# US 8,786,295 B2 · Current sensing apparatus and method for a capacitance-sensing device > Agents: this data is available as JSON from the patentgrep API and MCP server — see https://patentgrep.com/llms.txt - Number: US 8,786,295 B2 - Document ID: US8786295B2 - Status: Granted - Kind: B2 - Application number: 13/091,067 - Filed: 2011-04-20 - Published: 2012-10-25 - Granted: 2014-07-22 - Earliest claimed: 2011-04-20 - Assignees: Cypress Semiconductor Corporation - Applicants: Lokesh Chandra; Edward Grivna - Inventors: Lokesh Chandra; Edward Grivna - Web page: https://patentgrep.com/patent/US8786295B2 ## CPC - G06F 3/044: by capacitive means - G06F 3/04166: {Details of scanning methods, e.g. sampling time, grouping of sub areas or time sharing with display driving (Synchronisation with the driving of the display or the backlighting unit to avoid interferences generated internally G06F3/04184)} - H03K 2217/960735: characterised by circuit details - G06F: ELECTRIC DIGITAL DATA PROCESSING (computer systems based on specific computational models G06N) - H03K: PULSE TECHNIQUE (measuring pulse characteristics G01R; modulating sinusoidal oscillations with pulses H03C; transmission of digital information H04L; discriminator circuits detecting phase difference between two signals by counting or integrating cycles of oscillation H03D3/04; automatic control, starting, synchronisation or stabilisation of generators of electronic oscillations or pulses where the type of generator is irrelevant or unspecified H03L; coding, decoding or code conversion, in general H03M) ## Abstract A capacitance-sensing device including a current-to-voltage converter and an analog-to-digital converter is described. A sense element is coupled to an input of the current-to-voltage converter. The current-to-voltage converter is configured to convert current changes in the coupled sense element to an output voltage and to maintain a constant voltage at the input. The analog-to-digital converter is configured to convert the output voltage generated by the current-to-voltage converter to a digital value. ## Claims ### Claim 1 An apparatus, comprising: a current-to-voltage converter having an input coupled to receive a current representing a capacitance of a sense element, wherein the current-to-voltage converter is configured to convert the current to an output voltage; a switch system coupled between the sense element and the input of the current-to-voltage converter, wherein the switch system is configured to either connect the sense element to, or disconnect the sense element from, the input of the current-to-voltage converter; and an analog-to-digital converter (ADC) coupled to receive the output voltage of the current-to-voltage converter, wherein the ADC is configured to convert the output voltage to a digital value, and wherein the current-to-voltage converter is configured to maintain an approximately constant voltage at the input of the current-to-voltage converter. ### Claim 2 (depends on 1) The apparatus of claim 1, wherein the current-to-voltage converter is a transimpedance amplifier that comprises an operational amplifier with a negative feedback. ### Claim 3 (depends on 2) The apparatus of claim 2, wherein the input of the current-to-voltage converter is a negative input of the operational amplifier of the transimpedance amplifier. ### Claim 4 (depends on 3) The apparatus of claim 3, further comprising a programmable current source coupled to the negative input of the operational amplifier, wherein the programmable current source is configured to generate a programmable current to offset a current drawn by parasitic capacitance of the sense element. ### Claim 5 (depends on 4) The apparatus of claim 4, wherein the programmable current is approximately equal to the current drawn by the parasitic capacitance of the sense element. ### Claim 6 (depends on 4) The apparatus of claim 4, wherein the programmable current source and the negative input of the operational amplifier are coupled to the sense element via an analog mux bus. ### Claim 7 (depends on 2) The apparatus of claim 2, wherein a reference voltage is coupled to a positive input of the operational amplifier of the transimpedance amplifier. ### Claim 8 (depends on 7) The apparatus of claim 7, wherein the reference voltage is approximately equal to the constant voltage. ### Claim 9 (depends on 1) The apparatus of claim 1, wherein the switch system is configured to connect and disconnect the sense element at a switch frequency. ### Claim 10 (depends on 1) The apparatus of claim 1, further comprising a low pass filter coupled between the current-to-voltage converter and the ADC. ### Claim 11 A method, comprising: sensing current changes in a sense element coupled to an input of a capacitance-sensing device; generating a programmable current to offset a current drawn by a parasitic capacitance of the sense element; converting the sensed current changes to an output voltage by the capacitance-sensing device; and measuring capacitance changes of the sense element by converting the output voltage to a digital value. ### Claim 12 (depends on 11) The method of claim 11, further comprising maintaining an approximately constant voltage at the input of the capacitance-sensing device. ### Claim 13 (depends on 11) The method of claim 11, wherein the programmable current is programmed to be equal to the current drawn by the parasitic capacitance of the sense element. ### Claim 14 (depends on 11) The method of claim 11, further comprising switching the sense element at a switch frequency. ### Claim 15 (depends on 14) The method of claim 14, wherein switching the sense element at the switch frequency further comprises: connecting the sense element to the input of the capacitance-sensing device; charging the sense element; disconnecting the sense element from the input of the capacitance-sensing device; and discharging the sense element. ### Claim 16 (depends on 11) The method of claim 11, further comprising waiting for 15 to 20 μs after the converting the sensed current changes to the output voltage before said converting the output voltage to the digital value. ### Claim 17 (depends on 11) The method of claim 11, wherein said converting the output voltage to the digital value is performed substantially instantaneously of said sensing and said converting.