Providing a baseline capacitance for a capacitance sensing channel
A capacitance-sensing circuit may include a channel input associated with measuring a capacitance of a unit cell of a capacitive sense array. The capacitance-sensing circuit may also include a capacitive hardware baseliner that is coupled to the channel input. The capacitive hardware baseliner includes a programmable baseline resistor, and a buffer with an input coupled to the programmable baseline resistor and an output coupled to the channel input. The capacitive hardware baseliner generates a baseline current based on a time constant of the channel input associated with the measuring of the capacitance of the element of the capacitive sense array using the programmable baseline resistor. The capacitive hardware baseliner provides the baseline current at the channel input to provide a charge for a sense capacitor. A change in the charge of the sense capacitor is provided by the baseline current indicating a presence of a touch object proximate to the element.
1. A capacitance-sensing circuit comprising:
a channel input associated with measuring a capacitance of an element of a capacitive sense array; and
a capacitive hardware baseliner coupled to the channel input, wherein the capacitive hardware baseliner comprises:
a capacitor;
a programmable baseline resistor; and
a buffer with an input coupled to the programmable baseline resistor and an output coupled to the channel input, wherein the capacitive hardware baseliner generates a baseline current using the capacitor and the programmable baseline resistor to control a time constant of the capacitive hardware baseliner based on a time constant of the channel input associated with the measuring of the capacitance of the element of the capacitive sense array, wherein the capacitive hardware baseliner provides the baseline current at the channel input to provide a charge for a sense capacitor, wherein a change in the charge of the sense capacitor provided by the baseline current indicates a presence of a touch object proximate to the element.
2. The capacitance-sensing circuit of claim 1 , wherein the channel input is a receive (RX) channel coupled to an RX electrode of the element, wherein the RX channel comprises a first input coupled to the RX electrode and an output of a current gain component, and wherein a second input of the RX channel is coupled to a same voltage source as the buffer.
3. The capacitance-sensing circuit of claim 1 , the capacitive hardware baseliner further comprising:
a current mirror coupled to the buffer; and
a plurality of current gain components coupled to the current mirror, the current mirror to output a plurality of mirrored output currents to the plurality of current gain components.
4. The capacitance-sensing circuit of claim 3 , further comprising:
a second channel input to measure a capacitance of a second element of the capacitive sense array; and
a second current gain component coupled to the current mirror to apply a second gain factor to one of the plurality of mirrored output currents to generate a second baseline current and to provide the second baseline current to the second channel input.
5. The capacitance-sensing circuit of claim 4 , wherein the gain factor and the second gain factor are different gain factors.
6. The capacitance-sensing circuit of claim 1 , wherein the capacitive hardware baseliner further comprises a current gain component coupled to the buffer.
7. The capacitance-sensing circuit of claim 1 , wherein an input of the buffer is coupled to a slew rate control component.
8. The capacitance-sensing circuit of claim 1 , wherein an input of the buffer is coupled to a programmable time delay component associated with matching a current transient profile of the baseline current with a current transient profile associated with the channel input.
9. A method comprising:
generating, by a capacitive baseline circuit associated with a capacitive sense array, an output current by using a capacitor and a set resistance value of a programmable resistor to control a time constant of the capacitive baseline circuit based on a time constant at a channel input of the capacitive sense array;
buffering, by a buffer coupled to the programmable resistor, an input channel associated with the capacitive sense array, the output current to generate a buffered current;
applying a gain factor to the buffered current to generate a baseline current; and
providing the baseline current at the channel input associated with the capacitive sense array to provide a charge for a sense capacitor, wherein a change in the charge of the sense capacitor that is provided by the baseline current indicates a presence of a touch object at the capacitive sense array.
10. The method of claim 9 , further comprising:
generating a plurality of mirrored output currents based on the output current;
providing the plurality of mirrored output currents to a plurality of current gain components to generate a plurality of baseline currents; and
providing the plurality of baseline currents to different input channels associated with the capacitive sense array.
11. The method of claim 10 , further comprising applying a different gain factor to each of the plurality of mirrored output currents using the plurality of current gain components.
12. A capacitive sense array system comprising:
a plurality of elements, each element comprising one or more electrodes;
a plurality of channel inputs to measure capacitances of the plurality of elements; and
a capacitive hardware baseliner coupled to the plurality of channel inputs, wherein the capacitive hardware baseliner comprises:
a capacitor;
a programmable baseline resistor; and
a buffer with an input coupled to the programmable baseline resistor and an output coupled to the one of the plurality of channel inputs, wherein the capacitive hardware baseliner generates a plurality of baseline currents using the capacitor and the programmable baseline resistor to control a time constant of the capacitive hardware baseliner based on a time constant of the channel inputs associated with the measuring of the capacitances of the elements of the capacitive sense array, wherein the capacitive hardware baseliner provides the plurality of baseline currents at the plurality of channel inputs to provide a charge for a plurality of sense capacitors, wherein a change in one of the plurality of sense capacitors that is provided with the baseline current indicates a presence of a touch object proximate to the corresponding elements.
13. The capacitive sense array system of claim 12 , wherein the capacitive hardware baseliner further comprises a current gain component coupled to the buffer.
14. The capacitive sense array system of claim 13 , wherein the plurality of channel inputs are receive (RX) channels coupled to RX electrodes of the plurality of elements, wherein one of the RX channels comprises a first input coupled to one of the RX electrodes and an output of the current gain component, and wherein a second input of the one of the RX channels is coupled to a same voltage source as the buffer.
15. The capacitive sense array system of claim 13 , wherein the capacitive hardware baseliner further comprises:
a current mirror coupled to the buffer; and
a plurality of current gain components coupled to the current mirror, the current mirror to output a plurality of mirrored output currents to the plurality of current gain components.
16. The capacitive sense array system of claim 15 , wherein each of the plurality of current gain components applies a different gain factor to a corresponding mirrored output current of the plurality of mirrored output currents to generate a corresponding baseline current of the plurality of baseline currents.