Impedance Measuring Circuit with Capacitive DC Blocking
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Solution Overview
Problem
Impedance measuring circuits face challenges in accurately measuring impedance in human bodies due to the use of protective resistors, which reduce signal amplitude, and capacitors, which can hinder DC point determination and lead to unstable voltage levels.
Innovation Solution
The circuit employs capacitors connected in series with the target to remove DC voltage components and includes a voltage level setting circuit to stabilize the DC voltage level, allowing for accurate impedance measurement while ensuring compliance with current limits, using a DA converter to generate adjustable AC signals and an amplification circuit with a differential difference amplifier and low pass filter to manage signal components.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Reliability
If a protective resistor is used to limit current, then current safety is improved, but output signal amplitude decreases
Solution Approach 1:
The patent introduces a capacitor as an intermediary component connected in series with the protective resistor. The capacitor blocks DC current while allowing AC signals to pass through, thereby maintaining current safety through the resistor while preserving signal amplitude by preventing DC voltage drops from attenuating the AC measurement signal.
Solution Approach 2:
The patent changes the electrical parameters of the circuit by introducing capacitive reactance in series with the resistive current limiting element. This parameter change allows the circuit to differentiate between DC current (blocked by capacitor for safety) and AC measurement signals (passed through for measurement), thus improving both current safety and signal amplitude simultaneously.
2Measurement precision
If a capacitor is connected in series to remove DC voltage component, then noise removal is improved, but DC point determination becomes difficult
Solution Approach 1:
The patent uses the capacitor as a mediator that naturally blocks DC components while passing AC components. By combining this with the protective resistor, the circuit establishes a reference DC point at the capacitor-resistor junction, making DC point determination straightforward while maintaining effective noise removal through capacitive coupling.
3Measurement precision
If gain of amplifier is increased to compensate for small signal amplitude, then signal amplification is improved, but noise component increases
Solution Approach 1:
The patent converts the potentially harmful effect of the protective resistor (signal attenuation) into a benefit by combining it with a capacitor. This combination prevents DC voltage drops from reducing AC signal amplitude, thereby eliminating the need for high gain amplification that would otherwise amplify both signal and noise, thus improving signal-to-noise ratio.
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 increases output signal amplitude, prevents DC current flow, and stabilizes voltage levels, enabling precise impedance measurement even at low currents, effectively addressing the limitations of protective resistors and capacitors in existing circuits.
Implementation Method 1
a capacitor may be connected in series to the impedance in the target, thereby removing a DC voltage component that includes a noise component and detecting an AC signal component
Data Source
AI summary
An impedance measuring circuit has an amplifier connected to a target and to amplify an input AC voltage with a gain corresponding to an impedance in the target and to output the voltage, and a voltage level setting circuit to set a DC voltage level in an output node of the amplifier when a first capacitor is connected between one end of the target and an output node of the amplifier and a second capacitor is connected between another end of the target and a node with a voltage correlated with the input AC voltage.