Capacitive MEMS microphone with built-in self-test
View Patent ↗A digital microphone includes built-in self-test features. The features may include capability to apply different bias voltages to a MEMS capacitor sensor of the digital microphone, simulating application of different sound pressures to the digital microphone. The features may also include a digital oscillator, for applying a test signal to an analog front end of the microphone.
1. A method of testing a digital microphone, comprising:
applying, at different times, a plurality of different bias voltages to a MEMS capacitor of the digital microphone, the MEMS capacitor serving as a sensor for the digital microphone, the digital microphone including a preamplifier, an analog-to-digital converter (ADC), and a sigma delta block, arranged in a sequence;
measuring outputs of an analog front end of the digital microphone resulting from the application of the plurality of different bias voltages to the MEMS capacitor; and
applying a digital oscillation signal to an element of the analog front end of the digital microphone, and measuring an output of the analog front end;
wherein the digital oscillation signal is applied as an input to the sigma delta block, and the sigma delta block receives a negative feedback signal from the ADC.
2. The method of claim 1 , wherein the different bias voltages are applied using a single bit digital-to-analog converter (DAC).
3. The method of claim 2 , wherein the single bit DAC includes a low pass filter.
4. The method of claim 3 , wherein the sigma delta block comprises an nth order digital sigma delta block with partial feedback.
5. The method of claim 4 , wherein the ADC comprises a sigma delta block with feed forward paths, a plurality of integrators arranged in a series, and negative feedback paths.
6. The method of claim 1 , wherein an attenuation capacitor is coupled in series with the MEMS capacitor, for purposes of creating a gain that is a linear function of electrostatic pressure.
7. The method of claim 1 wherein the digital oscillation signal is applied to the element of the analog front end during a test, with a signal from the sigma delta block applied to the preamplifier during the test.
8. The method of claim 1 , wherein the sigma delta block comprises an nth order digital sigma delta block with partial feedback.
9. The method of claim 1 , wherein the ADC comprises a sigma delta block with feed forward paths, a plurality of integrators arranged in a series, and negative feedback paths.
10. A digital microphone with built-in self-test, comprising:
a MEMS capacitor, the MEMS capacitor serving as a sensor for the digital microphone;
at least one charge pump configured to provide a plurality of different bias voltages to the MEMS capacitor;
a processing chain comprised of a preamplifier, an analog-to-digital converter (ADC), and a one bit sigma delta block, in series; and
a multiplexer configured to pass a signal from the MEMS capacitor to the preamplifier in a first mode and to pass a signal from the sigma delta block in a second mode.
11. The digital microphone of claim 10 , further comprising an attenuation capacitor coupled in series with the MEMS capacitor.
12. The digital microphone of claim 11 , further comprising a single bit digital-to-analog converter (DAC) with low pass filter, coupling the at least one charge pump and the MEMS capacitor.
13. The digital microphone of claim 12 , further comprising a digital oscillator, the digital oscillator configured to generate signals at different frequencies and with different amplitudes, the digital oscillator coupled to an input of the one bit sigma delta block.
14. The digital microphone of claim 13 , wherein an output of the one bit sigma delta block is coupled to the single bit DAC.
15. The digital microphone of claim 12 , wherein the one bit sigma delta block comprises an nth order digital sigma delta block with partial feedback.
16. The digital microphone of claim 15 , wherein the nth order digital sigma delta block includes a plurality of integrators arranged in a sequence.
17. The digital microphone of claim 15 , wherein the ADC comprises a sigma delta block with feed forward paths, a plurality of integrators arranged in a series, and negative feedback paths.
18. The digital microphone of claim 10 , wherein the one bit sigma delta block comprises an nth order digital sigma delta block with partial feedback.
19. The digital microphone of claim 18 , wherein the nth order digital sigma delta block includes a plurality of integrators arranged in a sequence.
20. The digital microphone of claim 10 , wherein the ADC comprises a sigma delta block with feed forward paths, a plurality of integrators arranged in a series, and negative feedback paths.