Admittance-to-Digital-Impedance Converter for Wide-Frequency-Range Impedance Measurement System

• Published in: IEEE transactions on instrumentation and measurement Vol. 70; pp. 1 - 11
• Main Authors: Lin, Hao-Wei, Su, Chia-Ning, Chen, Tse-An, Chen, Yi-Chen, Syu, Mei-Jywan, Wei, Chia-Ling
• Format: Journal Article
• Language: English
• Published: New York IEEE 2021; The Institute of Electrical and Electronics Engineers, Inc. (IEEE)
• Subjects: Admittance; Albumins; biomedical integrated circuits; Converters; divider; Electrical impedance; Electrodes; Frequency measurement; Frequency synthesizers; Impedance; Impedance measurement; impedance measurement system; Semiconductor device measurement; Signal processing; signal processing chip; Sine waves; Synthesis
• ISSN: 0018-9456; 1557-9662
• DOI: 10.1109/TIM.2021.3073710

An admittance-to-digital-impedance converter (ADIC) for impedance measurement is proposed. The ADIC was integrated in a chip with a programmable sinusoidal frequency synthesizer, allowing an operating range from 0.1 Hz to 10 kHz. The frequency synthesizer generates sinusoidal voltage signals to a sample under test (SUT), and the resulting current signal from the SUT is sensed and sent to the ADIC for postprocessing. In fact, this sensed signal is proportional to the SUT admittance, rather than to its impedance. Hence, the proposed ADIC has two functions: taking the reciprocal of the sensed signal to make it proportional to impedance and converting it into a digital signal. Furthermore, two novel circuits are also proposed: a low-leakage magnitude detector that makes it possible to measure at sub-1-Hz frequencies and a stepped-pulse divider that performs an accurate dividing function. The chip was fabricated by using a 0.35-<inline-formula> <tex-math notation="LaTeX">\mu \text{m} </tex-math></inline-formula> 2P4M mixed-signal process. According to the measured results, the maximum errors in the magnitude and phase measurements were 1.6% and 1.8°, respectively, and the proposed system was verified by measuring the concentrations of albumin solutions.