Compact Fully-Differential CMOS Current Driver for Bioimpedance Measurements

Abstract

A fully-differential CMOS current driver, suitable to provide the excitation signal in an on-chip bioimpedance measurement system, is presented. The circuit, able to convert the voltage waveform produced by a sinusoidal signal generator into a highly-linear sinusoidal current, consists of a linearized voltage-to-current (V-to-I) converter and a common-mode feedback (CMFB) network. The transconductance, maximum output current, and DC level of the output voltage can be adjusted in a straightforward manner, whereas the maximum output current can be scaled if required by only modifying the gain of two current mirrors. The circuit has been designed in 180 nm CMOS technology to operate with a 1.8-V supply. The simulated output resistance of the current driver has been found to be <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$62.9\ \mathrm{M}\Omega$</tex> at low frequencies and it is kept above <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$250\ \mathrm{k}\Omega$</tex> for a frequency equal to 1 MHz. A maximum output current of <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$3.7\ \mu\mathrm{A}$</tex> and 1 MHz with a THD of −40 dB is obtained when a <tex xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">$150-\text{mV}_{pp}$</tex> input voltage is applied.