Sigma-Delta ADC Offset Compensation Using Dummy First Conversion
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Solution Overview
Problem
Sigma-delta ADCs suffer from offset voltage errors that reduce precision, particularly in high-resolution, high-speed applications like image sensors, due to fabrication variations and existing offset cancellation methods being time-consuming or requiring complex circuitry.
Innovation Solution
The method involves compensating for offset voltage by generating a dummy sample during the first conversion to measure and store the offset error in an integrator or decimation filter, which is then used to cancel subsequent errors in the oversampling process, utilizing a sigma-delta modulator and decimation filter with control logic to manage this process.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If existing offset cancellation methods are used, then offset errors are reduced, but the conversion time increases significantly
Solution Approach 1:
The patent performs offset measurement during the first conversion cycle before actual signal conversions begin. The integrator is pre-charged with the offset voltage by conducting a dummy conversion with zero input signal, so that subsequent conversions start with offset already compensated, eliminating time delays during actual measurements
Solution Approach 2:
The system uses its own integrator and conversion mechanism to measure and compensate for offset without requiring external calibration equipment or separate measurement circuits. The first conversion serves dual purposes: measuring offset and preparing the integrator for subsequent accurate conversions
2Measurement precision
If complex offset compensation circuitry is added, then precision is improved, but device complexity increases
Solution Approach 1:
The integrator is designed to perform multiple functions: it integrates the input signal during normal conversions, measures offset during the first conversion, and stores the offset compensation value. This multi-functionality eliminates the need for separate offset measurement circuits or additional compensation components
Solution Approach 2:
The system uses feedback by feeding the offset voltage measured during the first conversion back into the integrator for subsequent conversions. The integrator output from the first conversion, which contains the offset information, is fed back and used to compensate for offset in all following conversions
3Measurement precision
If oversampling is used to achieve high resolution, then measurement precision improves, but offset errors accumulate in the sample stream
Solution Approach 1:
The offset is measured and compensated before the actual oversampling conversions begin. By pre-charging the integrator with offset compensation during the first conversion, the system eliminates offset errors from the outset, preventing their accumulation in the oversampled stream rather than attempting to correct them afterward
Data Source
AI summary
An oversampling analog-to-digital converter (ADC) may include a quantizer that adds an offset error to each oversampling sample. If not reduced, the offset error may limit the performance of the ADC. The existing methods to eliminate the offset may increase a circuit size and slow the operation of the ADC. An oversampling ADC that can reduce, or remove, the offset error is disclosed. The disclosed ADC can be small and fast and still remove the offset. Accordingly, the disclosed ADC may be used in high performance applications, such as a high-speed image sensor.