Capacitance to frequency converter

• US 8,536,902 B1
• Filed: 11/21/2011
• Issued: 09/17/2013
• Est. Priority Date: 07/03/2007
• Status: Active Grant

• Current Assignee: Cypress Semiconductor Corporation (Infineon Technologies AG)
• Original Assignee: Cypress Semiconductor Corporation (Infineon Technologies AG)
• Inventors: Kremin, Viktor, Maharyta, Andriy
• Primary Examiner(s): Donovan, Lincoln
• Assistant Examiner(s): CHENG, DIANA
• Application Number:
  US13/301,764
• Time in Patent Office: 666 Days
• Field of Search: 327/101, 327/336, 327/337, 331/1.A, 331/1.R, 331/8, 331/16, 331/17, 331/108.R, 324/658
• US Class Current: 327/101
• CPC Class Codes: G01D 5/24 by varying capacitance
• CPC Class Codes: G01R 27/2605 Measuring capacitance capacitive sensors G01D5/24
• CPC Class Codes: H03K 17/955 using a capacitive detector
• CPC Class Codes: H03K 2217/960725 Charge-transfer
• CPC Class Codes: H03K 2217/96074 Switched capacitor

Abstract

A capacitance to frequency converter includes a switching capacitor circuit, a charge dissipation circuit, a comparator, and a signal generator. The switching capacitor circuit charges a sensing capacitor and transfers charge from the sensing capacitor to a circuit node of the charge dissipation circuit. The comparator is coupled to the charge dissipation circuit to compare a potential at the circuit node to a reference voltage. The signal generator is coupled to an output of the comparator and to the charge dissipation circuit. The signal generator is responsive to the output of the comparator to generate a signal fed back to control the charge dissipation circuit. A frequency of the signal is proportional to a capacitance of the sensing capacitor.

Claims

1. A method of converting a capacitance value to a frequency value, the method comprising:

reciprocally transferring charge between a first capacitor having the capacitance value and a second capacitor;

selectively modulating a frequency of a generated signal in response to the charge transferred between the first capacitor and the second capacitor; and

selectively transferring the charge relative to the second capacitor in response to the generated signal.

2. The method of claim 1, wherein selectively modulating the frequency of the generated signal comprises:

comparing a voltage potential on the second capacitor to a reference voltage potential;

generating a modulation signal dependent upon whether the voltage potential is greater or less than the reference voltage; and

modulating the frequency of the generated signal based on the modulation signal.

3. The method of claim 2, wherein modulating the frequency of the generated signal comprises modulating a period between fixed pulse lengths.

4. The method of claim 3, wherein modulating the frequency of the generated signal further comprises:

selectively enabling a pulse generator in response to the modulation signal;

generating the generated signal with a minimal fixed period when the modulation signal is in a first state; and

generating the generated signal having a fixed logic level between the fixed length pulses while the modulation signal is in a second state.

5. The method of claim 4, wherein modulating the period between the fixed length pulses comprises completing generation of a current one of the fixed length pulses prior to reverting to the fixed logic after the modulation signal transitions to the second state.

6. The method of claim 1, wherein reciprocally transferring charge between the first capacitor having the capacitance value and the second capacitor comprises two non-overlapping switching phases.

7. A capacitance to frequency converter comprising:

a switching capacitor circuit to transfer charge between a first capacitor having the capacitance and a second capacitor;

a modulator coupled to the switching capacitor to selectively modulate a frequency of a generated signal in response to the charge transferred between the first capacitor and the second capacitor; and

a charge dissipation circuit coupled to the modulator to selectively transfer the charge relative to the second capacitor in response to the generated signal.

8. The capacitance to frequency converter of claim 7, wherein selectively modulating the frequency of the generated signal comprises:

comparing a voltage potential on the second capacitor to a reference voltage potential;

generating a modulation signal dependent upon whether the voltage potential is greater or less than the reference voltage; and

modulating the frequency of the generated signal based on the modulation signal.

9. The capacitance to frequency converter of claim 8, wherein modulating the frequency of the generated signal comprises modulating a period between fixed puke lengths.

10. The capacitance to frequency converter of claim 9, wherein modulating the frequency of the generated signal further comprises:

selectively enabling a pulse generator in response to the modulation signal;

generating the generated signal with a minimal fixed period when the modulation signal is in a first state; and

generating the generated signal having a fixed logic level between the fixed length pulses while the modulation signal is in a second state.

11. The capacitance to frequency converter of claim 10, wherein modulating the period between the fixed length pulses comprises completing generation of a current one of the fixed length pulses prior to reverting to the fixed logic level after the modulation signal transitions to the second state.

12. The capacitance to frequency converter of claim 7, wherein reciprocally transferring charge between the first capacitor having the capacitance value and the second capacitor comprises two non-overlapping switching phases.

13. An apparatus comprising:

a switching capacitor circuit to transfer charge between a first capacitor having the capacitance and a second capacitor;

a comparator coupled to the second capacitor to compare a potential on the second capacitor to a first voltage and to generate a modulation signal;

a pulse generator coupled to the comparator to receive the modulation signal and to generate a feedback signal responsive to the modulation signal; and

a charge dissipation circuit coupled to the pulse generator to selectively transfer the charge relative to the second capacitor in response to the feedback signal.

14. The apparatus of claim 13, wherein the pulse generator generates a zero level fixed length pulse having a period between adjacent fixed length pulses modulated by the output of the comparator.

15. The apparatus of claim 14, further comprising a frequency measurement circuit coupled to the pulse generator to measure a frequency of the feedback signal.

16. The apparatus of claim 13, wherein the charge dissipation circuit comprises:

the second capacitor;

a resistor; and

a switch coupled in series with the resistor to selectively transfer the charge relative to the second capacitor in response to the feedback signal.

17. The apparatus of claim 16, wherein the switching capacitor circuit comprises:

a second switch to the first capacitor to selectively transfer charge relative to the first capacitor; and

a third switch coupled between the first capacitor and the second capacitor to selectively transfer the charge between the first capacitor and the second capacitor.

18. The apparatus of claim 13, wherein the comparator comprises an analog comparator.

19. The apparatus of claim 13, wherein an enable input of the pulse generator is coupled to the comparator to receive the modulation signal.

20. The apparatus of claim 13, further comprising a clocking circuit coupled to operate the second switch and the third switch in two non-overlapping phases.