Oscillator Pin Circuit for External Resistor Frequency Stability
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Solution Overview
Problem
Oscillators with on-chip resistors face accuracy issues due to temperature-dependent non-linear variations and drift over time, while external resistors are prone to noise and stability problems when coupled to internal circuitry, leading to frequency errors and jitter.
Innovation Solution
A system that couples an external resistor to an oscillator chip pin, using an on-chip resistor and additional internal resistors to stabilize the circuit, and includes a capacitor in parallel with the external resistor to filter noise, with trimming mechanisms for components like capacitors and current mirrors to correct for frequency errors across temperature and process variations.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of manufacture
If an on-chip resistor is used in the oscillator circuit, then the oscillator can be integrated within the chip, but the frequency accuracy deteriorates due to temperature-dependent non-linear variations and drift over time
Solution Approach 1:
A buffer amplifier is introduced as an intermediary component between the on-chip resistor and the oscillator circuitry. This buffer amplifier isolates the oscillator circuit from the resistor's temperature-dependent variations and drift, allowing the use of integrated on-chip resistors while maintaining frequency accuracy. The buffer amplifier acts as a mediator that decouples the resistor's imperfections from the oscillator's frequency determination.
2Measurement precision
If an external resistor is used to improve frequency accuracy, then frequency precision improves, but the circuit becomes prone to noise and stability problems when coupled to internal circuitry
Solution Approach 1:
A buffer amplifier is introduced as an intermediary between the external resistor and the internal oscillator circuitry. This buffer amplifier isolates the high-precision external resistor from the noisy internal circuit environment, allowing the resistor to provide accurate frequency determination while the buffer protects it from noise and stability problems caused by coupling to internal circuitry.
3Measurement precision
If an external capacitor is coupled to the pin in parallel with the external resistor, then noise-induced errors are reduced, but the device complexity increases
Solution Approach 1:
The noise filtering function is extracted from the main oscillator circuit and implemented through a separate external capacitor coupled in parallel with the external resistor. This external capacitor forms an RC filter that removes noise-induced errors from the frequency determination process, while being externally coupled to minimize its impact on the internal circuit complexity.
Data Source
AI summary
In an example, a system includes an oscillator circuit on a chip. The oscillator circuit includes a charging current generator including a current mirror, an amplifier, and an on-chip resistor, where the on-chip resistor is coupled to a pin on the chip. The oscillator circuit also includes oscillator circuitry coupled to the charging current generator, where the oscillator circuitry includes a comparator, a phase generator, a first capacitor coupled to a first resistor, and a second capacitor coupled to a second resistor. The system also includes an external resistor coupled to the pin, where the external resistor is external to the chip. The system includes an external capacitor coupled to the pin, where the external capacitor is external to the chip.