A 350μW CMOS MSK Transmitter and 400μW OOK Super-Regenerative Receiver for Medical Implant Communications

A 350μW MSK direct modulation transmitter and a 400μW OOK super-regenerative receiver (SRR) are implemented in 90nm CMOS technology. The transceiver tunes 24MHz in frequency steps smaller than 2kHz and is designed to meet the specifications of the Medical Implant Communications Service (MICS) standard in the 402–405MHz band. The transmitter meets MICS mask specifications with data rates up to 120kbps, and the receiver has a sensitivity better than –99dBm with a data rate of 40kbps or –93dBm with a data rate of 120kbps. Introduction In 1999, the FCC allocated the frequency band of 402– 405MHz to the MICS standard for short-range (∼2m) data communications with implanted medical devices [1]. The band is divided into ten 300kHz channels, and a maximum of 25μW of effective isotropic radiation power (EIRP) can be transmitted over each. Frequency stability must be within 100ppm (∼40kHz) over the temperature range of 25–45◦C. In this work, we maximally exploit the low output power and modest temperature stability requirements to reduce overall power consumption. The resulting power dissipation is over an order of magnitude lower than commercially available units [2]. The critical insight behind our transceiver architecture is that temperature regulation in the human body naturally reduces oscillator frequency drift. This allows us to replace the frequency synthesizer with a distributed feedback loop between the base-station and the implant. The base-station periodically measures DCO frequency drift and sends correction data to the implant. By eliminating the crystal oscillator (XO), the power, volume, cost, and system startup time are reduced, allowing more efficient duty cycling since XOs generally take milliseconds to stabilize. The transmitter and receiver (Fig. 1) center around a differential Colpitts digitally-controlled oscillator (DCO) with a switching current source [3](Fig. 2). We exploit the low output power requirement of the MICS standard by incorporating a PCB loop antenna as the inductive element in the DCO’s resonant tank. The antenna’s high Q results in better noise performance, and its power dissipation is not simply wasted as thermal loss, but radiated to transmit information. FSK data is transmitted by directly modulating the DCO with digital data, and a SRR architecture is used to achieve excellent sensitivity with minimal power consumption and allow a direct connection between the antenna and the oscillator. The DCO can tune within 1kHz (∼2.5ppm) of a desired frequency over the range of 391–415MHz by sub-ranging the total ∗The authors acknowledge the support of the Focus Center for Circuit & System Solutions (C2S2), one of five research centers funded under the Focus Center Research Program, a Semiconductor Research Corporation Program. PCB Antenna SPI DCO NC<4:0>, NM<4:0>, NF<7:0>