A MEMS Interface IC With Low-Power and Wide-Range Frequency-to-Voltage Converter for Biomedical Applications

Abstract

This paper presents an interface circuit for capacitive and inductive MEMS biosensors using an oscillator and a charge pump based frequency-to-voltage converter. Frequency modulation using a differential crossed coupled oscillator is adopted to sense capacitive and inductive changes. The frequency-to-voltage converter is designed with a negative feedback system and external controlling parameters to adjust the sensitivity, dynamic range, and nominal point for the measurement. The sensitivity of the frequency-to-voltage converter is from 13.28 to 35.96 mV/MHz depending on external voltage and charging current. The sensitivity ranges of the capacitive and inductive interface circuit are 17.08 to 54.4 mV/pF and 32.11 to 82.88 mV/mH, respectively. A capacitive MEMS based pH sensor is also connected with the interface circuit to measure the high acidic gastric acid throughout the digestive tract. The sensitivity for pH from 1 to 3 is 191.4 mV/pH with 550 μV(pp) noise. The readout circuit is designed and fabricated using the UMC 0.18 μm CMOS technology. It occupies an area of 0.18 mm (2) and consumes 11.8 mW.

Frequency Modulation based Resistive Sensing for Wearable Galvanic Skin Response

“…A sine-to-square (STS) converter circuit converts ∆Fvco from the VCO into time- period variation (∆Tvco). A frequency-to-voltage converter (FVC) circuit is designed to convert ∆Tvco as the voltage changes (∆V ) [11].…”

“…It is composed of two capacitors C1 and C2, transistors (M p13, M n14, M n15, and M p16) acting as switches, and two current sources I Ch and IDis. The description and operations of the FVC block are explained in [11].…”

“…In this paper, a frequency modulation based readout circuit is used for detecting galvanic skin resistance. A frequencyto-voltage converter (FVC) is used to convert the frequency shifts due to skin resistance into voltage change with adjustable sensitivity [11]. The feedback control system minimizes the overall power consumption and presents a low power alternative for resistive readout circuits.…”

A Capacitance-to-Time Converter-Based Electronic Interface for Differential Capacitive Sensors

“…Basically, their behavior can be simply described as a planar capacitor (i.e., C = ε·A/d, being ε the relative dielectric constant, A the active surface, and d the distance between capacitor metal plates) whose mechanical features (i.e., A and/or d) are temporarily changed by the physical phenomena to be detected. Furthermore, in several sensory systems, differential capacitive sensing configurations also provide a suitable reduction of the common-mode noise and the parasitic component effects [4][5][6][7][8][9][10][11][12][13][14][15].…”

“…In the literature, capacitive sensor interfaces mainly concern Capacitance-to-Voltage (C-V) and Capacitance-to-Time (C-T) analogue conversion techniques [15][16][17][18][19][20][21][22][23][24]. In particular, the first topology commonly employs voltage/transimpedance amplifiers, charge/chopper amplifiers, and switched 2 of 14 capacitors showing limited/reduced detection ranges, sensitivities, and resolutions mainly due to noise issues.…”