Compact Fully-Differential CMOS Current Driver for Bioimpedance Measurements

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 62.9 M Omega at low frequencies and it is kept above 250 k Omega for a frequency equal to 1 MHz. A maximum output current of 3.7 mu A and 1 MHz with a THD of 40 dB is obtained when a 150-mV pp input voltage is applied.