RC Calibration Circuit Using Comparator Pulse Counting
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Solution Overview
Problem
Existing RC calibration methods are sensitive to temperature, time-consuming, and suffer from accuracy and cost issues due to manufacturing inaccuracies and the need for complex analog circuitry, particularly in communication apparatuses where precise frequency selectivity is required.
Innovation Solution
A calibration circuit using a comparator and current sources to measure the RC time constant by counting clock pulses until the voltage across a capacitor matches the voltage across a resistor, with optional switching arrangements and filter capacitors to improve accuracy and speed, allowing for fast and accurate calibration.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If oscillator-based RC calibration is used, then temperature sensitivity is reduced through chopping, but calibration time becomes very long and switching time errors occur
Solution Approach 1:
The patent extracts the calibration measurement from an oscillator context and places it in a comparator-based time constant measurement context. By removing the oscillator element, the patent eliminates the temperature sensitivity issues and long calibration times associated with oscillator-based methods while retaining the essential calibration function.
Solution Approach 2:
The patent replaces the oscillator-based mechanical/time-based calibration system with a comparator-based electrical measurement system. This substitution allows for faster calibration by directly measuring voltage equality rather than waiting for oscillation cycles, significantly reducing calibration time while maintaining or improving temperature stability.
2Measurement precision
If comparator-based calibration with large switching devices is used, then current source matching is improved, but thermal noise increases and calibration time becomes long
Solution Approach 1:
The patent changes the operating parameters of the comparator and switching devices to optimize the trade-off between matching accuracy and thermal noise. By adjusting parameters such as switching duration, frequency, and device sizing, the patent achieves good current source matching while keeping thermal noise within acceptable limits and calibration time reasonable.
3Adaptability or versatility
If digitally tunable resistors and capacitors are used in LPF, then frequency selectivity can be adjusted, but manufacturing inaccuracies cause cut-off frequency deviation
Solution Approach 1:
The patent implements a feedback-based calibration system that measures the actual RC time constant and uses this information to adjust the digitally tunable components. This feedback loop compensates for manufacturing inaccuracies and temperature variations, ensuring accurate cut-off frequency despite variations in resistor and capacitor values.
Solution Approach 2:
The patent replaces direct reliance on precise manufacturing of passive components with an active measurement and adjustment system. By using the comparator-based time constant measurement and digital tuning, the system achieves frequency accuracy without depending on high-precision manufacturing of resistors and capacitors.
4Measurement precision
If RC calibration circuit with filter capacitor is used, then switching noise is reduced, but circuit area and complexity increase
Solution Approach 1:
The patent applies filtering selectively and partially - using a filter capacitor only when necessary for specific measurement conditions or frequency ranges. This partial application of filtering reduces switching noise impact without requiring always-on complex filtering circuitry, thereby balancing measurement precision with circuit simplicity and area efficiency.
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
This approach provides a fast and accurate method for determining RC time constants, reducing calibration time and improving accuracy while minimizing the need for complex and area-intensive analog components, thus addressing the limitations of existing methods.
Implementation Method 1
a capacitor (C) connected between a second current source (403) and the reference voltage
Implementation Method 2
the product of resistance and capacitance is normally referred to as the time constant
Implementation Method 3
a comparator circuit (406) arranged to compare voltages across the resistor and the capacitor
Implementation Method 4
The controller is arranged to control the discharge switch to discharge the capacitor, change state of the switch to enable charging of the capacitor and count clock signal pulses
Data Source
AI summary
A circuit for calibration measurements comprises a first and a second current source arranged to provide current outputs; a resistor connected between the first current source and a reference voltage; a capacitor connected between the second current source and the reference voltage; a discharge switch connected in parallel with the capacitor and arranged to selectively discharge the capacitor; a comparator circuit arranged to compare voltages across the resistor and the capacitor and output a signal when voltage across the capacitor reaches the voltage across the resistor; and a controller having a clock signal input and connected to the output of the comparator circuit. The controller is arranged to control the discharge switch to discharge the capacitor, change state of the switch to enable charging of the capacitor and count clock signal pulses until the comparator provides the signal when voltage across the capacitor reaches the voltage across the resistor, wherein the controller is arranged to determine a calibration measurement from counted number of clock signal pulses. A method, computer program and electronic device are also disclosed.