Two-Stage Current Sensing Amplifier for High Common Mode Voltage
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Solution Overview
Problem
Current sensing systems face challenges in amplifying small differential voltages with high common mode voltages, requiring high precision resistors that increase costs and complexity, especially in half bridge PWM drivers where common mode rejection ratios are difficult to achieve.
Innovation Solution
A current sensing system comprising a control circuitry, a switching network, an impedance element, and two-stage amplifiers, where the first stage amplifier provides differential gain without amplifying common mode voltage, reducing the CMRR requirement for the second stage amplifier and allowing for simpler, less precise resistor matching.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If a differential amplifier is used to amplify small differential voltage with high common mode voltage, then measurement precision is improved, but device complexity and cost increase due to requirement for extremely accurate resistors
Solution Approach 1:
The amplifier circuit is divided into two separate stages: a first stage amplifier that handles the high common mode voltage and a second stage amplifier that provides the differential gain. This segmentation allows each stage to be optimized for its specific function, reducing the overall complexity and cost while maintaining measurement precision.
Solution Approach 2:
The first stage amplifier acts as an intermediary between the sense resistor and the second stage amplifier. It conditions the signal by handling the high common mode voltage and presenting a lower common mode voltage to the second stage, thereby reducing the CMRR requirement for the second stage and allowing for less precise resistors.
2Measurement precision
If extremely accurate resistors are used to minimize common mode signal amplification, then measurement precision is improved, but manufacturing cost increases significantly
Solution Approach 1:
The amplifier is segmented into two stages with different resistor precision requirements. The first stage uses resistors that do not require extreme precision, while the second stage uses standard precision resistors. This segmentation significantly reduces the overall manufacturing cost while maintaining adequate measurement precision.
Solution Approach 2:
The circuit transforms the common mode voltage parameter from a high value at the input to a lower value at the output of the first stage amplifier. This parameter change reduces the common mode voltage level that the second stage must handle, thereby reducing the CMRR requirement and allowing for less precise resistors.
3Ease of operation
If voltage divider is used to reduce high common mode voltage, then ease of operation is improved, but measurement precision deteriorates due to resistor mismatch in differential voltage dividers
Solution Approach 1:
The first stage amplifier serves as an intermediary that actively reduces the common mode voltage through amplification rather than passive division. This active reduction mechanism avoids the resistor mismatch problems inherent in passive voltage dividers while maintaining differential voltage measurement precision.
Data Source
AI summary
A current sensing system constituted of: an impedance element; a switching network arranged to alternately couple a first end of the impedance element between a supply voltage and return, the impedance element arranged to develop a voltage there across reflecting a current flow to a load coupled to the second end of the impedance element; a first stage amplifier, a first and second input thereof respectively coupled to the first and second end of the impedance element, a power supply input thereof coupled to a voltage greater than the supply voltage and a return thereof coupled to the first end of the impedance element, the amplifier having a first and second output, the potential difference reflecting the impedance element voltage times a first stage gain; and a second stage amplifier, a first and second input thereof respectively coupled to a first and second output of the first stage amplifier.