Auto-calibration method for inertial MEMS sensors
A sensor module includes a pattern generator configured to generate a variable frequency self-test signal. The sensor module includes an inertial sensor including a self-test electrode configured to receive the frequency sweep self-test signal. The inertial sensor is configured to generate an analog sensor signal based on the self-test signal. The sensor module includes an analog to digital converter configured to generate a digital sensor signal based on the analog sensor signal and a demodulator including a first input configured to receive the digital sensor signal, a second input configured to receive the self-test signal, and an output configured to output a demodulated signal. The sensor module includes a first low pass filter coupled to the output of the demodulator and configured to generate a baseband signal. The sensor module includes a calibration circuit configured to identify different MEMS characteristics, like resonance frequency, Q-factor, or sensitivity based on the baseband signal.
1 . A method, comprising:
generating, with an inertial sensor, an analog sensor signal by applying a self-test signal to a self-test electrode of an inertial sensor;
converting the analog sensor signal to a digital sensor signal;
generating, with a first demodulator, a demodulated signal by passing the self-test signal and the digital sensor signal to a first demodulator;
generating a baseband signal by passing the first demodulated signal through a low pass filter;
identifying, with a calibration circuit, a zero-crossing frequency and a first peak frequency in the baseband signal; and
calibrating the inertial sensor based on the zero-crossing frequency and the first peak frequency.
2 . The method of claim 1 , wherein calibrating the inertial sensor includes calibrating a digital notch filter based on the zero-crossing frequency and the first peak frequency.
3 . The method of claim 1 , comprising calculating a Q factor of the inertial sensor based on the first peak frequency and the zero-crossing frequency.
4 . The method of claim 1 , wherein the zero-crossing frequency is a resonant frequency of the inertial sensor.
5 . The method of claim 1 , comprising identifying, with the calibration circuit, a second peak frequency I the baseband signal, wherein the first peak frequency is a positive peak frequency, the second peak frequency is a negative peak frequency, and the zero-crossing frequency is between the first peak frequency and the second peak frequency.
6 . The method of claim 5 , wherein the inertial sensor is a gyroscope, the method comprising calculating a Q factor of the inertial sensor based on the first peak frequency, the second peak frequency, and the zero-crossing frequency.
7 . The method of claim 6 , comprising calculating the Q factor based on a drive frequency of the inertial sensor.
8 . The method of claim 1 , comprising passing the self-test signal to the self-test electrode via a first variable delay element.
9 . The method of claim 8 , comprising passing the self-test signal to the demodulator via a second variable delay element.
10 . The method of claim 9 , wherein calibrating the inertial sensor includes adjusting at least one of the first and the second variable delay element.
11 . The method of claim 1 , wherein the self-test signal is frequency sweep signal.
12 . The method of claim 1 , wherein the self-test signal is a variable frequency self-test signal.
13 . A method, comprising:
applying a frequency sweep self-test signal to a test electrode of an inertial sensor and to a first demodulator of the inertial sensor;
generating a first digital signal with the inertial sensor based on the frequency sweep self-test signal;
generating a demodulated signal by demodulating the first digital signal and the frequency sweep self-test signal;
generating a baseband signal by passing the demodulated signal through a digital low-pass filter; and
identifying a resonant frequency of the inertial sensor by analyzing the baseband signal with a calibration circuit.
14 . The method of claim 13 , wherein identifying the resonant frequency includes identifying a zero-crossing frequency of the baseband signal.
15 . The method of claim 14 , comprising:
identifying, with the calibration circuit, a first peak frequency of the baseband signal; and
calculating a Q factor of the inertial sensor based on the resonant frequency and the first peak frequency.
16 . The method of claim 14 , comprising:
identifying, with the calibration circuit, a positive frequency of the baseband signal;
identifying, with the calibration circuit, a negative frequency of the baseband signal; and
calculating a Q factor of the inertial sensor based on the resonant frequency, the positive peak frequency, and the negative peak frequency.
17 . A device comprising:
a pattern generator configured to generate a variable frequency self-test signal;
an inertial sensor including a self-test electrode configured to receive the variable frequency self-test signal, the inertial sensor configured to generate an analog sensor signal based on the variable frequency self-test signal;
an analog to digital converter configured to generate a digital sensor signal based on the analog sensor signal;
a demodulator including a first input configured to receive the digital sensor signal, a second input configured to receive the self-test signal, and an output configured to output a demodulated signal;
a first low pass filter coupled to the output of the demodulator and configured to generate a baseband signal; and
a calibration circuit configured to identify a resonant frequency of the inertial sensor by analyzing the baseband signal.
18 . The device of claim 17 , wherein the calibration circuit is configured to identify the resonant frequency as a zero-crossing frequency of the baseband signal.
19 . The device of claim 18 , wherein the calibration circuit is configured to identify a first peak frequency of the baseband signal and to calculate a Q factor of the inertial sensor based on the resonant frequency and the first peak frequency.
20 . The device of claim 19 , wherein the calibration circuit is configured to identify a positive frequency of the baseband signal, to identify a negative frequency of the baseband signal, and to calculate a Q factor of the inertial sensor based on the resonant frequency, the positive peak frequency, and the negative peak frequency.
21 . The device of claim 17 , wherein the variable frequency self-test signal includes pseudo random sequences.