SAR A/D Converter Internal Diagnostics for Fault Bit Detection
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Solution Overview
Problem
Existing A/D converters in safety/critical applications require external test circuits to ensure output bits toggle independently, which occupy PCB space and introduce failure modes, especially in systems with isolated input channels.
Innovation Solution
An A/D converter with an internal diagnostic circuit, featuring a capacitor array, voltage comparator, and successive approximation register, which allows for internal testing by varying the equivalent capacitance and sampling charge to diagnose fault bits without external circuits.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If an external test circuit is used to test A/D converter output bits, then fault detection capability is improved, but PCB area occupation and system complexity increase
Solution Approach 1:
The diagnostic circuit is merged with the A/D converter by integrating the capacitor array and switch network directly into the converter architecture. The same capacitor array used for normal conversion operations is repurposed for diagnostic testing, eliminating the need for separate external test circuits and reducing PCB area occupation.
Solution Approach 2:
The capacitor array serves dual functions: it performs normal A/D conversion operations during regular operation and enables diagnostic testing during test modes. This multi-functionality allows fault detection capability to be improved without adding dedicated test components, thereby avoiding increased PCB area occupation.
2Reliability
If an external test circuit is used to test A/D converter output bits, then fault detection capability is improved, but device complexity increases
Solution Approach 1:
The diagnostic functionality is merged with the existing A/D converter structure by utilizing the same capacitor array and control switches. This integration approach improves fault detection capability while avoiding the increased device complexity that would result from adding separate external test circuitry.
Solution Approach 2:
The A/D converter performs self-diagnosis by using its own internal capacitor array and control mechanisms to test its output bits. This self-service approach enables fault detection without requiring external test equipment, thereby improving reliability while maintaining device complexity at acceptable levels.
3Reliability
If multiple instances of external test circuit are used for isolated input channels, then fault detection coverage is improved, but PCB area and system complexity increase significantly
Solution Approach 1:
Each isolated input channel's A/D converter incorporates its own integrated diagnostic capability using its internal capacitor array. This universal approach provides fault detection coverage for all channels without requiring multiple external test circuits, thereby improving reliability while avoiding the significant PCB area increase that would result from external testing for each channel.
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 internal diagnostic circuit enables fault detection within the A/D converter, eliminating the need for external test circuits and ensuring reliability in safety/critical applications by identifying unsuitable converters before deployment.
Implementation Method 1
a capacitor array having respective first terminals selectively coupled to a reference voltage or ground via a plurality of first switches and having respective second terminals coupled to a sample and hold (S/H) output
Implementation Method 2
a voltage comparator having a first input coupled to the S/H output and having a second input coupled to the bias voltage. The voltage comparator is configured to output a comparison voltage responsive to a sampled charge at the S/H output and the bias voltage
Implementation Method 3
a successive approximation register coupled to receive the comparison voltage and configured to output an approximate digital code responsive to the comparison voltage
Data Source
AI summary
An analog to digital (A/D) converter includes a capacitor array having respective first terminals selectively coupled to a reference voltage or ground via a plurality of switches and having respective second terminals coupled to a sample and hold (S/H) output. The A/D converter also includes a voltage comparator having a first input coupled to the S/H output and having a second input coupled to a bias voltage. The voltage comparator is configured to output a comparison voltage responsive to a sampled charge at the S/H output and the bias voltage. The A/D converter also includes a successive approximation register coupled to receive the comparison voltage and configured to output an approximate digital code responsive to the comparison voltage, wherein the approximate digital code is varied by controlling an equivalent capacitance of the capacitor array.