Capacitive sensor testing
Sensor devices and methods are provided where a test signal is applied to a capacitive sensor. Furthermore, a bias voltage is applied to the capacitive sensor via a high impedance component. A path for applying the test signal excludes the high impedance component. Using this testing signal, in some implementations a capacity imbalance of the capacitive sensor may be detected.
1. A sensor device, comprising:
a capacitive sensor,
a voltage source circuit configured to provide a bias voltage,
wherein the voltage source circuit is coupled to a terminal of the capacitive sensor via a high-impedance component providing an ohmic resistance of at least 1 MΩ,
a test signal generator circuit configured to provide a test signal wherein the test signal generator circuit is coupled to the terminal of the capacitive sensor via a path not comprising the high-impedance component,
an evaluation circuit configured to evaluate a response of the capacitive sensor to the test signal, wherein the evaluation circuit is configured to determine a capacitance imbalance of the capacitive sensor based on the response; and
a bridge circuit coupled in parallel with the high-impedance component, and configured, in a low-impedance mode, to couple the voltage source circuit to the terminal of the capacitive sensor via a connection excluding the high-impedance component.
2. The sensor device of claim 1 , wherein the path comprises an alternating current (AC) coupling circuit configured to block direct current (DC) signals.
3. The sensor device of claim 2 , wherein the AC coupling circuit comprises a capacitance.
4. The sensor device of claim 3 , wherein the voltage source circuit comprises a charge pump circuit comprising the capacitance of the AC coupling circuit.
5. The sensor device of claim 1 , wherein the capacitive sensor is a differential sensor comprising a first variable capacitance and a second variable capacitance, wherein the terminal of the capacitive sensor is coupled to a node between the first variable capacitance and the second variable capacitance.
6. The sensor device of claim 5 , wherein the capacitive sensor comprises a movable membrane arranged between a first back plate and a second back plate, wherein the terminal of the capacitive sensor is coupled to the movable membrane.
7. The sensor device of claim 1 , wherein the capacitive sensor comprises a differential output having a first output terminal and a second output terminal, the sensor device further comprising:
a first further high impedance component having an ohmic resistance of at least 1 MΩ coupled between the first output terminal and a reference voltage,
a second further high impedance component having an ohmic resistance of at least 1 MΩ coupled between the second output terminal and the reference voltage,
a first amplifier coupled to the first terminal; and
a second amplifier coupled to the second terminal.
8. The sensor device of claim 7 , wherein the first and second amplifiers comprise high impedance input terminals.
9. The device of claim 7 , further comprising AC coupling circuits between the first and second amplifiers and the first and second output terminals of the capacitive sensor, respectively.
10. The device of claim 1 , wherein the evaluation circuit is configured to determine at least one of a pull-in voltage or a release voltage of the capacitive sensor based on the response.
11. A method, comprising:
applying a bias voltage to a terminal of a capacitive sensor via a high impedance component having an ohmic resistance of at least 1 MΩ,
applying a test signal to the capacitive sensor via a path excluding the high impedance component,
setting a bias voltage based on a response of the capacitive sensor to the test signal, and
entering a low-impedance mode in which the bias voltage applied to the terminal of the capacitive sensor excludes the high impedance component, wherein the low-impedance mode is entered via use of a bridge circuit coupled in parallel with the high-impedance component, wherein the bridge circuit, in the low-impedance mode, is configured to couple the bias voltage to the capacitive sensor via a connection excluding the high-impedance component.
12. The method of claim 11 , wherein applying the test signal comprises applying the test signal via an AC coupling circuit.
13. The method of claim 11 , further comprising determining a capacitance imbalance of the capacitive sensor based on a response of the capacitive sensor to the test signal.
14. The method of claim 11 , further comprising calibrating the capacitive sensor based on a response of the capacitive sensor to the test signal.
15. The method of claim 11 , further comprising:
sweeping the bias voltage over a range of bias voltages, and
determining at least one of a pull-in voltage and a release voltage based on a response of the capacitive sensor to the test signal during the sweeping.
16. A sensor device, comprising:
a capacitive sensor,
a voltage source circuit configured to provide a bias voltage,
wherein the voltage source circuit is coupled to a terminal of the capacitive sensor via a high-impedance component providing an ohmic resistance of at least 1 MΩ,
a test signal generator circuit configured to provide a test signal wherein the test signal generator circuit is coupled to the terminal of the capacitive sensor via a path not comprising the high-impedance component, wherein the path comprises an alternating current (AC) coupling circuit configured to block direct current (DC) signals, wherein the AC coupling circuit comprises a capacitance, and
a bridge circuit coupled in parallel to the high-impedance component, and configured, in the low-impedance mode, to couple the voltage source circuit to the terminal of the capacitive sensor via a connection excluding the high-impedance component,
the voltage source circuit comprising a charge pump circuit comprising the capacitance of the AC coupling circuit.
17. The method of claim 11 , in association with a sensor device that comprises:
a capacitive sensor,
a voltage source circuit configured to provide a bias voltage,
wherein the voltage source circuit is coupled to a terminal of the capacitive sensor via a high-impedance component providing an ohmic resistance of at least 1 MΩ,
a test signal generator circuit configured to provide a test signal wherein the test signal generator circuit is coupled to the terminal of the capacitive sensor via a path not comprising the high-impedance component, wherein the path comprises an alternating current (AC) coupling circuit configured to block direct current (DC) signals, wherein the AC coupling circuit comprises a capacitance,
an evaluation circuit configured to evaluate a response of the capacitive sensor to the test signal,
a bridge circuit coupled in parallel to the high-impedance component, and configured, in the low-impedance mode, to couple the voltage source circuit to the terminal of the capacitive sensor via a connection excluding the high-impedance component,
the method further comprising measuring a physical quantity produced by the capacitive sensor, and
wherein the physical quantity is measured concurrently while applying the test signal.
18. The method of claim 17 , further comprising setting a bias voltage on the capacitive sensor based on a response of the capacitive sensor to the test signal, wherein the bias voltage value is based on a value determined by the evaluation circuit.
19. The method of claim 17 , wherein while measuring the physical quantity a charge on the capacitive sensor remains constant on a time scale of variations of an output signal due to the test signal.