Abstract
Abstract. In this work an Application Specific Integrated Circuit (ASIC) for an implantable electrochemical biosensor system (SMART implant, Stett et al., 2014) is presented. The ASIC drives the measurement electrodes and performs amperometric measurements for determining the oxygen concentration, potentiometric measurements for evaluating the pH-level as well as temperature measurements. A 10-bit pipeline analog to digital (ADC) is used to digitize the acquired analog samples and is implemented as a single stage to reduce power consumption and chip area. For pH measurements, an offset subtraction technique is employed to raise the resolution to 12-bits. Charge integration is utilized for oxygen and temperature measurements with the capability to cover current ranges between 30 nA and 1 μA. In order to achieve good performance over a wide range of supply and process variations, internal reference voltages are generated from a programmable band-gap regulated circuit and biasing currents are supplied from a wide-range bootstrap current reference. To accommodate the limited available electrical power, all components are designed for low power operation. Also a sequential operation approach is applied, in which essential circuit building blocks are time multiplexed between different measurement types. All measurement sequences and parameters are programmable and can be adjusted for different tissues and media. The chip communicates with external unites through a full duplex two-wire Serial Peripheral Interface (SPI), which receives operational instructions and at the same time outputs the internally stored measurement data. The circuit has been fabricated in a standard 0.5-μm CMOS process and operates on a supply as low as 2.7 V. Measurement results show good performance and agree with circuit simulation. It consumes a maximum of 500 μA DC current and is clocked between 500 kHz and 4 MHz according to the measurement parameters. Measurement results of the on-chip ADC show a Differential Non Linearity (DNL) lower than 0.5 LSB, an Integral Non Linearity (INL) lower than 1 LSB and a Figure of Merit (FOM) of 6 pJ/conversion.
Highlights
An integral part of all types of active implantable medical devices, such as cochlear and brain implants, is the electronic module
Erated by the measurement electrodes, which corresponds to the measured pH value and lays between 0.7 and 2.3 V, is first converted to a 10 bit “coarse” word, according to the conversion result, a known offset generated by the on-chip 5-bit Digital to Analog Converter (DAC) is subtracted from the original input
In the case of O2 and temperature measurement, the measured analog signals are currents, where for the former a three electrode measurement setup is implemented and current flowing between the working electrode and the counter electrode is of interest (Kubon et al, 2010)
Summary
An integral part of all types of active implantable medical devices, such as cochlear and brain implants, is the electronic module. For monitoring of neuronal and metabolic activity a readout chip has to be implemented, which controls the data acquisition and management. For applications where large batteries and cabling is not suitable, stringent requirements on the readout chip in terms of size and energy efficiency are placed. The system contains a Read-Out Application Specific Integrated Circuit (RO-ASIC, or ROIC) which controls the measurement electrodes connected directly to the tissue or material to be characterized. A microcontroller sends instructions to the ROIC, receives the results and relays them to the power and data management ASIC. This frontend chip is responsible for power supply regulation of the implant and for the transmission of data via an inductive interface to an external reader unit placed outside the body. The aim here is not to go through all the numerous analog and digital circuit blocks individually, but rather to give a description of the chip’s functionality and structure, highlighting issues of energy efficiency, programmability and reliability of operation, in addition to the design techniques employed to approach these aspects
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