Programmable CMOS current signal generator for simultaneous multi-sine bioimpedance analysis

A fully-differential CMOS current signal generator, suitable for on-chip simultaneous multi-sine bioimpedance spectroscopy, is presented. The proposal is based on generating sinusoidal voltage signals, which are converted into currents and summed in a multiple-input current driver. The oscillators rely on a transconductor-capacitor ($G_m$-C) structure, which allows low-power and wide-frequency-range features. Each input channel of the current driver is a linearized voltage-to-current converter, to deliver a highly-linear multi-sine excitation current. A common-mode feedback (CMFB) network is used to set the DC component of the output voltage, also leading to a high output impedance. The output current can be digitally programmed by means of a 3-bit control signal, which allows measuring a wide range of impedances under test. The circuit has been designed and fabricated in 180 nm CMOS technology to operate with a 1.8-V supply. The output resistance of the current driver has been found to be above 1 M$\Omega$ at low frequencies for the maximum output current of $62.5\mu A$ and it is kept higher than approximately 20 k$\Omega$ for a frequency equal to 1 MHz and the same output current level. The output current can be tuned in the range [9.7,62.5] $\mu$A ensuring that the individual frequency components present a THD lower than $-$40 dB.