Offset cancellation device for micro-electromechanical system
The present disclosure is directed to a system that includes a sensor and a signal conditioner coupled to the sensor. The signal conditioner includes signal processing circuitry coupled to the sensor and offset cancellation circuitry. The offset cancellation circuitry includes a sign detector configured to output a high signal or a low signal based on a sign of an output signal from the signal processing circuitry, an integrator coupled to the sign detector, and a divider coupled to the integrator and to an input of the signal processing circuitry.
1. A method, comprising:
receiving an output signal from a micro-electromechanical structure (MEMS) sensor; and
removing a bias from the output signal of the MEMS sensor, the removing including:
combining the output signal of the MEMS sensor with a cancellation signal;
during a first time period, adjusting the cancellation signal by a first scalar;
during a second time period, adjusting the cancellation signal by a second scalar that is smaller than the first scalar; and
during a third time period, adjusting the cancellation signal by a third scalar that is smaller than the second scalar.
2. The method of claim 1 wherein the adjusting the cancellation signal during the first, second and third time periods includes incrementing the cancellation signal by steps equal in magnitude to the first scalar in the first time period, the second scalar in the second time period, and the third scalar in the third time period, the steps having the same sign as the combined output signal and cancellation signal during the first, second and third time periods, respectively.
3. The method of claim 1 wherein the adjusting during the first time period occurs at a first frequency, the adjusting during the second time period occurs at a second frequency, and the adjusting during the third time period occurs at a third frequency, the first frequency being greater than the second frequency, and the second frequency being greater than the third frequency.
4. The method of claim 1 wherein the adjusting the cancellation signal includes:
integrating the cancellation signal over a first interval during the first time period;
integrating the cancellation signal over a second interval during the second time period; and
integrating the cancellation signal over a third interval during the third time period.
5. The method of claim 4 wherein the first interval is less than the second interval, and the second interval is less than the third interval.
6. The method of claim 4 , further comprising:
dynamically adjusting at least one of the first interval, the second interval, and the third interval.
7. A method, comprising:
generating, by a micro-electromechanical structure (MEMS) sensor, a sensing signal including an offset;
receiving, by a signal detector, the sensing signal;
generating, by a feedback loop coupled to the signal detector, a cancellation signal;
combining, by a combiner, the sensing signal with the cancellation signal; and
adjusting the cancellation signal by at least one scalar,
wherein the adjusting the cancellation signal by at least one scalar includes:
during a first time period, adjusting the cancellation signal by a first scalar;
during a second time period, adjusting the cancellation signal by a second scalar that is smaller than the first scalar; and
during a third time period, adjusting the cancellation signal by a third scalar that is smaller than the second scalar.
8. The method of claim 7 wherein the generating the sensing signal includes generating the sensing signal as at least one of an electrical, mechanical, or optical signal.
9. The method of claim 7 wherein the MEMS sensor comprises an accelerometer having a sensing range of +/−240 g of acceleration and a sensitivity of about 17 least significant bits (LSB)/g.
10. The method of claim 7 wherein the combiner comprises at least one of an adder, a subtractor, a compiler, or a summer.
11. The method of claim 7 wherein the generating, by the feedback loop, the cancellation signal includes:
amplifying, by a charger amplifier, an output of the combiner;
converting an output of the charge amplifier into a digital output signal;
filtering, by a digital low pass filter, the digital output signal; and
generating the cancellation signal based on an output of the digital low pass filter.
12. The method of claim 11 wherein the generating the cancellation signal based on the output of the digital low pass filter includes:
generating a digital offset signal based on the output of the digital low pass filter; and
generating the cancellation signal by converting, by a digital to analog converter, the digital offset signal into an analog offset signal.
13. The method of claim 11 wherein the digital low pass filter has a sampling frequency of about 16 kHz and a step response period of about 1.5 ms.
14. The method of claim 7 wherein the adjusting the cancellation signal during the first, second and third time periods includes incrementing the cancellation signal by steps equal in magnitude to the first scalar in the first time period, the second scalar in the second time period, and the third scalar in the third time period, the steps having the same sign as the combined sensing signal and cancellation signal during the first, second and third time periods, respectively.