Matched filter for a first order sigma delta capacitance measurement system and a method to determine the same
View Patent ↗A processing system that includes a sigma-delta converter and a filter unit that applies a matched filter to the output of the sigma-delta converter. The processing system drives sensor electrodes for capacitive sensing and receives resulting signals with the sensor electrodes in response. The processing system applies these resulting signals to sigma-delta converters. The matched filter boosts the signal-to-noise ratio of the signal received from the sigma-delta converter, thereby improving the ability to sense presence of an input object. The filter unit may apply different, customized matched filters for different capacitive pixels to improve the signal-to-noise ratio of each capacitive pixel in a customized manner.
1. A processing system for generating filtered digital data, the processing system comprising:
a sigma-delta converter configured to:
receive a first resulting signal, and
apply sigma-delta conversion to the first resulting signal to generate a first sigma-delta quantized signal; and
a filter logic unit configured to:
determine a first matched filter based on an impedance-characteristic of one or more transmission lines through which the first resulting signal has traveled; and
apply the first matched filter to the first sigma-delta quantized signal to generate a first filtered sigma-delta quantized signal.
2. The processing system of claim 1 , wherein:
the first matched filter is configured to approximate a substantially noise-free resulting signal.
3. The processing system of claim 1 , wherein:
the first matched filter is based on a discrete time cumulative sum signal.
4. The processing system of claim 3 , wherein:
the discrete time cumulative sum signal comprises a series of values, each value being a cumulative sum of a baseline sigma-delta quantized signal up to a phase index corresponding to the value.
5. The processing system of claim 3 , wherein:
the discrete time cumulative sum signal comprises a series of values, each value being a cumulative sum of a time-reversed baseline sigma-delta quantized signal up to a phase index corresponding to the value.
6. The processing system of claim 5 , wherein:
the first matched filter comprises a time-reversed version of the discrete time cumulative sum signal with high frequencies filtered out, the high frequencies substantially equal to or greater than a clock signal frequency that is associated with the baseline sigma-delta quantized signal.
7. The processing system of claim 1 , wherein:
the sigma-delta converter is further configured to:
receive a second resulting signal, and
apply sigma-delta conversion to the second resulting signal to generate a second sigma-delta quantized signal; and
the filter logic unit is further configured to:
determine a second matched filter based on an impedance-characteristic of one or more transmission lines through which the second resulting signal has traveled; and
apply the second matched filter to the second sigma-delta quantized signal to generate a second filtered sigma-delta quantized signal, the second matched filter differing from the first matched filter in a manner that is based on differences in impedance characteristics associated with the first resulting signal and the second resulting signal.
8. The processing system of claim 1 , wherein:
the first matched filter is periodic; and
the filter logic unit is further configured to store the first matched filter as a plurality of constants in a phase-indexed lookup table.
9. The processing system of claim 1 , wherein the filter logic is further configured to:
apply a second matched filter that is phase-shifted with respect to the first matched filter by approximately 90 degrees to the first sigma-delta quantized signal to generate a quadrature-phase filtered sigma-delta quantized signal.
10. A method for generating filtered digital data, the method comprising:
receiving a first resulting signal;
applying sigma-delta conversion to the first resulting signal to generate a first sigma-delta quantized signal; and
determining a first matched filter based on an impedance-characteristic of one or more transmission lines through which the first resulting signal has traveled; and
applying the first matched filter to the first sigma-delta quantized signal to generate a first filtered sigma-delta quantized signal.
11. The method of claim 10 , wherein:
the first matched filter is configured to approximate a substantially noise-free resulting signal.
12. The method of claim 10 , wherein:
the first matched filter is based on a discrete time cumulative sum signal.
13. The method of claim 12 , wherein:
the discrete time cumulative sum signal comprises a series of values, each value being a cumulative sum of a baseline sigma-delta quantized signal up to a phase index corresponding to the value.
14. The method of claim 12 , wherein:
the discrete time cumulative sum signal comprises a series of values, each value being a cumulative sum of a time-reversed baseline sigma-delta quantized signal up to a phase index corresponding to the value.
15. The method of claim 14 , wherein:
the first matched filter comprises a time-reversed version of the discrete time cumulative sum signal with high frequencies filtered out, the high frequencies substantially equal to or greater than a clock signal frequency that is associated with the baseline sigma-delta quantized signal.
16. The method of claim 10 , further comprising:
receiving a second resulting signal;
applying sigma-delta conversion to the second resulting signal to generate a second sigma-delta quantized signal;
determining a second matched filter based on an impedance-characteristic of one or more transmission lines through which the second resulting signal has traveled; and
applying the second matched filter to the second sigma-delta quantized signal to generate a second filtered sigma-delta quantized signal, the second matched filter differing from the first matched filter in a manner that is based on differences in impedance characteristics associated with the first resulting signal and the second resulting signal.
17. The method of claim 10 , wherein:
the first matched filter is periodic; and, the method further comprises:
storing the first matched filter as a plurality of constants in a phase-indexed lookup table.
18. The method of claim 10 , further comprising:
applying a second matched filter that is phase-shifted with respect to the first matched filter by approximately 90 degrees to the first sigma-delta quantized signal to generate a quadrature-phase filtered sigma-delta quantized signal.
19. An input device, comprising:
a plurality of sensing elements; and
a processing system coupled to the plurality of sensing elements, the processing system comprising:
a sigma-delta converter configured to:
receive a first resulting signal with a first sensing element of a plurality of sensing elements, and
apply sigma-delta conversion to the first resulting signal to generate
a first sigma-delta quantized signal; and
a filter logic unit configured to:
determine a first matched filter based on an impedance-characteristic of one or more transmission lines through which the first resulting signal has traveled; and
apply the first matched filter to the first sigma-delta quantized signal to generate a first filtered sigma-delta quantized signal.
20. The input device of claim 19 , wherein:
the first matched filter is configured to approximate a substantially noise-free resulting signal received with the first sensing element.