Half-bridge for capacitive sensing

An input device for receiving user input, the input device comprising:

• a first comparator including a first comparator input, a first comparator output, and a first reference voltage;
• a first capacitive element including a first capacitance, a first electrode, and a second electrode;
• a first node coupling the first comparator input to the second electrode;
• a digitally-controlled voltage waveform generator for generating a varying waveform, the digitally-controlled voltage waveform generator including a digitally controlled voltage waveform generator output;
• a first switch configured to selectively couple the digitally-controlled voltage waveform generator output to the first node;
• a first biasing element coupled to the first electrode;
• a second capacitive element including a second capacitance and a third electrode coupled to the first electrode such that a first output signal of the first comparator is dependent on a ratio of capacitances comprising the first capacitance and the second capacitance;
• and
• a controller coupled to the first comparator output, the controller programmed to control the digitally-controlled voltage waveform generator such that the varying waveform is controllably varied with a searching algorithm to cause a voltage at the first comparator input to be near the first reference voltage, and such that the varying waveform more quickly causes the voltage at the first comparator input to repeatedly cross the first reference voltage and trip the first comparator output, and wherein the controller is further configured to determine a measure of the ratio of capacitances from when the first comparator output tripped and generate position information for objects in a sensing region from the measure of the ratio of capacitances, wherein:
• at least one of the first capacitive element and the second capacitive element is a variable capacitive element.

A method for providing a user input to an electronic device in response to an object in a sensing region, the method generating a first comparator output signal dependent on a ratio including a first capacitance (C1) of a first capacitive element and a second capacitance (C2) of a second capacitive element, the method comprising the steps of:

• setting an initial voltage across the first capacitive element;
• generating a varying voltage waveform (Vw);
• applying Vw to at least one of the first capacitive element and the second capacitive element to generate a first voltage (Vn1) such that Vn1 exhibits values above and below a first reference voltage (Vref1) at different times, wherein a change in Vn1 is substantially proportional to the ratio including the first capacitance and the second capacitance, and wherein Vw is controllably varied with a searching algorithm to cause Vn1 to be near and more quickly cross Vref1;
• comparing Vn1 to Vref1;
• generating the first comparator output signal, wherein the first comparator output signal changes state in response to Vn1 crossing from one of being below Vref1 to being above Vref1 and from being above Vref1 to being below Vref1, and wherein a change in Vw is representative of a first ratio including the first capacitance and the second capacitance when Vn1 is substantially equal to Vref1;
• determining when the first comparator output signal changes state;
• determining a measure of the ratio from when the first comparator output signal changes state;
• and
• generating an indicia of object position in the sensing region to facilitate user input using the measure of the ratio.