Differential Programmable Gain Instrumentation Amplifier
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Solution Overview
Problem
Traditional programmable gain instrumentation amplifiers (PGIAs) often fail to meet stringent performance requirements due to limitations in linearity and speed, particularly when using current conveyors with global feedback, which can result in insufficient performance for data acquisition systems.
Innovation Solution
The input stage of a PGIA is implemented using precision current conveyors differentially, with a pair of operational amplifiers and field effect transistors in feedback loops, along with multiple gain resistors and double multiplexers for gain switching, to enhance linearity and speed, and an output circuit with composite differential I/V converters for improved signal-to-noise ratio and common mode rejection.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If traditional current conveyors with global feedback are used in PGIA, then device complexity is reduced, but linearity and speed performance deteriorate
Solution Approach 1:
The patent divides the single global feedback loop into multiple independent local feedback loops, with each operational amplifier having its own feedback path. This segmentation allows each stage to be optimized independently, improving linearity and speed performance while maintaining manageable circuit complexity.
Solution Approach 2:
The patent implements different feedback configurations for different stages of the amplifier. The first operational amplifier uses a specific feedback network optimized for input signal conditioning, while the second operational amplifier uses a different feedback configuration optimized for output signal processing, allowing each stage to have optimal local characteristics.
2Device complexity
If traditional current conveyors with global feedback are used in PGIA, then device complexity is reduced, but speed performance deteriorates
Solution Approach 1:
By segmenting the feedback paths, the patent reduces the bandwidth requirements for each individual feedback loop compared to a single global feedback loop. This allows each operational amplifier to operate at higher speeds without requiring the entire system to support the full bandwidth, thereby improving overall operating speed.
Solution Approach 2:
The patent implements preliminary signal conditioning in the first operational amplifier stage, preparing the signal in advance for the second stage. This preliminary action reduces the processing burden on subsequent stages, allowing them to operate faster and improving the overall speed of the instrumentation amplifier.
3Manufacturing precision
If precision current conveyors with differential configuration are used, then linearity is improved, but device complexity increases
Solution Approach 1:
The patent combines the functions of multiple operational amplifiers and feedback networks into a unified differential configuration. By merging the input and output stages into a coordinated differential architecture, the patent achieves improved linearity while the overall structure remains systematically organized and manageable.
Solution Approach 2:
The differential configuration serves multiple functions simultaneously: it provides differential signal amplification, common-mode rejection, and improved linearity. This multi-functionality reduces the need for additional separate circuits, thereby limiting the increase in device complexity despite the enhanced performance.
4Measurement precision
If composite differential I/V converters are used in output circuit, then signal-to-noise ratio is improved, but device complexity increases
Solution Approach 1:
The patent implements a nested architecture where the composite differential I/V converter is integrated within the existing operational amplifier stages. The current-to-voltage conversion is performed in a nested manner within the feedback loops, allowing the noise performance benefits to be achieved while reusing existing circuit elements and minimizing additional complexity.
Data Source
AI summary
In one embodiment, a programmable gain instrumentation amplifier (PGIA) comprises a pair of current conveyors, each current conveyor having a respective sense node and a respective voltage input, with a gain-setting resistor coupled between the respective sense nodes, and current being sensed on both sides of the gain setting resistor. In one embodiment, each current conveyor comprises a corresponding operational amplifier (op-amp) having a non-inverting input configured as the respective voltage input that may receive a respective input voltage signal, an output and an inverting input, with a respective current conveying element, which may be a FET, configured in a feedback loop between the output and the inverting input. Each current conveyor may be configured to sense a corresponding current flowing through its respective FET, with the corresponding currents forming a differential output current of the PGIA. The respective input voltage signals received by the respective voltage inputs may form a differential input voltage of the PGIA. In one set of embodiments, the PGIA may be implemented with a differential voltage output, leading to an increased output swing for higher signal-to-noise ratio and increased symmetry for a higher common mode rejection ratio.