Isolating Micro-Machined Accelerometer Zero-G Bias Shifts under Static Thermal Cycles
Static thermal zero-g bias shifts in MEMS accelerometers are isolated by enforcing prolonged thermal dwell periods to decouple stress creep from thermal gradients.
Continuous-Time Demodulation
Signal Recovery
Extraction of low-frequency sensor data from a modulated high-frequency carrier wave is performed using analog circuitry before any digital sampling occurs. This technique, known as continuous-time demodulation, avoids the aliasing and quantization noise that can occur when signals are digitized before processing. The method is commonly used in capacitive and inductive sensing applications where the physical variable changes the amplitude of a carrier signal.
This approach preserves the full bandwidth of the sensor and minimizes latency.
Circuit Architecture
Implementations typically rely on an analog multiplier or a switching mixer followed by a low-pass filter to extract the baseband signal. The switching mixer is driven by a reference clock that is synchronized with the excitation source of the sensor, ensuring phase-coherent detection. This multiplication shifts the desired information to direct current while moving the high-frequency carrier and its harmonics to higher frequencies.
These unwanted high-frequency components are then rejected by the low-pass filter, leaving a clean analog signal that represents the measured physical variable.
Metrological Precision
Phase alignment between the sensor signal and the reference clock is the most critical parameter for maintaining measurement accuracy. Any phase shift reduces the demodulated output amplitude and introduces sensitivity to quadrature errors, which degrades the linearity of the system. Calibration protocols must include a phase-tuning step to compensate for delays in the cabling and the sensor electronics.
This adjustment ensures that the demodulation occurs at the peak of the signal envelope, maximizing the signal-to-noise ratio.
Operational Limit
High-frequency carrier limits are determined by the bandwidth of the analog multiplier and the transition times of the switching elements. At very high frequencies, charge injection and clock feedthrough introduce offset errors that drift with temperature. This drift requires periodic calibration or the use of compensation circuits to maintain the specified measurement accuracy.