Digital Control Loop for Fast SMU Current Saturation Recovery

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Solution Overview

Problem

Source-measure units (SMUs) face limitations in flexibility and accuracy due to analog control loops, particularly during range switching and transient events, which can lead to unpredictable behavior and slow recovery from loading events.

Innovation Solution

Implementing a digital control loop with a digital loop controller that checks measured current and voltage against compliance limits, uses weighted averaging for noise rejection, and employs a feed-forward algorithm to rapidly adjust output signals during transient events, reducing the accuracy requirements on digital-to-analog converters and enhancing stability and speed.

Engineering Contradictions & Design Principles

VSEngineering Contradiction Analysis

1Device complexity

If analog control loops are used in SMUs, then the system can operate with simpler hardware architecture, but the flexibility and accuracy during range switching and transient events deteriorate

Engineering Contradiction:

Improvehardware architecture complexityVSAvoidflexibility during range switching

Core Design Contradiction:

Device complexityVSAdaptability or versatility

Solution Approach 1:

The patent replaces analog control loops with a digital control loop implemented in firmware or software. The digital loop controller reads measurements from ADCs, compares them to compliance limits, and adjusts DAC outputs accordingly. This substitution eliminates the need for complex analog circuitry while providing superior flexibility and accuracy during range switching and transient events, as the digital controller can rapidly adapt to changing conditions without the limitations of analog components.

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

2Device complexity

If analog control loops are used in SMUs, then the system structure can be simpler, but the response speed during transient events deteriorates

Engineering Contradiction:

Improvecontrol loop structureVSAvoidresponse speed during transient events

Core Design Contradiction:

Device complexityVSSpeed

Solution Approach 1:

The patent implements a digital control loop that operates at higher speeds than analog loops can achieve during transient events. The digital controller can process measurements and adjust outputs in discrete time steps with precise timing control, enabling faster response to compliance violations. The digital architecture allows for rapid calculation and adjustment without the bandwidth limitations and phase shifts inherent in analog control circuits.

Inventive Principle:

Principle #28Mechanics substitution (Replace mechanical system)

Solution Approach 2:

The digital control loop operates in periodic cycles, continuously reading ADC measurements, processing compliance limit comparisons, and updating DAC outputs at defined intervals. This periodic operation ensures consistent timing and allows the system to maintain high response speeds by optimizing the control cycle duration, providing predictable and repeatable performance during transient events.

Inventive Principle:

Principle #19Periodic action

3Reliability

If analog control loops with compliance limits are used, then the system can limit output signals, but the recovery time from loading events increases

Engineering Contradiction:

Improvecompliance limit enforcementVSAvoidrecovery time from saturation

Core Design Contradiction:

ReliabilityVSLoss of time

Solution Approach 1:

The patent implements a digital feedback control loop that continuously monitors ADC measurements against compliance limits and immediately adjusts DAC outputs when violations occur. The digital controller provides rapid feedback with precise control over the adjustment magnitude and timing, enabling faster recovery from loading events compared to analog loops. The feedback mechanism operates in discrete steps that can be optimized for minimum recovery time while maintaining reliable compliance limit enforcement.

Inventive Principle:

Principle #23Feedback

Solution Approach 2:

The digital control loop is designed to detect compliance violations and initiate corrective action at the earliest possible moment in the control cycle. By checking measurements against limits and preparing correction values in advance within each digital control period, the system minimizes the time delay between violation occurrence and corrective output adjustment, thereby reducing recovery time from saturation conditions.

Inventive Principle:

Principle #10Preliminary action

Data Source

PatentUS8653840B2Fast current saturation recovery for a digital source measure unit (SMU)

Publication Date: 2014.02.18 NATIONAL INSTRUMENTS CORP

AI summary

A source-measure unit (SMU) may be implemented with respective digital control loops for output voltage and output current. The output voltage and output current may be measured with dedicated ADCs (analog-to-digital converters). The readings obtained by the ADCs may be compared to a setpoint, which may be set in a digital loop controller (DLC). The DLC may be used to produce an output to drive a DAC (digital-to-analog converter) until the output voltage and/or output current and/or a function thereof reach the respective desired levels. The DLC may perform a threshold check to determine if the output current is outside a specified measurable range, and generate an override signal to drive the DAC to rapidly return the output current to the measurable range. Once the current is within the measurable range, the DAC may once again be driven according to the respective digital control loops for the output voltage and the output current.