Apparatus and method for reducing parasitic capacitive coupling and noise in fingerprint sensing circuits

Abstract

A fingerprint sensing circuit for reducing noise and parasitic capacitive coupling is disclosed in one embodiment of the invention as including a plurality of transmitting elements to sequentially emit a probing signal. A digital ground is provided to ground digital components in the fingerprint sensing circuit. A quiet ground, separate from and quieter than the digital ground, is provided to ground transmitting elements that are not transmitting the probing signal. Similarly, control logic is provided to connect, to the quiet ground, transmitting elements that are not transmitting the probing signal, while disconnecting, from the quiet ground, transmitting elements that are emitting the probing signal. The quiet ground helps to reduce the adverse effects of parasitic capacitive coupling and noise on the inactive transmitting elements.

The invention claimed is: 
     
       1. A fingerprint sensing circuit for reducing parasitic capacitive coupling and noise when scanning a fingerprint, the fingerprint sensing circuit comprising:
 a plurality of transmitting elements configured to sequentially transmit a probing signal; 
 a digital ground to provide a ground for digital components in the fingerprint sensing circuit; 
 a quiet ground, different from the digital ground; 
 a grounding switch electrically connected between the quiet ground and a respective transmitting element; and 
 control logic configured to selectively connect each transmitting element to the quiet ground through the operation of the respective grounding switch. 
 
     
     
       2. The fingerprint sensing circuit of  claim 1 , further comprising a plurality of switches configured to selectively connect each transmitting element to the quiet ground and disconnect each transmitting element from the quiet ground. 
     
     
       3. The fingerprint sensing circuit of  claim 2 , wherein the control logic controls the operation of the switches. 
     
     
       4. The fingerprint sensing circuit of  claim 1 , wherein the plurality of transmitting elements are arranged in a substantially linear array. 
     
     
       5. The fingerprint sensing circuit of  claim 4 , wherein the control logic is further configured to connect transmitting elements that are adjacent to a transmitting element transmitting the probing signal, to the digital ground. 
     
     
       6. The fingerprint sensing circuit of  claim 1 , wherein the digital ground and quiet ground are connected to one another at a ground plane. 
     
     
       7. The fingerprint sensing circuit of  claim 1 , further comprising a receiving element configured to sense the probing signal from the plurality of transmitting elements. 
     
     
       8. A method for reducing parasitic capacitive coupling and noise in a fingerprint sensing circuit, the method comprising:
 providing a plurality of transmitting elements to sequentially transmit a probing signal; 
 providing a digital ground to ground digital components in the fingerprint sensing circuit; 
 providing a quiet ground, different from the digital ground; 
 providing a grounding switch electrically connected between the quiet ground and a respective transmitting element; 
 selectively connecting, to the quiet ground, each transmitting element through the operation of the respective grounding switch; and 
 disconnecting, from the quiet ground, transmitting elements that are transmitting the probing signal. 
 
     
     
       9. The method of  claim 8 , further comprising providing a plurality of switches to selectively connect and disconnect each transmitting element from the quiet ground. 
     
     
       10. The method of  claim 9 , further comprising providing control logic to control the operation of the switches. 
     
     
       11. The method of  claim 8 , further comprising arranging the plurality of transmitting elements in a substantially linear array. 
     
     
       12. The method of  claim 11 , further comprising providing control logic to connect transmitting elements that are adjacent to a transmitting element emitting the probing signal, to the digital ground. 
     
     
       13. The method of  claim 8 , further comprising connecting the digital ground and quiet ground to one another at a ground plane. 
     
     
       14. The method of  claim 8 , further comprising providing a receiving element to sense the probing signal emitted by the plurality of transmitting elements. 
     
     
       15. A fingerprint sensing circuit for reducing parasitic capacitive coupling and noise when scanning a fingerprint, the fingerprint sensing circuit comprising:
 a plurality of receiving elements configured to sequentially sense a probing signal; 
 a digital ground configured to provide a ground for digital components in the fingerprint sensing circuit; 
 a quiet ground, substantially separate from the digital ground, configured to provide a ground for selected receiving elements that are not currently sensing the probing signal; 
 a grounding switch electrically connected between the quiet ground and a respective receiving element; and 
 control logic configured to selectively disconnect a receiving element that is currently sensing the probing signal from the quiet ground, while selectively connecting receiving elements that are not sensing the probing signal to the quiet ground. 
 
     
     
       16. The fingerprint sensing circuit of  claim 15 , further comprising a plurality of switches to selectively connect and disconnect each receiving element from the quiet ground. 
     
     
       17. The fingerprint sensing circuit of  claim 16 , wherein the control logic controls the operation of the switches. 
     
     
       18. The fingerprint sensing circuit of  claim 15 , wherein the plurality of receiving elements are arranged in a substantially linear array. 
     
     
       19. The fingerprint sensing circuit of  claim 18 , wherein the control logic is further configured to connect, to the digital ground, receiving elements that are adjacent to a receiving element sensing the probing signal. 
     
     
       20. The fingerprint sensing circuit of  claim 15 , wherein the digital ground and quiet ground are connected to one another at a ground plane. 
     
     
       21. A biometric sensing circuit comprising:
 a plurality of probe signal transmitting elements;   a plurality of response signal receiving elements;   a digital ground connected to digital components in the fingerprint sensing circuit;   a quiet ground, substantially separate from the digital ground;   a respective grounding switch electrically connected between the quiet ground and a respective transmitting element or the quiet ground and a respective response signal receiving element; and   control logic configured to selectively connect at least one of each respective transmitting element to the quiet ground through the operation of the respective grounding switch and each respective response signal receiving element to the quiet ground through the operation of the respective grounding switch.    
     
     
       22. The sensing circuit of claim 21, further comprising a plurality of switches configured to selectively connect each respective probe signal transmitting element to the quiet ground and disconnect other probe signal transmitting elements from the quiet ground or to selectively connect each respective response signal receiving element to the quiet ground and disconnect other response signal receiving elements from the quiet ground.  
     
     
       23. The sensing circuit of claim 21, wherein the control logic controls the operation of the switches.  
     
     
       24. The sensing circuit of claim 21, wherein the plurality of transmitting elements are arranged in a substantially linear array.  
     
     
       25. The sensing circuit of claim 24, wherein the control logic is further configured to connect probe signal transmitting elements that are adjacent to an active probe signal transmitting element transmitting the probing signal, to the digital ground or to connect response signal receiving elements that are adjacent to an active response signal receiving element receiving the response signal, to the digital ground.  
     
     
       26. The sensing circuit of claim 21, wherein the digital ground and quiet ground are connected to one another at a ground plane.  
     
     
       27. The sensing circuit of claim 21, further comprising a receiving element configured to sense the probing signal from the plurality of transmitting elements.  
     
     
       28. The sensor of claim 21 wherein the sensing circuit comprises a biometric sensing circuit.  
     
     
       29. The sensor of claim 28 wherein the biometric sensing circuit comprises a fingerprint sensing circuit.