, a digital audio player, a PDA, a digital camcorder or camera, and a portable computer.

21. The electronic device according to claim 19, wherein said first and second control capacitances are structured to produce a balancing electrical quantity that is such as to maintain substantially constant a common-mode voltage present on said first sense input and said second sense input;

said common-mode electrical quantity being a common-mode amount of charge, and said balancing electrical quantity being an amount of charge equal in absolute value and opposite in sign to said common-mode amount of charge.

22. The electronic device according to claim 19, wherein said differential capacitive sensor has an equivalent electrical circuit having a first and a second sense capacitor connected respectively between the input and said first sense input and said second sense input, and having a capacitance at rest in common, the first and second control capacitances having a capacitance substantially equal to the capacitance at rest.

23. The electronic device according to claim 19, further comprising an active second common-mode control circuit provided with a feedback loop;

said feedback loop being configured to generate a feedback voltage of a value such as to contribute to controlling said common-mode electrical quantity.

24. The electronic device according to claim 23, wherein said feedback loop comprises:

feedback-amplifier means connected at input to said first sense terminal and second sense terminal and having an output configured to generate said feedback voltage; and

first and second feedback capacitances connected between said output and, respectively, said first sense input and said second sense input.

25. A method for reading a differential capacitive sensor through an interface circuit provided with a first sense input and a second sense input electrically connected to said differential capacitive sensor, comprising:

providing a read signal to an input of the differential capacitive sensor;

sensing a capacitive unbalancing of said differential capacitive sensor;

generating an output signal related to said capacitive unbalancing; and

controlling, using a passive first common-mode control circuit, a common-mode electrical quantity present on said first sense input and said second sense input, the controlling including providing a drive signal to a capacitive circuit coupled to the first and second sense inputs, the drive signal having a control amplitude variation opposite to a read amplitude variation of the read signal.

26. The read method according to claim 25, wherein the controlling comprises generating a balancing electrical quantity such as to balance said common-mode electrical quantity and to maintain substantially constant a common-mode voltage present on said first sense input and said second sense input;

said common-mode electrical quantity being a common-mode amount of charge, and said balancing electrical quantity being an amount of charge equal in absolute value and opposite in sign to said common-mode amount of charge.

27. The read method according to claim 26, wherein said differential capacitive sensor has an equivalent electrical circuit, and said capacitive circuit has a circuit configuration substantially identical to said equivalent electrical circuit, the read signal having a read amplitude variation, and the driving signal having a control amplitude variation opposite to said read amplitude variation.

28. The read method according to claim 27, wherein said control amplitude variation of said driving signal is equal in absolute value and opposite in sign to said read amplitude variation.