Capacitance detection circuit including voltage compensation function

• US 7,652,486 B2
• Filed: 01/17/2008
• Issued: 01/26/2010
• Est. Priority Date: 01/17/2008
• Status: Expired due to Fees

• Current Assignee: NXP USA, Inc. (NXP Semiconductors NV)
• Original Assignee: Freescale Semiconductor, Inc. (NXP Semiconductors NV)
• Inventors: Shikata, Eiji
• Primary Examiner(s): Dole; Timothy J
• Assistant Examiner(s): HOQUE, FARHANA AKHTER
• Application Number:
  US12/015,515
• Publication Number:
  US 20090184723A1
• Time in Patent Office: 740 Days
• Field of Search: 324658-690
• US Class Current: 324/661
• CPC Class Codes: G01P 15/125 by capacitive pick-up
• CPC Class Codes: G01R 27/2605 Measuring capacitance capacitive sensors G01D5/24

Abstract

A capacitance detection circuit that compensates for the fluctuation of a reference voltage with a simple structure. A C-V circuit for a sensor element generates a detection signal by amplifying a capacitance change value of the sensor element by a predetermined gain based on a reference voltage. A voltage compensation circuit, which is connected to the C-V circuit and supplies a reference voltage to the C-V circuit, reduces the gain relative to a deviation amount when the reference voltage fluctuates by a predetermined deviation amount.

Claims

1. A capacitance detection circuit for detecting a capacitance change value of a sensor element, the capacitance detection circuit comprising:

a C-V circuit, connected to the sensor element, for amplifying the capacitance change value of the sensor element by a predetermined gain based on a reference voltage to generate a detection signal; and

a voltage compensation circuit, connected to the C-V circuit, for supplying the reference voltage to the C-V circuit, wherein when the reference voltage fluctuates by a predetermined deviation amount, the voltage compensation circuit reduces the gain relative to the deviation amount;

wherein the C-V circuit includes;

an operational amplifier having a first input terminal connected to the sensor element, a second input terminal connected to the voltage compensation circuit, and an output terminal that outputs the detection signal; and

a feedback capacitor connected between the first input terminal and output terminal of the operational amplifier;

the sensor element includes;

a first variable capacitor having a movable electrode connected to the first input terminal of the operational amplifier and a fixed electrode that receives a first drive signal;

a second variable capacitor having a movable electrode connected to the movable electrode of the first variable capacitor and a fixed electrode that receives a second drive signal; and

a parasitic capacitor element formed in the sensor element and functioning as a parasitic capacitor connected to the first input terminal of the operational amplifier;

wherein the sensor element and feedback capacitor set a first load capacitance applied to the first input terminal of the operational amplifier; and

the voltage compensation circuit includes a plurality of balance capacitors that set a second load capacitance, which is applied to the second input terminal of the operational amplifier, to substantially the same value as the first load capacitance;

wherein the plurality of balance capacitors include;

a first balance capacitor having a first electrode, which is connected to the second input terminal of the operational amplifier, and a second electrode, which receives the first drive signal;

a second balance capacitor having a first electrode, which is connected to the first electrode of the first balance capacitor, and a second electrode, which receives the second drive signal;

a third balance capacitor having a first electrode, which is connected to the second input terminal of the operational amplifier, and a second electrode, which receives the reference voltage; and

a fourth balance capacitor having a first electrode, which is connected to the second input terminal of the operational amplifier, and a second electrode, which is connected to the second electrode of the third balance capacitor.

2. The capacitance detection circuit of claim 1, wherein:

a sum of the first variable capacitor and the second variable capacitor has a first capacitance, the parasitic capacitor has a second capacitance, and the feedback capacitor has a third capacitance; and

a sum of the first balance capacitor and the second balance capacitor is set to the first capacitance, the third balance capacitor is set to the second capacitance, and the fourth balance capacitor is set to the third capacitance.

3. The capacitance detection circuit of claim 1, further comprising:

a first switch element connected in parallel to the feedback capacitor; and

a second switch element connected in parallel to the fourth balance capacitor and activated and deactivated simultaneously with the first switch element.

4. The capacitance detection circuit of claim 1, further comprising:

a voltage divider, connected to the second electrode of the third balance capacitor and the second electrode of the fourth balance capacitor, for dividing the reference voltage by a predetermined ratio and applying the divided reference voltage to the second electrode of the third balance capacitor.

5. The capacitance detection circuit of claim 4, wherein:

a sum of the first variable capacitor and the second variable capacitor has a first capacitance, the parasitic capacitor has a second capacitance, and the feedback capacitor has a third capacitance; and

a sum of the first balance capacitor and the second balance capacitor is set to the first capacitance, the third balance capacitor is set to the second capacitance, and the fourth balance capacitor is set to the third capacitance.

