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 high voltage node;
a low voltage node;
an acoustic element in communication with a bottom surface of a substrate, configured to produce an acoustic input into the substrate, the acoustic element comprising:
an ultrasonic transducer;
a drive circuit configured to couple the ultrasonic transducer to the high voltage node and generate the acoustic input; and
a sense circuit coupled to the low voltage node and configured to obtain an acoustic output from the substrate; and
a controller configured to increase a voltage bias of the sense circuit when the drive circuit is active to a voltage approximately equal to a voltage bias of the high voltage node.
2. The acoustic imaging sensor of claim 1 , wherein the acoustic imaging sensor is a biometric imaging sensor.
3. The acoustic imaging sensor of claim 1 , wherein the acoustic imaging sensor is configured to capture an image of a fingerprint.
4. The acoustic imaging sensor of claim 1 , wherein the substrate is formed from an optically transparent material.
5. The acoustic imaging sensor of claim 1 , wherein the substrate is formed from one of the group consisting of glass, sapphire, and metal.
6. The acoustic imaging sensor of claim 1 , wherein the ultrasonic transducer is formed from a material configured to expand in response to a high voltage pulse.
7. The acoustic imaging sensor of claim 1 , wherein the high voltage node is coupled to a high voltage source configured to output 100 volts.
8. The acoustic imaging sensor of claim 1 , wherein the low voltage node is coupled to a low voltage source configured output less than 10 volts.
9. The acoustic imaging sensor of claim 1 , wherein the controller is configured to raise the voltage bias of the sense circuit to a voltage greater than or equal to one hundred volts.
10. The acoustic imaging sensor of claim 9 , wherein raising the voltage bias of the sense circuit to a voltage approximately equal to the voltage bias of the high voltage node comprises ground shifting the sense circuit.
11. The acoustic imaging sensor of claim 9 , wherein ground shifting the sense circuit comprises applying a direct current voltage bias to the low voltage node and to a circuit ground of the sense circuit.
12. A method of driving an acoustic element comprising a high voltage drive circuit and a low voltage sense circuit, the method comprising:
increasing a voltage bias of the low voltage sense circuit approximately equal to a voltage bias of the high voltage drive circuit such that a potential difference between a voltage source of the low voltage sense circuit and a circuit ground of the low voltage sense circuit does not exceed a selected threshold; and
applying a high voltage drive signal to an ultrasonic transducer coupled to the high voltage drive circuit.
13. The method of claim 12 , wherein the acoustic element comprises an acoustic transducer formed from a material configured to expand in response to a high voltage pulse.
14. The method of claim 12 , wherein the high voltage drive circuit is coupled to a high voltage source configured to output 100 volts.
15. The method of claim 14 , wherein the low voltage sense circuit is coupled to a low voltage node configured to output approximately 3.3 volts.
16. The method of claim 12 , wherein increasing a voltage bias of the low voltage sense circuit comprises raising the voltage bias of the low voltage sense circuit to a voltage greater than or equal to one hundred volts.
17. The method of claim 16 , wherein raising the voltage bias of the low voltage sense circuit to a voltage approximately equal to the voltage of the high voltage drive circuit comprises ground shifting the low voltage sense circuit.
18. The method of claim 17 , wherein ground shifting the sense circuit comprises applying a direct current voltage bias to the low voltage node and to a circuit ground of the sense circuit.
19. A method of reading an output of an acoustic element of an acoustic fingerprint imaging system comprising a high voltage drive circuit and a low voltage sense circuit, the method comprising:
increasing a voltage bias of the low voltage sense circuit to a voltage approximately equal to a voltage bias of the high voltage drive circuit such;
applying a high voltage drive signal to an ultrasonic transducer coupled to the high voltage drive circuit
removing the voltage bias of the low voltage sense circuit such that a potential difference between a voltage source of the low voltage sense circuit and a circuit ground of the low voltage sense circuit does not exceed a selected threshold;
decreasing a voltage bias of the high voltage drive circuit such that the potential difference between a voltage source of the high voltage drive circuit and a circuit ground of the high voltage drive circuit is approximately zero volts;
applying a reset voltage signal to the low voltage sense circuit; and
measuring a voltage output from the low voltage sense circuit.
20. The method of claim 19 , wherein the acoustic element comprises an acoustic transducer formed from a material configured to expand in response to a high voltage pulse.
21. The method of claim 19 , wherein the high voltage drive circuit is coupled to a high voltage source configured to output 100 volts.
22. The method of claim 19 , wherein the low voltage sense circuit is coupled to a low voltage node configured to output approximately 3.3 volts.