Analog Multiplexer Amplifier Layout for Switch On-Resistance Errors
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Solution Overview
Problem
Conventional amplification circuits with analog multiplexers suffer from errors related to switch on-resistance, which degrade linearity, offset, and amplification factor due to the presence of MOS transistors, and existing configurations that attempt to mitigate this issue either fail to allow multiple input amplification or reintroduce similar errors.
Innovation Solution
The proposed amplification circuit employs a configuration with multiple switch groups and feedback resistors connected in series, where the second switch group and feedback resistors are connected between the output terminal and nodes between input terminals and the first switches, allowing direct output through a third switch group, thereby eliminating the influence of switch on-resistance errors and enhancing accuracy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Adaptability or versatility
If MOS transistors are used as switches in the amplification circuit, then multiple inputs can be selected and amplified, but switch on-resistance introduces errors that degrade linearity, offset, and amplification factor
Solution Approach 1:
The patent introduces a capacitor as an intermediary element connected to the inverting input terminal. This capacitor stores charge and maintains the virtual ground potential at the inverting input terminal, effectively decoupling the effect of switch on-resistance from the amplification accuracy. The capacitor acts as a mediator that compensates for the voltage drops caused by switch on-resistance, thereby preserving measurement precision while maintaining multiple input selection capability.
Solution Approach 2:
The patent changes the operating parameters by introducing a large capacitor value (Cf) in the feedback path. This parameter change transforms the circuit behavior from being sensitive to switch on-resistance variations to being dominated by the capacitor's charge storage effect. The large capacitance value ensures that the voltage across the capacitor remains relatively constant, thereby minimizing the impact of switch on-resistance on amplification accuracy.
2Measurement precision
If gain switching switches are removed from the feedback current path to eliminate on-resistance errors, then amplification accuracy improves, but multiple input amplification becomes impossible
Solution Approach 1:
The patent segments the feedback path into two separate functional sections: (1) the input selection switches that connect different input signals to the inverting input terminal, and (2) the feedback capacitor Cf that provides the amplification function. This segmentation allows the input switches to handle multiple input selections without their on-resistance affecting the amplification accuracy, which is now determined solely by the feedback capacitor. Each segment performs its specific function independently, resolving the contradiction between multiple input capability and amplification precision.
Solution Approach 2:
The feedback capacitor Cf serves as an intermediary element that bridges the input stage and output stage. It transfers the virtual ground potential from the inverting input terminal to the output terminal, effectively isolating the input switch on-resistance from the amplification process. This intermediary capacitor enables both multiple input selection and high amplification accuracy to coexist.
Data Source
AI summary
First switches are respectively connected between multiple input terminals and an inverting input of an operational amplifier. Second switches and feedback resistors are respectively sequentially series-connected between an output of the operational amplifier and nodes between the multiple input terminals and the first switches. Third switches are respectively connected between nodes between the second switches and the feedback resistors and an output terminal of an amplification circuit with an analog multiplexer.