ADC Leakage Correction Circuit for Reset Switch Linearity Errors
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Solution Overview
Problem
Successive approximation analog to digital converters experience significant linearity errors due to voltage-dependent leakage current from reset switches, particularly at strong CMOS model corners and elevated temperatures when using deep submicron processes.
Innovation Solution
An analog leakage current correction circuit is introduced, comprising an operational amplifier and carefully matched MOS transistors, which replicates and cancels the leakage current at the input sense nodes of the comparator, maintaining operational independence from process variations and temperature.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Ease of operation
If reset switches are used to reset comparator input nodes, then the converter can perform successive approximation conversions, but voltage-dependent leakage current is injected into the input sense nodes causing linearity errors
Solution Approach 1:
A correction circuit is introduced as an intermediary component between the reset switches and the comparator input nodes. This circuit actively compensates for the leakage current by injecting an equal and opposite current, thereby eliminating the harmful effect while preserving the reset function. The correction circuit includes current mirrors and control logic that detect and counterbalance the leakage current from reset switches.
Solution Approach 2:
The leakage current, which is inherently harmful, is converted into a beneficial effect by using it as a reference signal for the correction circuit. The circuit measures the leakage current and uses this information to generate a compensating current that exactly cancels the harmful leakage, thereby transforming the problem into a solution.
2Productivity
If deep submicron processes are used for fabrication, then device scaling and integration are improved, but leakage current increases particularly at strong CMOS model corners and elevated temperatures
Solution Approach 1:
The correction circuit implements a feedback mechanism that continuously monitors the leakage current from reset switches and dynamically adjusts the compensating current accordingly. The feedback loop ensures that the correction remains effective across process variations, temperature changes, and different operating conditions, maintaining linearity accuracy despite the increased leakage inherent in deep submicron processes.
Solution Approach 2:
The correction circuit adapts to changing parameters such as temperature and process corner variations by dynamically adjusting its operation. The circuit's control logic modifies the compensation current based on detected leakage levels, ensuring effective correction across strong, weak, and nominal CMOS model corners and over the full temperature range.
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 correction circuit effectively reduces linearity errors by approximately 4 millivolts, equivalent to x bits of error for a 12-bit, 1.8-volt scale analog to digital converter, improving the converter's accuracy and reliability.
Implementation Method 1
An analog leakage current correction circuit is introduced, comprising an operational amplifier and carefully matched MOS transistors, which replicates and cancels the leakage current at the input sense nodes of the comparator
Data Source
AI summary
An analog to digital converter includes leakage current correction circuitry to cancel leakage current injected by a reset switch employing a dummy PMOS switch with a shape factor substantially similar to that of the reset switch. An operational amplifier replicates the voltage of the comparator sense input node to the drain of the dummy transistor to create the same operating point as the reset switch. The resulting leakage current is then repeated and fed back to the node to cancel the offending leakage current.