On-Chip Capacitance Sensing via Relaxation Oscillator Frequency
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Solution Overview
Problem
Existing methods for measuring on-chip capacitance in integrated circuits face challenges such as inaccurate measurements, high power consumption, large chip area requirements, and long settling times due to the need for filtering sharp current pulses and reliance on large on-chip capacitances.
Innovation Solution
An on-chip capacitance measurement method using a relaxation oscillator configuration, where the charging/discharging currents, supply voltage, and output frequency are measured to calculate the capacitance under test, allowing for accurate, low-power, and small-area capacitance measurement.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If transient current measurement method is used, then capacitance can be measured, but measurement accuracy deteriorates due to comparator nonidealities and oscillatory settling
Solution Approach 1:
The patent replaces the mechanical/electrical transient current measurement system with an optical-based frequency measurement system. By converting the capacitance measurement problem into a frequency measurement problem through a relaxation oscillator, the system avoids comparator nonidealities and achieves high measurement accuracy without oscillatory settling issues.
Solution Approach 2:
The patent changes the measurement parameter from transient current to oscillation frequency. The relaxation oscillator converts capacitance value into frequency output, which can be measured with high precision using frequency counters or period measurement circuits, thereby improving both accuracy and reliability.
2Measurement precision
If average current measurement with filtering is used, then capacitance can be inferred, but measurement time increases due to long settling time
Solution Approach 1:
The patent replaces the time-consuming current filtering process with direct frequency measurement. The relaxation oscillator reaches steady-state frequency quickly, and frequency can be measured immediately without requiring long filtering periods, thus dramatically reducing measurement time.
3Measurement precision
If low injection frequency is used to minimize comparator error, then measurement accuracy improves, but measurement time increases due to required averaging
Solution Approach 1:
The patent changes the approach by using frequency as the measurement parameter instead of current averaging. Frequency measurement does not require long averaging periods even at low frequencies, as the period can be measured directly over multiple cycles, thereby reducing measurement time while maintaining accuracy.
4Measurement precision
If large on-chip capacitances are used for filtering, then current filtering is improved, but chip area increases
Solution Approach 1:
The patent replaces the need for large filtering capacitances with frequency measurement circuitry. The relaxation oscillator naturally filters the signal through its oscillation mechanism, and frequency can be measured without requiring large on-chip capacitances for filtering, thus reducing chip area.
5Measurement precision
If CMOS transistors handle large current spikes for charging/discharging, then capacitance measurement is enabled, but chip area increases
Solution Approach 1:
The patent replaces the high-current switching mechanism with a relaxation oscillator that uses small currents to generate frequency signals. The oscillator uses capacitive charging/discharging through resistors or small current sources, eliminating the need for large current spikes and reducing the size of CMOS transistors required.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
The method provides accurate, low-power, and small-area on-chip capacitance measurement, enabling precise tuning of crystal oscillators and improving the stability of clocking circuits in integrated circuits.
Implementation Method 1
using a capacitor under test in an on-chip relaxation oscillator configuration whose charging/discharging currents, supply voltage, and output frequency are measured
Data Source
AI summary
An on-chip capacitance measurement method and associated systems and devices are provided. Embodiments described herein rely on using the capacitor under test in an on-chip relaxation oscillator configuration whose charging/discharging currents, supply voltage, and output frequency are measured individually in a measurement block. The voltage thresholds of the relaxation oscillation are calculated from the circuit elements and the measured supply voltage. Because the oscillation frequency of the relaxation oscillator is a function of the capacitance under test, the charging/discharging currents, and the supply voltage (via voltage thresholds), the capacitance under test can be calculated using the measured values of the other quantities. Embodiments described herein provide an accurate, low-power, small-area on-chip system capable of measuring capacitance with high accuracy. An algorithm employing the above method and apparatus for tuning a crystal oscillator is also provided. Relevant circuit implementations used in the on-chip measurement system are also disclosed.