Active sensing element for acoustic imaging systems
An acoustic fingerprint imaging system having a plurality of acoustic elements, each acoustic element including a transducer, and independent drive and sense circuitry is disclosed. Drive circuitry may require higher voltage than low voltage sense circuitry. Many embodiments described herein include a ground shifting controller to apply a voltage bias to the low voltage sense circuitry during a drive operation, in order to prevent electrical damage to the sense circuitry.
1. An acoustic imaging sensor comprising:
a piezoelectric element operable in a drive mode and a sense mode;
a sense circuit coupled to the piezoelectric element and configured to obtain a voltage signal from the piezoelectric element when in the sense mode, the voltage signal corresponding to at least a portion of a pixel of an acoustic image; and
a ground-shifting controller configured to increase a ground and a source of the sense circuit approximately to a drive voltage when in the drive mode.
2. The acoustic imaging sensor of claim 1 , further comprising:
a drive voltage node; and
a drive circuit configured to couple the piezoelectric element to the drive voltage node when in the drive mode.
3. The acoustic imaging sensor of claim 2 , wherein the ground-shifting controller is configured to connect the drive voltage node to a system ground when in the sense mode.
4. The acoustic imaging sensor of claim 2 , wherein the drive voltage node is coupled to a high voltage source.
5. The acoustic imaging sensor of claim 2 , wherein the sense circuit is coupled to a low voltage source.
6. The acoustic imaging sensor of claim 1 , wherein:
the piezoelectric element is a first piezoelectric element; and
the acoustic imaging sensor further comprises:
an array of piezoelectric elements, including the first piezoelectric element, coupled to an interior surface of a substrate.
7. The acoustic imaging sensor of claim 6 , wherein the array of piezoelectric elements is configured to capture an image of a fingerprint in contact with an exterior surface of the substrate.
8. The acoustic imaging sensor of claim 7 , wherein the substrate is formed from one of the group consisting of glass, plastic, sapphire, and metal.
9. The acoustic imaging sensor of claim 1 , wherein the piezoelectric element is positioned below a display.
10. The acoustic imaging sensor of claim 1 , wherein the piezoelectric element is positioned below a button.
11. The acoustic imaging sensor of claim 1 , wherein the piezoelectric element comprises a lead-free piezoelectric material.
12. A method of operating an acoustic sensor element in a drive mode, the method comprising:
increasing a ground of a low-voltage sense circuit coupled to the acoustic sensor element to a voltage approximately equal to a high-voltage drive signal;
increasing a source of the low-voltage sense circuit such that a potential difference between the source and the ground of the low-voltage sense circuit does not exceed a selected low-voltage threshold; and
after increasing the ground and source of the low-voltage sense circuit, applying the high-voltage drive signal to the acoustic sensor element.
13. The method of claim 12 , wherein the high-voltage drive signal has a sinusoidal waveform.
14. The method of claim 12 , further comprising increasing the source of the low-voltage sense circuit above the ground.
15. The method of claim 12 , wherein the ground and source are biased to the same voltage.
16. The method of claim 12 , wherein when the acoustic sensor element is in a sense mode, the method comprises:
decreasing the ground of the low-voltage sense circuit to a system ground; and
decreasing the source of the low-voltage sense circuit to a voltage above the system ground such that a potential difference between the source and the ground of the low-voltage sense circuit does not exceed a selected threshold.
17. A method of operating an acoustic imaging sensor, the method comprising:
operating in a drive mode by:
increasing a ground of a low-voltage sense circuit coupled to an acoustic sensor element array to a voltage approximately equal to a high-voltage drive signal;
increasing a source of the low-voltage sense circuit such that a potential difference between the source and the ground of the low-voltage sense circuit does not exceed a selected threshold; and
applying the high-voltage drive signal to at least one acoustic sensor element of the acoustic sensor element array; and
operating in a sense mode, after applying the high-voltage drive signal to the at least one acoustic sensor element, by:
decreasing the ground of the low-voltage sense circuit;
decreasing the source of the low-voltage sense circuit such that the potential difference between the source and the ground does not exceed the selected threshold; and
obtaining at least one voltage signal from the at least one acoustic sensor element.
18. The method of claim 17 , further comprising amplifying the at least one voltage signal.
19. The method of claim 18 , further comprising correlating the at least one voltage signal to at least one pixel of an output image.
20. The method of claim 18 , wherein:
the high-voltage drive signal is generated by a drive circuit; and
the method further comprising, in the sense mode, grounding a voltage supply line of the drive circuit.