A 570-kbps ASK demodulator without external capacitors for low-frequency wireless bio-implants
Introduction
The implantable micro-stimulator is one of the recent major medical research topics. It is widely used in bladder leakage control [1], muscle nerve stimulation [2], and cochlear implants [3]. The implanted device can be powered by an RF transcutaneous magnetic coupling method using an external transmitter coil to power and communicate with the implanted devices. One of the most important issues for implantable devices is the size. Notably, living tissues have a high absorption rate if the frequency of the wireless RF induction is high, which will cause the temperature of the living tissues to rise drastically. Therefore, all of the RF coupling methods used for biomedical implants are really low, e.g., 13.56 MHz, or even 2 MHz. Meanwhile, most of the prior implanted devices adopted ASK (amplitude shift keying) modulation because of the simplicity of the demodulation circuit [4]. However, those prior designs contained a large capacitor to cope with the low RF frequency [5], [6], [7]. The capacitors are even larger than 10 pF. Such a large capacitor either occupies a huge area in the SOC (system-on-chip) or becomes a discrete component on a PCB. Either way will increase the size of the implants. Reducing the number of the these large components is important for integration of an SOC. Thus, we propose an ASK demodulator design containing no large capacitor at all such that a much smaller size can be achieved. The entire circuit is carried out using TSMC (Taiwan Semiconductor Manufacturing Company) 2P4M CMOS technology. The area of the proposed ASK demodulator is , which is smaller than all of the prior works.
Moreover, a high carrier frequency and the higher modulation index usually imply a high data rate. However, the high carrier frequency might increase the power consumption of the overall system. Furthermore, high frequency signal could be absorbed by the tissues as we mentioned in the previous paragraph such that the transmission efficiency is decreased and the tissue might be hurt. Thus, the carrier frequency is often chosen to be less than 15 MHz for most of the implantable applications [4]. Besides, because the implantable devices usually employ wireless link to obtain the required power from an external transmitter, a small modulation index is much more preferred to improve the stability of the received power when the ASK modulation is used. Thus, it is preferred that high data rate can be achieved at such a low modulation index and a low carrier frequency for the bio-implant. The measurement results reveal that a 570 kbps data rate can be accomplished by the proposed ASK demodulator for 10% modulation index at 2 MHz carrier frequency. The physical measurements of the prototype chip justify the outstanding performance of the proposed design.
Section snippets
ASK demodulator without external capacitors
The entire micro-stimulator system is given in Fig. 1. The external control module includes a PC, a class E amplifier, and a transmitter coil. ASK modulation protocol is employed to transfer the external control data and power to the internal stimulation chip by the class E amplifier. An on-chip power regulator is required to supply a stable VDD output voltage to the internal core by regulating the power generated by the on-board coupling coil. The baseband circuit decodes the command from the
Simulation and measurement
The chip is designed by using TSMC (Taiwan Semiconductor Manufacturing Company) 2P4M CMOS process. The die photo of the analog circuit of the implantable neural chip is shown in Fig. 6, where “Clock & POR Gen.” block denotes the clock generator and the power-on-reset generator of the implantable chip. The area of the proposed ASK demodulator is . The maximum power dissipation of the whole chip (including the circuit in Fig. 6) is measured to be less than 12 mW. Fig. 7 is the
Conclusion
A small-area solution for an implantable ASK demodulator is presented in this work. The MOS count and the capacitor size compared with prior works are clearly analyzed. The data rate for the proposed ASK demodulator is measured to be 570 kbps for a 10% modulation index at 2 MHz carrier frequency. The measurement results on silicon justify the proposed ASK demodulator.
Acknowledgments
The authors would like to express their deepest gratefulness to CIC (Chip Implementation Center) of NAPL (National Applied Research Laboratories), Taiwan, for their thoughtful chip fabrication service. Moreover, this research was partially supported by National Health Research Institutes under Grant NHRI-EX95-9319EI and NHRI-EX97-9732EI. The authors also like to thank “Aim for Top University Plan” project of NSYSU and Ministry of Education, Taiwan, for partially supporting the research.
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