Simple oscillators-based readout circuit for low-power biomedical implant system

Abstract

This paper reports a wireless sensor readout circuit for continuous physiological parameters monitoring including a potentiostat, a data generation unit and a frequency-shift-keying (FSK) modulator unit with the low drop-out (LDO) regulator for biomedical implant system. The potentiostat can generate an output potential of 0.7 V for the data generation unit. The data generation unit is designed based on a relaxation oscillator scheme and can be used to sense a current signal from any amperometric biomedical sensor and convert the signal to a square waveform in which the frequency of the square wave signal is proportional to the sensor current. FSK modulation scheme has been selected for wireless transmission. Designed with a very simple ring oscillator, this modulator integrates the modulation functionality into the oscillator itself by using the data signal to control the oscillation frequency. The prototype circuits have been fabricated in a 0.35 μm bulk complementary metal-oxide semiconductor (CMOS) process. Working with a regulated 1.8 V supply, the potentiostat consumes only 2 μA of current while the data generation unit can generate around 15.7 kHz output frequency with an input current of 1 μA. The FSK modulator consumes a total current of around 19 μA for a carrier frequency around 1 MHz. An off-chip demodulator is constructed to demodulate the data signal from the FSK modulator and the demodulated signal has less than 1.6 % variation of frequency.