Capacitance Measurement Circuit With Two-Stage Current-Mirror Sensing
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Solution Overview
Problem
Existing capacitance measurement technologies struggle with high measurement errors and inadequate accuracy for small capacitance values, particularly below 200 fF, and replacing components for higher accuracy increases circuit costs.
Innovation Solution
A capacitance measurement circuit utilizing an analog front-end circuit with parallel current mirror circuits, an ADC, and a controller to perform two-stage capacitance measurement, including a rough measurement to determine the connection number of current mirror circuits and a fine measurement to improve accuracy.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional capacitance measurement methods are used, then the measurement can be performed with simple circuit components, but the measurement accuracy is insufficient for small capacitance values below 200 fF
Solution Approach 1:
The measurement process is divided into two stages: a first measurement stage and a second measurement stage. The first measurement stage performs a rough measurement to determine the capacitance range, while the second measurement stage performs a precise measurement for the final result. This segmentation allows the system to achieve high measurement accuracy for small capacitance values without requiring complex high-precision components throughout the entire measurement process.
Solution Approach 2:
The circuit dynamically adjusts the connection configuration of current mirror circuits based on the measurement stage. During the first measurement stage, a first connection number of current mirror circuits is connected, and during the second measurement stage, a second connection number of current mirror circuits is connected. This dynamic reconfiguration optimizes the measurement process for different stages, achieving high precision without permanently requiring the most complex configuration.
2Measurement precision
If components with higher accuracy are replaced, then the measurement accuracy for small capacitance values improves, but the circuit cost increases greatly
Solution Approach 1:
The measurement process is divided into two stages: a first measurement stage and a second measurement stage. The first measurement stage performs a rough measurement to determine the capacitance range, while the second measurement stage performs a precise measurement for the final result. This segmentation allows the system to achieve high measurement accuracy for small capacitance values without requiring complex high-precision components throughout the entire measurement process.
Solution Approach 2:
The system uses standard components but applies them in a partially optimized manner. Instead of using high-precision components throughout, the system uses standard components in a two-stage measurement process where the second stage provides the necessary precision only when needed, based on the range determination from the first stage.
3Measurement precision
If a single measurement stage is used, then the measurement process is simple and fast, but the measurement error is high for small capacitance values
Solution Approach 1:
The measurement process is divided into two stages: a first measurement stage and a second measurement stage. The first measurement stage performs a rough measurement to determine the capacitance range, while the second measurement stage performs a precise measurement for the final result. This segmentation allows the system to achieve high measurement accuracy for small capacitance values without requiring complex high-precision components throughout the entire measurement process.
Solution Approach 2:
The first measurement stage performs a preliminary rough measurement to determine the capacitance range before the second precise measurement stage. This preliminary action allows the system to quickly eliminate large errors in the first stage, enabling the second stage to focus on achieving high precision without needing to handle the full range of possible values, thus reducing overall measurement time.
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
The circuit achieves high accuracy in detecting small capacitance values by reducing measurement errors and improving precision, allowing for capacitance measurement within a large range with a maximum error of 3 LSBs in the 1-pF measurement range.
Implementation Method 1
The analog front-end circuit includes M numbers of current mirror circuits connected in parallel
Implementation Method 2
an analog front-end circuit, a parasitic capacitor
Data Source
AI summary
A capacitance measurement circuit includes an analog front-end (AFE) circuit with current mirror circuits, a parasitic capacitor, an AD converter, an output shift register, and a controller. The controller disconnects a capacitor to be measured and connects one current mirror circuit to record an AFE output voltage VN collected at an inverting input terminal of the AD converter, then connects the capacitor to be measured to collect an AFE output voltage VP at a non-inverting input terminal of the AD converter, and converts a value of (VP−VN) into a first digital signal; determines, based on the value of the first digital signal, a connection number m of the current mirror circuits, controls the analog front-end circuit to connect m current mirror circuits, and repeat the steps to obtain a second digital signal; and shift the second digital signal based on the connection number m to obtain a capacitance measurement value.