Fingerprint sensing arrangement and a method for filtering a fingerprint pattern signal

1. A method for providing a filtered fingerprint pattern signal indicative of fingerprint pattern of a user's finger with a fingerprint sensing device comprising an array of sensing elements for sensing the fingerprint pattern, each sensing element is configured to provide a sensing signal indicative of a local fingerprint pattern feature,

wherein the method comprises:

receiving analog sensing signals from each of a set of sensing elements comprising at least four sensing elements,

computing a Hadamard transform of each set of sensing signals for filtering the set of sensing signals for common mode noise suppression, and to provide a set of filtered output signals, each of the filtered output signals comprises a linear combination of the set of sensing signals in which each sensing signal has a respective coefficient, wherein the coefficients in each linear combination sum up to zero, and wherein the linear combinations are different from each other, wherein the set of filtered output signals is obtained by discarding only a single sum output from the Hadamard transform, the single sum output that is discarded comprising a sum of the set of sensing signals resulting from a sum operator corresponding to Hadamard matrix elements of the same sign, and

converting the set of filtered output signals to a set of filtered digital sensing signals indicative of the user's fingerprint pattern.

2. The method according to claim 1 , wherein the number of sensing elements in the set of sensing elements is an even number of sensing elements.

3. The method according to claim 1 , wherein the set of sensing elements is a first set of sensing elements, the method comprising:

receiving analog sensing signals from a second set of sensing elements comprising at least four sensing elements, the sensing elements of the second set are not comprised in the first set of sensing elements,

filtering the set of analog sensing signals from the second set of sensing elements to provide a further set of filtered output signals, each of the further filtered output signals comprises a linear combination of the set of sensing signals from the second set of sensing elements in which each sensing signal has a respective coefficient, wherein the coefficients in each linear combination sum up to zero, and wherein the linear combinations are different from each other,

converting the second set of filtered output signals to a second set of filtered digital sensing signal indicative of the user's fingerprint pattern,

combining the first set of filtered digital sensing signal with the second set of filtered digital sensing signal.

4. The method according to claim 3 , wherein the number of sensing elements in the first set is equal to the number of sensing elements in the second set.

5. The method according to claim 3 , wherein the first set of sensing signals and the second set of sensing signals are acquired substantially simultaneously.

6. The method according to claim 1 , comprising:

shifting the set of sensing element to a further set of sensing element overlapping with at least one sensing element from the first set of sensing element, and

providing a further filtered digital sensing signal based on sensing signals from the further set of sensing elements.

7. A fingerprint sensing arrangement for sensing a fingerprint pattern of a user's finger for providing a fingerprint pattern signal, the fingerprint sensing arrangement comprising:

an array of sensing elements for sensing the fingerprint pattern, each sensing element is configured to provide a sensing signal indicative of a local fingerprint pattern feature,

a filter bank configured to suppress common mode noise, and configured to receive analog sensing signals from each of a set of sensing elements comprising at least four sensing elements,

the filter bank is further configured to compute a Hadamard transform of each set of sensing signals to calculate a set of filtered output signals, each of the filtered output signals comprises a linear combination of the set of sensing signals in which each sensing signal is associated with a respective coefficient, wherein the coefficients in each linear combination sum up to zero and wherein the linear combinations are different from each other, wherein the set of filtered output signals is obtained by discarding only a single sum output from the Hadamard transform, the single sum output that is discarded comprising a sum of the set of sensing signals resulting from a sum operator corresponding to Hadamard matrix elements of the same sign, and

an analog to digital converter configured to convert the set of filtered output signals to set of filtered digital sensing signal indicative of the user's fingerprint pattern.

8. The fingerprint sensing arrangement according to claim 7 , wherein the filter bank comprises a plurality of adding circuits and a plurality of subtraction circuits for calculating the set of filtered output signals.

9. The fingerprint sensing arrangement according to claim 7 , wherein each sensing element comprises:

a sensing structure for capacitive coupling with the finger, each sensing structure being covered by a dielectric structure, and

sensing circuitry for providing the sensing signals which are indicative of the capacitive coupling between the sensing structure and the finger in response to a change in potential difference between a sensing structure potential of the sensing structure and a finger potential of the finger.