Impedance Measurement Circuit with Chopper Calibration
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Solution Overview
Problem
Conventional impedance measurement systems face accuracy issues due to non-linearity, mismatch, and offset errors, particularly in biomedical applications requiring precise measurements across a wide range of impedances.
Innovation Solution
The system employs a DSP unit to digitize and store offset error values during calibration, which are then subtracted from impedance measurements, using a high-pass filter to reject DC offset and a chopper for error compensation, enhancing measurement accuracy by separating current and voltage electrodes.
Engineering Contradictions & Design Principles
Engineering Contradiction Analysis
1Measurement precision
If conventional impedance measurement systems use integrated circuit design, then the system is compact and functional, but measurement accuracy deteriorates due to non-linearity, mismatch, and offset errors
Solution Approach 1:
The system divides the impedance measurement function into separate components: a current generator unit, an analogue impedance readout unit, an ADC unit, and a DSP unit. This segmentation allows each component to be optimized independently, with the DSP unit performing complex error compensation calculations that would be difficult to implement in a fully integrated circuit.
Solution Approach 2:
The patent introduces a DSP unit as an intermediary between the ADC unit and the final measurement output. This intermediary component receives the digitized impedance signal, retrieves stored offset error values from memory, performs subtraction to compensate for errors, and produces the corrected impedance measurement, thereby overcoming the limitations of direct integrated circuit design.
2Measurement precision
If offset error compensation is implemented through calibration and subtraction, then measurement accuracy improves, but system complexity and calibration procedures increase
Solution Approach 1:
The system performs offset error measurement and storage during a calibration phase before actual impedance measurements are taken. The DSP unit measures the offset error of the analogue impedance readout unit and ADC unit, stores these values in memory, and then uses them to compensate for errors in subsequent measurements. This preliminary action simplifies the measurement phase while achieving high accuracy.
Solution Approach 2:
The system implements a feedback mechanism where the measured offset error is fed back into the measurement process. The DSP unit continuously retrieves the stored offset error values and subtracts them from the digitized impedance signals, creating a closed-loop error compensation system that maintains measurement accuracy across varying operating conditions.
3Measurement precision
If high precision measurements are required across wide impedance ranges, then measurement capability improves, but error from non-linearity and mismatch increases
Solution Approach 1:
The system changes the parameter being measured and compensated - specifically, it measures and compensates for offset error as a separate parameter before calculating the final impedance value. The DSP unit subtracts the offset error from the digitized impedance signal, allowing accurate measurement of small impedance changes (milliohm range) superimposed on large baseline impedances (kilo-ohm range) without the measurements being corrupted by systematic errors.
Data Source
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Figure 5A~5B
AI summary
The invention relates to a circuit (40) for impedance measurement comprising a switch matrix (43) connected to a chopper (46). The switch matrix (43) comprises a plurality of switches (S1 to S6) and is configured to receive an analogue signal (IN1, IN2). In operation, during a calibration period (PH1), the circuit (40) is configured to activate the chopper (46) and to control the switch matrix (43) in order to disconnect the received analog signal (IN1, IN2) from the chopper inputs (IN1', IN2') and to connect a voltage reference signal (VR) to the chopper inputs (IN1', IN2'). In operation, during a measurement period (PH2), the circuit (40) is configured to deactivate the chopper (46) and to control the switch matrix (43) in order to disconnect the voltage reference signal (VR) from the chopper inputs (IN1', IN2') and to connect the received analog signal (IN1, IN2) to the chopper inputs (IN1', IN2'). The invention also relates to a system and to a method for impedance measurement.