High signal to noise ratio capacitive sensing analog front-end
Capacitance sensing circuits and methods are provided. The capacitance sensing circuit includes a capacitance-to-voltage converter configured to receive a signal from a capacitance to be sensed and to provide an output signal representative of the capacitance, an output chopper configured to convert the output signal of the capacitance-to-voltage converter to a sensed voltage representative of the capacitance to be sensed, an analog accumulator configured to accumulate sensed voltages during an accumulation period of NA sensing cycles and to provide an accumulated analog value, an amplifier configured to amplify the accumulated analog value, and an analog-to-digital converter configured to convert the amplified accumulated analog value to a digital value representative of the capacitance to be sensed. The analog accumulator may include a low pass filter having a frequency response to filter wideband noise.
1. A capacitance sensing circuit comprising:
a differential capacitance-to-voltage converter configured to receive a first signal from a sense capacitor and a second signal from a cancellation capacitor, the differential capacitance-to-voltage converter further configured to provide a differential output signal representative of a change in capacitance at the sense capacitor;
an output chopper configured to convert the differential output signal of the differential capacitance-to-voltage converter to a sensed voltage representative of the change in capacitance;
an analog accumulator configured to accumulate sensed voltages during an accumulation period of a number of sensing cycles and to provide an accumulated analog value;
an amplifier configured to amplify the accumulated analog value; and
an analog-to-digital converter configured to convert the amplified accumulated analog value to a digital value representative of the change in capacitance.
2. A capacitance sensing circuit as defined in claim 1 , wherein the analog accumulator includes a filter having a changeable frequency response to filter wideband noise.
3. A capacitance sensing circuit as defined in claim 2 , wherein the changeable frequency response of the filter is variable during the accumulation period.
4. A capacitance sensing circuit as defined in claim 1 , wherein the differential capacitance-to-voltage converter comprises a differential amplifier and wherein the first signal from the sense capacitor and second signal from the cancellation capacitor are received at a same input of the differential amplifier.
5. A capacitance sensing circuit as defined in claim 1 , wherein the differential capacitance-to-voltage converter comprises a differential amplifier and wherein the first signal from the sense capacitor is received at a first input of the differential amplifier and the second signal from the cancellation capacitor is received at a second input of the differential amplifier.
6. A capacitance sensing circuit as defined in claim 1 , wherein the amplifier is configured to adjust a range of the accumulated analog value to a range compatible with an input to the analog-to-digital converter.
7. A capacitance sensing circuit as defined in claim 6 , wherein the amplifier is configured to hold the accumulated analog value during a subsequent accumulation period.
8. A capacitance sensing circuit as defined in claim 7 , wherein the number of sensing cycles in the accumulation period is selectable.
9. A capacitance sensing circuit as defined in claim 8 , wherein the amplifier is adjusted to provide a constant overall gain for different accumulation periods.
10. A capacitance sensing circuit as defined in claim 1 , further comprising a controller configured to provide timing signals to the differential capacitance-to-voltage converter, the output chopper, the analog accumulator and the analog-to-digital converter.
11. A capacitance sensing circuit as defined in claim 1 , further comprising an input chopper configured to convert a DC voltage to an AC signal applied to the sense capacitor and the cancellation capacitor.
12. A capacitance sensing circuit as defined in claim 1 , wherein the sense capacitor is an element of a capacitive touch screen.
13. A capacitance sensing circuit comprising:
a capacitance-to-voltage converter including a differential amplifier having a first input configured to receive a common mode voltage and a second input configured to receive a signal which sums a sense signal from a sense capacitor and a cancellation signal from a cancellation capacitor, said differential amplifier further configured to provide an output signal representative of a change in capacitance of the sense capacitor;
a circuit configured to convert the output signal of the capacitance-to-voltage converter to a sensed voltage representative of the change in capacitance;
an analog accumulator and filter configured to accumulate sensed voltages during an accumulation period of a number of sensing cycles and to filter the sensed voltages, wherein the analog accumulator and filter provides an accumulated analog value;
an amplifier configured to amplify the accumulated analog value; and
an analog-to-digital converter configured to convert the amplified accumulated analog value to a digital value representative of the change in capacitance.
14. A capacitance sensing circuit as defined in claim 13 , wherein the analog accumulator and filter includes a programmable frequency response to filter wideband noise.
15. A capacitance sensing circuit as defined in claim 14 , wherein parameters of the programmable frequency response are variable during the accumulation period.
16. A capacitance sensing circuit as defined in claim 14 , wherein the programmable frequency response is provided by one of a low pass filter or a triangular analog FIR filter.
17. A capacitance sensing circuit as defined in claim 13 , wherein a range of the accumulated analog value is adjustable to be compatible with a range of an input to the analog-to-digital converter.
18. A capacitance sensing circuit as defined in claim 13 , wherein the amplifier is configured to hold the accumulated analog value during a subsequent accumulation period.
19. A capacitance sensing circuit as defined in claim 13 , wherein the number of sensing cycles in the accumulation period is selectable.
20. A capacitance sensing circuit as defined in claim 13 , further comprising an input chopper configured to convert a DC voltage to a first AC signal applied to the sense capacitor and a second AC signal, which is an inversion of the first AC signal, applied to the cancellation capacitor.
21. A capacitance sensing circuit as defined in claim 13 , wherein the sense capacitor is an element of a capacitive touch screen.
22. A capacitance sensing circuit comprising:
a capacitance-to-voltage converter including a differential amplifier having a first input configured to receive a sense signal from a sense capacitor and a second input configured to receive a cancellation signal from a cancellation capacitor, said differential amplifier further configured to provide an output signal representative of a change in capacitance of the sense capacitor;
a circuit configured to convert the output signal of the capacitance-to-voltage converter to a sensed voltage representative of the change in capacitance;
an analog accumulator and filter configured to accumulate sensed voltages during an accumulation period of a number of sensing cycles and to filter the sensed voltages, wherein the analog accumulator and filter provides an accumulated analog value;
an amplifier configured to amplify the accumulated analog value; and
an analog-to-digital converter configured to convert the amplified accumulated analog value to a digital value representative of the change in capacitance.
23. A capacitance sensing circuit as defined in claim 22 , wherein the analog accumulator and filter includes a programmable frequency response to filter wideband noise.
24. A capacitance sensing circuit as defined in claim 23 , wherein parameters of the programmable frequency response are variable during the accumulation period.
25. A capacitance sensing circuit as defined in claim 23 , wherein the programmable frequency response is provided by one of a low pass filter or a triangular analog FIR filter.
26. A capacitance sensing circuit as defined in claim 22 , wherein a range of the accumulated analog value is adjustable to be compatible with a range of an input to the analog-to-digital converter.
27. A capacitance sensing circuit as defined in claim 23 , wherein the amplifier is configured to hold the accumulated analog value during a subsequent accumulation period.
28. A capacitance sensing circuit as defined in claim 24 , wherein the number of sensing cycles in the accumulation period is selectable.
29. A capacitance sensing circuit as defined in claim 25 , further comprising an input chopper configured to convert a DC voltage to a first AC signal applied to the sense capacitor and a second AC signal applied to the cancellation capacitor.
30. A capacitance sensing circuit as defined in claim 26 , wherein the sense capacitor is an element of a capacitive touch screen.