Guard Drive Circuit Topology for Stable Low-Current Measurements
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Solution Overview
Problem
Conventional electrical test and measurement systems face challenges in maintaining frequency stability and preventing phase characteristic changes due to feedback paths in guard amplifiers, leading to inadequate shielding and slow guard response during transient conditions, especially in low-current measurements.
Innovation Solution
The implementation of a guard drive circuit with a gain less than unity, utilizing an attenuator circuit with a resistor divider to stabilize the coupling path and prevent oscillation, allowing the guard to be driven in phase with the measurement signal without limiting bandwidth.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a feedback path is present from a guard amplifier to the measurement terminal, then the guard can be driven to the same voltage as the measurement terminal, but frequency stability cannot be maintained and phase characteristics change
Solution Approach 1:
The patent removes the feedback path from the guard amplifier to eliminate the oscillation problem while maintaining guard effectiveness through alternative means (guard drive circuit without feedback to measurement terminal)
Solution Approach 2:
The patent introduces an intermediary approach by using a guard drive circuit that does not directly feedback to the measurement terminal, thereby preventing phase shifts while still maintaining guard voltage control through a different signal path
2Stability of the object's composition
If resistance is added in series with the guard amplifier, then frequency stability is provided, but the guard's shielding effectiveness is decimated
Solution Approach 1:
The patent removes the series resistance from the guard amplifier circuit to restore shielding effectiveness while maintaining frequency stability through the eliminated feedback path approach
3Stability of the object's composition
If series guard resistance is used, then frequency stability is achieved, but the guard voltage deviates from the measurement node voltage under transient conditions
Solution Approach 1:
The patent removes the series resistance that caused transient voltage deviations, allowing the guard to respond faster to transient conditions while maintaining frequency stability through the alternative feedback-free design
4Stability of the object's composition
If a large resistor is used in the guard amplifier, then stability is achieved, but guard bandwidth is reduced and the guard lags behind the measurement terminal
Solution Approach 1:
The patent removes the large series resistance from the guard amplifier circuit, eliminating the bandwidth limitation and guard lag while maintaining stability through the feedback-free architecture
Data Source
AI summary
A test and measurement device including a source configured to output a source signal, a source output configured to output the source signal to a connected cable, a guard drive circuit electrically coupled to the source and configured to receive the source signal and generated a guard drive signal, the guard drive circuit having a gain less than one, and a guard drive circuit output configured to output the guard drive signal to a connected guard.