Biometric skin contact sensor and methods of operating a biometric skin contact sensor
A capacitive biometric skin contact sensor configured to resolve the contours of skin in contact with the sensor, wherein the sensor comprises: an array of sensor pixels, wherein each sensor pixel comprises a thin film transistor and a capacitive sensing electrode connected to the thin film transistor; a plurality of gate drive channels, wherein each gate drive channel is arranged to provide a gate drive signal to one or more of the sensor pixels; a plurality of read-out channels, wherein each read-out channel is arranged to receive a read-out current from one or more of the sensor pixels, each read-out current being indicative of a proximity to a respective capacitive sensing electrode of a conductive object to be sensed; a current multiplexer connected to a plurality of the read-out channels to receive read-out currents therefrom; and a current mirror assembly connected to the multiplexer to receive an input current therefrom and to provide a selected gain to the input current; wherein the sensor is configured to control both: (i) the number of read-out currents selected by the multiplexer, and (ii) the selected gain to the input current.
1. A capacitive biometric skin contact sensor configured to resolve the contours of skin in contact with the sensor, wherein the sensor comprises:
an array of sensor pixels, wherein each sensor pixel comprises a thin film transistor and a capacitive sensing electrode connected to the thin film transistor;
a plurality of gate drive channels, wherein each gate drive channel is arranged to provide a gate drive signal to one or more of the sensor pixels;
a plurality of read-out channels, wherein each read-out channel is arranged to receive a read-out current from one or more of the sensor pixels, each read-out current being indicative of a proximity to a respective capacitive sensing electrode of a conductive object to be sensed;
a current multiplexer connected to a first plurality of the read-out channels to receive read-out currents therefrom, wherein the current multiplexer is configured to select one or more read-out currents from said first plurality of read-out channels; and
a current mirror assembly connected to the multiplexer to receive an input current therefrom indicative of the one or more read-out currents selected by the current multiplexer, and wherein the current mirror assembly is configured to provide a selected gain to the one or more read-out currents selected by the current multiplexer;
wherein the sensor is configured to control: (i) a sensor resolution by controlling the number of read-out currents selected by the multiplexer, and (ii) a sensor sensitivity by controlling the selected gain provided by the current mirror assembly.
2. The biometric sensor of claim 1 , wherein controlling the number of read-out currents selected by the multiplexer comprises at least one of: (i) selecting one of the read-out currents as an input current for the current mirror assembly, (ii) combining two or more of the read-out currents to provide an input current for the current mirror assembly, and (iii) multiplexing two or more of the read-out currents to provide a sequence of input currents for the current mirror assembly.
3. The biometric sensor of claim 1 , wherein the sensor is configured to control the selected gain based on at least one of: (i) the number of read-out currents selected by the multiplexer, and (ii) an amount of time for which the gate drive signal is applied to the one or more respective pixels.
4. The biometric sensor of claim 3 , wherein the sensor is configured to increase the selected gain when the number of read-out currents selected decreases.
5. The biometric sensor of claim 1 , wherein the sensor is configured to control the number of read-out currents selected by the multiplexer based on at least one of: (i) the selected gain, and (ii) the amount of time for which the gate drive signal is applied to the one or more respective pixels.
6. The biometric sensor of claim 1 , wherein the sensor is configured to control both: (i) the number of read-out currents selected by the multiplexer, and (ii) the selected gain, to provide a selected level of resolution and/or sensitivity for the sensor.
7. The biometric sensor of claim 6 , wherein the sensor is configured to decrease the number of different read-out currents combined by the multiplexer and/or to increase the selected gain to increase the resolution and/or sensitivity for the sensor.
8. The biometric sensor of claim 1 , wherein the multiplexer comprises a plurality of switches for controlling which of the read-out currents are selected.
9. The biometric sensor of claim 1 , wherein the biometric sensor is configured to select one or more values for the selected gain to the input current.
10. The biometric sensor of claim 1 , wherein the current mirror assembly comprises a plurality of field effect transistors, and wherein the biometric sensor is configured to control the selected gain to the input current by selecting which of the field effect transistors are used.
11. The biometric sensor of claim 10 , wherein the current mirror assembly comprises:
a first field effect transistor connected to the input current at both its gate region and its drain region; and
a plurality of second field effect transistors each connected to the input current at their gate region and configured to output an amplified current at their source region, wherein the outputs from each of the second field effect transistors are connected to provide an output current from the current mirror assembly.
12. The biometric sensor of claim 11 , wherein each of the second field effect transistors has at least one different property to provide different amounts of gain to the input current.
13. The biometric sensor of claim 11 , wherein the current mirror assembly comprises a plurality of switches for controlling which of the second field effect transistors output an amplified current.
14. The biometric sensor of claim 1 , wherein each sensor pixel comprises a reference capacitor, and wherein for each sensor pixel, the reference capacitor and the capacitive sensing electrode are connected to a gate region of the thin film transistor.
15. A method of operating a biometric sensor to resolve the contours of skin in contact with the sensor, wherein the sensor comprises:
an array of sensor pixels, wherein each sensor pixel comprises a thin film transistor and a capacitive sensing electrode connected to the thin film transistor;
a plurality of gate drive channels, wherein each gate drive channel is arranged to provide a gate drive signal to one or more of the sensor pixels;
a plurality of read-out channels, wherein each read-out channel is arranged to receive a read-out current from one or more of the sensor pixels, each read-out current being indicative of a proximity to a respective capacitive sensing electrode of a conductive object to be sensed;
a current multiplexer connected to a first plurality of the read-out channels to receive read-out currents therefrom, wherein the current multiplexer is configured to select one or more read-out currents from said first plurality of read-out channels; and
a current mirror assembly connected to the multiplexer to receive an input current therefrom indicative of the one or more read-out currents selected by the current multiplexer, and wherein the current mirror assembly is configured to provide a selected gain to the one or more read-out currents selected by the current multiplexer;
wherein the method comprises:
controlling: (i) a sensor resolution by controlling the number of read-out currents selected by the multiplexer, and (ii) a sensor sensitivity by controlling the selected gain provided by the current mirror assembly.
16. The method of claim 15 , wherein the method comprises receiving a control signal indicating a desired level of resolution and/or sensitivity for the biometric sensor, and controlling both: (i) the number of read-out currents selected by the multiplexer, and (ii) the selected gain to the input current, to provide the desired level of resolution and/or sensitivity.
17. The method of claim 16 , wherein the method comprises increasing the resolution and/or sensitivity by decreasing the number of read-out currents selected and/or increasing the selected gain.
18. The method of claim 15 , wherein the method comprises controlling the selected gain based on at least one of: (i) the number of read-out currents selected by the multiplexer, and (ii) an amount of time for which the gate drive signal is applied to the one or more respective pixels.
19. The method of claim 18 , wherein the method comprises increasing the selected gain when the number of read-out currents selected decreases.
20. A non-transitory computer program product comprising computer program instructions configured to program a controller to perform the method of claim 15 .