6. The capacitance detection circuit of claim 5, wherein:

the voltage divider includes;

a first resistor circuit having a first resistance; and

a second resistor circuit having a second resistance; and

the predetermined ratio is expressed by (second resistance/(first resistance+second resistance)); and

the first resistance and the second resistance are set to satisfy the following equation;

second resistance/first resistance=(second capacitance/third capacitance)−

1.

7. A capacitance detection circuit for detecting a capacitance change value of a sensor element, the capacitance detection circuit comprising:

a C-V circuit, connected to the sensor element, for amplifying the capacitance change value of the sensor element by a predetermined gain based on a reference voltage to generate a detection signal;

a voltage compensation circuit, connected to the C-V circuit, for supplying the reference voltage to the C-V circuit, wherein when the reference voltage is fluctuated by a certain deviation amount, the voltage compensation circuit reduces the gain relative to the deviation amount; and

a voltage divider, connected to the voltage compensation circuit, for dividing the reference voltage by a predetermined ratio and supplying the divided reference voltage to the voltage compensation circuit.

8. The capacitance detection circuit of claim 7, wherein the voltage compensation circuit sets the gain relative to the deviation amount of the reference voltage to one or less.

9. The capacitance detection circuit of claim 8, wherein the voltage compensation circuit sets the gain relative to the deviation amount of the reference voltage to substantially zero.

10. The capacitance detection circuit of claim 7, wherein:

the voltage compensation circuit reduces the deviation amount of the reference voltage, and corrects the reference voltage with the reduced deviation amount to generate a corrected reference voltage; and

the C-V circuit converts the capacitance change value of the sensor element to the detection value using the corrected reference voltage.

11. The capacitance detection circuit of claim 7, wherein:

the C-V circuit includes;

an operational amplifier having a first input terminal connected to the sensor element, a second input terminal connected to the voltage compensation circuit, and an output terminal that outputs the detection signal; and

a feedback capacitor connected between the first input terminal and output terminal of the operational amplifier;

the sensor element includes;

a first variable capacitor having a movable electrode connected to the first input terminal of the operational amplifier and a fixed electrode that receives a first drive signal;

a second variable capacitor having a movable electrode connected to the movable electrode of the first variable capacitor and a fixed electrode that receives a second drive signal; and

a parasitic capacitor element formed in the sensor element and functioning as a parasitic capacitor connected to the first input terminal of the operational amplifier;

the sensor element and feedback capacitor set a first load capacitance applied to the first input terminal of the operational amplifier; and

the voltage compensation circuit includes a plurality of balance capacitors that set a second load capacitance, which is applied to the second input terminal of the operational amplifier, to substantially the same value as the first load capacitance.

12. The capacitance detection circuit of claim 11, wherein the plurality of balance capacitors include:

a first balance capacitor having a first electrode, which is connected to the second input terminal of the operational amplifier, and a second electrode, which receives the first drive signal;

a second balance capacitor having a first electrode, which is connected to the first electrode of the first balance capacitor, and a second electrode, which receives the second drive signal;

a third balance capacitor having a first electrode, which is connected to the second input terminal of the operational amplifier, and a second electrode, which receives the reference voltage; and

a fourth balance capacitor having a first electrode, which is connected to the second input terminal of the operational amplifier, and a second electrode, which is connected to the second electrode of the third balance capacitor.

13. The capacitance detection circuit of claim 12, wherein:

a sum of the first variable capacitor and the second variable capacitor has a first capacitance, the parasitic capacitor has a second capacitance, and the feedback capacitor has a third capacitance; and

a sum of the first balance capacitor and the second balance capacitor is set to the first capacitance, the third balance capacitor is set to the second capacitance, and the fourth balance capacitor is set to the third capacitance.

14. The capacitance detection circuit of claim 12, wherein:

a first switch element connected in parallel to the feedback capacitor; and

a second switch element connected in parallel to the fourth balance capacitor and activated and deactivated simultaneously with the first switch element.

15. The capacitance detection circuit of claim 12, wherein the voltage divider is connected between the second electrode of the third balance capacitor and the second electrode of the fourth balance capacitor, for dividing the reference voltage by the predetermined ratio and applying the divided reference voltage to the second electrode of the third balance capacitor.

16. The capacitance detection circuit of claim 15, wherein:

a sum of the first variable capacitor and the second variable capacitor has a first capacitance, the parasitic capacitor has a second capacitance, and the feedback capacitor has a third capacitance; and

a sum of the first balance capacitor and the second balance capacitor is set to the first capacitance, the third balance capacitor is set to the second capacitance, and the fourth balance capacitor is set to the third capacitance.

17. The capacitance detection circuit of claim 16, wherein:

the voltage divider includes;

a first resistor circuit having a first resistance; and

a second resistor circuit having a second resistance;

the predetermined ratio is expressed by (second resistance/(first resistance+second resistance)); and

the first resistance and the second resistance are set to satisfy the following equation of;

second resistance/first resistance=(second capacitance/third capacitance)−

1.