Chopper-Stabilized Operational Amplifier for Zero Offset Sensing
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Solution Overview
Problem
Operational amplifiers exhibit significant voltage offset between their input terminals, leading to inaccuracies in current sensing in power stages, with a 1 millivolt offset causing a standard deviation of 0.4 amperes current offset in current sensing power MOSFETs.
Innovation Solution
A chopper circuit is integrated into the operational amplifier to periodically reverse the input terminals and outputs, using a differential amplifier, switching circuits, and an RC noise filter to generate an offset adjustment signal, effectively reducing the voltage offset to near zero by averaging the offset voltage over a 50% duty cycle.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Device complexity
If a conventional operational amplifier is used, then the circuit is simple, but the voltage offset between input terminals is significant (1 millivolt range)
Solution Approach 1:
The chopper circuit periodically switches the input terminals and outputs of the amplifier at a frequency much higher than the bandwidth of the operational amplifier. This periodic action causes the offset voltage to average to zero over each switching cycle, effectively reducing the DC offset while maintaining circuit functionality.
Solution Approach 2:
The chopper circuit acts as an intermediary between the input terminals and the amplifier. By introducing this intermediate switching stage, the offset voltage is modulated and then averaged out, allowing the amplifier to operate with significantly reduced offset without requiring a completely different amplifier design.
2Measurement precision
If the voltage offset is reduced by using a chopper circuit, then the current sensing accuracy is improved, but the device complexity increases
Solution Approach 1:
The chopper circuit uses periodic switching at a frequency much higher than the amplifier bandwidth to modulate the offset voltage. This high-frequency switching causes the offset to average to zero over each cycle, achieving current sensing accuracy with less than 0.04 A offset while using standard operational amplifier components.
Solution Approach 2:
The chopper circuit uses the amplifier's own output to drive the switching action. The output of the amplifier feeds back through the chopper switches to the input terminals, creating a self-regulating system where the amplifier's output automatically corrects its own offset without requiring external control circuits.
3Measurement precision
If the input terminals are periodically reversed, then the offset voltage is averaged to zero, but the switching circuit complexity increases
Solution Approach 1:
The switching circuit periodically reverses the connections of the input terminals and outputs at a high frequency much greater than the amplifier bandwidth. This periodic reversal causes the offset voltage to be modulated and averaged to zero over each switching cycle, achieving offset cancellation with a relatively simple switching topology.
Solution Approach 2:
The chopper circuit combines the offset cancellation function with the signal amplification function in a single integrated structure. The same switching network that reverses the input terminals also processes the amplified signal, merging multiple functions into one circuit block and reducing overall complexity.
Applied Scientific Principles
This section explains which scientific principles are used to turn an abstract innovation direction into a practical engineering solution.
Function Achieved in This Case
This approach reduces the voltage offset by a factor of 10, improving the accuracy of output current sensing to less than 0.04 A, compared to 0.4 A without the chopper circuit, enhancing the precision of current measurement in power stages.
Implementation Method 1
A chopper circuit is integrated into the operational amplifier to periodically reverse the input terminals and outputs, using a differential amplifier, switching circuits, and an RC noise filter to generate an offset adjustment signal, effectively reducing the voltage offset to near zero by averaging the offset voltage over a 50% duty cycle.
Data Source
AI summary
A circuit for minimizing a voltage offset between inverting and non-inverting input terminals of an operational amplifier circuit is provided. The circuit includes a chopper circuit connected to the inverting and non-inverting input terminals of the operational amplifier circuit, the chopper circuit including: an amplifier having differential outputs; and a switching circuit for periodically reversing the input terminals to the amplifier and periodically reversing the outputs of the amplifier to provide an output signal having an offset adjustment signal to the operational amplifier circuit to adjust the offset of the operational amplifier circuit.