Switched-Capacitor Amplifier Feedback for Low Input Current
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Solution Overview
Problem
SC amplifier arrangements used in instrumentation circuits introduce measurement errors due to input current, which is exacerbated by high output impedance of signal sources, leading to inaccuracies in voltage measurement.
Innovation Solution
Incorporating a compensation circuit with a compensation capacitor and a switching arrangement between the output and input of the SC amplifier, which is clock-controlled to feed back electric charge during a second clock phase, thereby reducing measurement errors by matching the capacitance of the compensation capacitor to the input capacitor based on the amplifier's gain factor.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Productivity
If a clock-controlled switch connects the input capacitor to the input in an SC amplifier arrangement, then the amplifier can operate with switched capacitor topology, but an input current is drawn from the input which causes measurement errors
Solution Approach 1:
The patent implements a feedback circuit that senses the input current drawn by the SC amplifier and generates a compensating current that is injected back into the input node. This feedback mechanism cancels out the harmful input current, eliminating measurement errors while preserving the switched capacitor amplifier's operational efficiency
Solution Approach 2:
The patent introduces an intermediary compensation circuit that includes a compensation capacitor and current injection mechanism. This intermediary component acts as a mediator between the SC amplifier and the input signal source, transferring the effect of input current to a compensation current that offsets the original harmful current without directly affecting the measurement
2Measurement precision
If the input capacitor capacitance is increased to reduce input current effect, then measurement accuracy improves, but the device complexity and component values increase
Solution Approach 1:
Instead of increasing capacitor values, the patent uses a feedback mechanism that actively compensates for the input current effect. This allows the use of smaller, more practical capacitor values while maintaining high measurement accuracy through dynamic current compensation rather than static capacitance scaling
3Reliability
If the output impedance of the signal source is high, then the signal source can provide better voltage isolation, but the input current of the SC amplifier causes larger measurement errors
Solution Approach 1:
The feedback compensation circuit is particularly effective for high-impedance signal sources because it actively counteracts the input current effect regardless of the source impedance. The compensation current is proportional to the input current, ensuring accurate compensation even when the high source impedance limits the available current and magnifies the measurement error
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 configuration significantly reduces measurement errors caused by input current, ensuring accurate voltage measurement by compensating for the charge removed during the first clock phase, even in non-ideal operational amplifier conditions.
Implementation Method 1
at least one compensation circuit, which is connected between the output and the input and which has a compensation capacitor and a switching arrangement, with the switching arrangement being designed to connect under clock control the compensation capacitor between the output and the input
Data Source
AI summary
An SC amplifier arrangement and a method for measuring an input voltage are described.