Delta-Sigma Modulator Digital Control Circuit
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Solution Overview
Problem
Digital control circuits with analog controlled variables require analog-to-digital conversion, increasing complexity and potentially degrading the properties of the control loop, especially due to limitations in accuracy and the need for complex anti-aliasing filters.
Innovation Solution
A digital control loop with a delta-sigma modulator for analog-to-digital conversion, where the averaging is handled by the low-pass filter behavior of the digital controller, reducing the need for external components and allowing integration on a chip with improved accuracy and reduced complexity.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a conventional analog-to-digital converter is used, then the conversion is straightforward, but the accuracy is limited by resolution requiring complex constructions for high accuracy
Solution Approach 1:
The patent replaces conventional analog-to-digital conversion mechanisms with a delta-sigma modulator that uses oversampling and digital filtering. Instead of relying on high-resolution flash converters or successive approximation converters that require complex circuitry, the invention uses a 1-bit delta-sigma modulator with oversampling ratio M, achieving high effective resolution through temporal averaging rather than spatial (parallel) complexity. The low-pass filter in the digital domain replaces the need for complex anti-aliasing filters and high-resolution converter architecture.
2Ease of manufacture
If a conventional analog-to-digital converter is used, then the conversion process is simple, but complex anti-aliasing filters with low cut-off frequency are required
Solution Approach 1:
The patent employs periodic oversampling at a rate M times the Nyquist frequency, converting the anti-aliasing requirement from a complex continuous-time filter into a simpler discrete-time digital low-pass filter. The periodic sampling process spreads the quantization noise over a wider frequency range, allowing the digital low-pass filter to achieve the same attenuation with a higher cut-off frequency and simpler structure. This periodic oversampling action transforms the filter design challenge into a manageable digital signal processing task.
3Reliability
If pulse width modulator is used, then the controlled variable contains signal components at modulator frequency multiples, but a filter must suppress these components risking phase shift that degrades stability
Solution Approach 1:
The patent changes the sampling frequency parameter to an integer multiple M of the modulator frequency. This parameter relationship ensures that the spectral images of the PWM signal fall exactly at the Nyquist frequency and its harmonics, where the digital low-pass filter provides maximum attenuation. By aligning the sampling grid with the modulator frequency, the invention achieves clean separation between the fundamental control signal and the PWM switching components, minimizing phase shift in the passband while effectively suppressing the switching harmonics.
4Measurement precision
If delta-sigma modulator is used, then high accuracy is achieved with moderate effort, but the modulator generates a bit stream signal requiring scaling and processing
Solution Approach 1:
The patent designs the digital controller to perform multiple functions: it acts as both the control algorithm executor and the digital low-pass filter for the delta-sigma modulator output. The same digital processing resources used for control calculations also perform the decimation and averaging of the bit stream. This multi-functionality eliminates the need for a separate dedicated filter block, reducing overall device complexity while maintaining high conversion accuracy. The controller's accumulator and divider resources are reused for both control computations and signal decimation.
Data Source
Figure 1
AI summary
The circuit (10) has a sensor (20) provided for detecting a control variable (x) of a control system (18) and for outputting an analog output signal that represents an actual value of the control variable. A delta-sigma modulator (22) is provided for converting the analog output signal into a digital actual value signal (r). The delta-sigma modulator converts the analog output signal of the sensor into a bit stream signal, where an error variable (e) is formed by a signal of a digital reference value sensor (12).