Low-Power System Clock Calibration Using Delta-Sigma Loop
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Solution Overview
Problem
Implantable medical devices, particularly those with small form factors like leadless pacemakers, face challenges in achieving high clocking accuracy while minimizing current drain due to the use of low-power oscillators, which suffer from inaccuracies related to long-term stability, temperature characteristics, and trim resolution.
Innovation Solution
A calibration routine using a delta-sigma loop is employed within the implantable medical device, where a high accuracy reference clock is periodically powered to correct the low-power system clock, integrating clock errors over time to adjust the trim value and reduce inaccuracies, allowing for accurate clock calibration while minimizing power consumption.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Use of energy by moving object
If a low-power oscillator is used for the system clock, then current drain is reduced, but clocking accuracy deteriorates due to poor long-term stability, temperature characteristics, and trim resolution
Solution Approach 1:
A high-accuracy reference clock is introduced as an intermediary component to periodically calibrate the low-power system clock. The reference clock serves as a mediator that transfers accuracy information to the system clock through a calibration routine, allowing the system clock to maintain accuracy without continuously consuming high power. The calibration process measures the frequency difference between the two clocks and adjusts the system clock's trim value accordingly.
Solution Approach 2:
The calibration routine operates periodically rather than continuously, with the reference clock being activated only at scheduled intervals to correct system clock drift. This periodic calibration approach maintains clocking accuracy while minimizing the time the high-power reference clock is active, thereby reducing overall power consumption of the clocking system.
2Measurement precision
If a high accuracy reference clock is continuously powered, then clocking accuracy is improved, but current drain increases
Solution Approach 1:
The reference clock is powered only during periodic calibration intervals rather than continuously. The system implements a calibration routine that activates the reference clock, performs frequency comparison and trim adjustment, then powers down the reference clock. This periodic operation maintains necessary clocking accuracy while dramatically reducing the average current drain compared to continuous operation.
Solution Approach 2:
The system clock performs self-calibration by automatically comparing its frequency against the reference clock and adjusting its own trim value through a feedback mechanism. The calibration routine measures the frequency difference and modifies the system clock's operating parameters to compensate for drift, enabling the system to maintain accuracy autonomously without continuous external intervention or high power consumption.
3Volume of moving object
If the housing form factor is reduced for unobtrusive implantation, then ease of implantation is improved, but battery capacity is reduced leading to increased current drain requirements
Solution Approach 1:
The calibration routine uses periodic activation of the high-accuracy reference clock instead of continuous operation. By confining the high-power reference clock operation to brief calibration intervals, the average current drain is reduced to levels compatible with small battery capacities in miniaturized implantable devices.
Solution Approach 2:
The patent replaces a mechanical/power-intensive continuous clocking system with a hybrid approach that uses a low-power oscillator for continuous operation and substitutes high-accuracy reference measurements only when needed. This substitution of continuous high-power operation with periodic low-power operation enables miniaturization while maintaining accuracy.
Data Source
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AI summary
Various techniques are described for periodically performing a calibration routine to calibrate a low-power system clock within an implantable medical device (IMD) based on a high accuracy reference clock also included in the IMD. The system clock is powered continuously, and the reference clock is only powered on during the calibration routine. The techniques include determining a clock error of the system clock based on a difference between frequencies of the system clock and the reference clock over a fixed number of clock cycles, and adjusting a trim value of the system clock to compensate for the clock error. Calibrating the system clock with a delta-sigma loop, for example, reduces the clock error over time. This allows accurate adjustment of the system clock to compensate for errors due to trim resolution, circuit noise and temperature.