Capacitance measurement

An input device comprising:

• a sensor electrode;
• and
• a capacitance measurement circuit comprising:
• a differential amplifier comprising:
• a first input and a second input, wherein said first input is coupled to a modulated reference voltage and said second input is coupled with said sensor electrode;
• an output;
• and
• a switch coupled with said second input, said switch having a closed state and an open state;
• a first feedback capacitance coupled between said output and said second input;
• a second feedback capacitance coupled between said output and said second input;
• and
• a reset mechanism coupled in parallel with said first feedback capacitance and said second feedback capacitance and configured to reset said first feedback capacitance to a first level of charge and to reset said second feedback capacitance to a second level of charge during a reset phase,
• wherein said reset mechanism is configured to reset said first feedback capacitance to said first level of charge when said switch is in the open state, and
• wherein during an absolute capacitance measurement phase said differential amplifier is configured to charge said sensor electrode while balancing voltages on said first input and said second input to a voltage level associated with said modulated reference voltage coupled with said first input and to integrate charge on said sensor electrode to measure capacitance corresponding to a coupling between said sensor electrode and an input object.

A method of capacitive measurement with a differential amplifier having an output, a first input, and a second input, said method comprising:

• during an absolute sensing cycle:
• resetting, using a reset mechanism in a capacitance measurement circuit, a first feedback capacitance to a first level of charge and a second feedback capacitance to a second level of charge, wherein said first feedback capacitance and said second feedback capacitance are disposed between said second input and said output,
• wherein said reset mechanism is coupled in parallel with said first feedback capacitance and said second feedback capacitance,
• wherein said second input is coupled with a switch having a closed state and an open state, and
• wherein said reset mechanism is configured to reset said first feedback capacitance to said first level of charge when said switch is in the open state;
• balancing charge between a sensor electrode and said first feedback capacitance and said second feedback capacitance such that a voltage of said sensor electrode equals a voltage of said first input and a voltage of said second input, and said sensor electrode is charged to a value proportional to its capacitance and said voltage of said second input;
• and
• utilizing said differential amplifier to integrate charge on said sensor electrode, such that an absolute capacitance is measured.