Current Sense Amplifier With On-Chip Filtering and Offset Cancellation
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Solution Overview
Problem
Current sense amplifiers face challenges in accurately amplifying low-frequency signals while rejecting high common mode voltage transients, often resulting in nonlinear behavior and offset errors due to mismatched legs and the need for large off-chip RC filters, which increase size and expense.
Innovation Solution
A single-stage differential current sense amplifier using a linearized differential transconductance amplifier with on-chip RC filtering, feedback loops to stabilize gain, and synchronized choppers to cancel leg mismatches, enabling level-shifting and stable gain irrespective of common mode voltage.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Object-affected harmful factors
If RC filters are used at the input to remove high frequency transients, then transient filtering is improved, but the filter size increases requiring large off-chip capacitors
Solution Approach 1:
The patent combines the RC filter components directly onto the integrated circuit chip, merging what would traditionally be separate off-chip components into a unified on-chip implementation. This integration eliminates the need for large external capacitors while maintaining the transient filtering function, directly resolving the contradiction between filtering effectiveness and component size.
2Ease of manufacture
If the amplifier legs are not perfectly matched, then manufacturing variations occur, but this causes offset errors in the differential output
Solution Approach 1:
The patent employs a feedback mechanism where the differential output is fed back through a resistor network to the inverting input of the operational amplifier. This feedback automatically compensates for mismatches between the amplifier legs by adjusting the output to maintain the desired differential relationship, thereby eliminating offset errors caused by manufacturing variations while maintaining ease of production.
3Adaptability or versatility
If the input common mode voltage is high relative to downstream operating voltages, then the amplifier can handle high side sensing, but the output must be level-shifted to lower voltages
Solution Approach 1:
The patent introduces an intermediary level-shifting stage between the high common mode voltage input and the downstream low voltage circuitry. This intermediary stage, implemented using precision resistors and operational amplifier configuration, translates the high voltage differential signal to the appropriate low voltage range for ADCs and other downstream devices, enabling high side sensing capability while maintaining compatibility with standard low voltage digital interfaces.
Data Source
AI summary
A single stage current sense amplifier is described that generates a differential output that is proportional to a current through a sense resistor. The voltage across the sense resistor is Vsense. The current sense amplifier includes a differential transconductance amplifier having high impedance input terminals. An on-chip RC filter filters transients in the Vsense signal. A feedback circuit for each leg of the amplifier causes a pair of input transistors to conduct a fixed constant current irrespective of Vsense, which stabilizes the transconductance. A gain control resistor (Re) is coupled across terminals of the pair of input transistors and has Vsense across it. The current through the gain control resistor is therefore Vsensex1/Re. A level shifting circuit coupled to each of the input transistors lowers a common mode voltage at an output of the amplifier. Chopper circuits at the input and output cancel any offset voltages.