A High-Voltage Compliant Microelectrode Array Driver for Neuro Prosthesis

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High-voltage switching
A fundamental component of this microelectrode driver is a high-voltage switch and it is necessary to characterize the implemented switch for different stimulation current levels. An analog CMOS switch can be an n-type or p-type MOSFET which is operated either in cut-off (OFF state) or ohmic region
(ON state). The switch can be characterized by a fixed linear transconductance, gds , which is strongly signal-dependent and expressed according to Eq. 8 [37] where, μn and μp are electron and hole mobility respectively in two types of transistors, Cox is the gate-oxide capacitance, W n and W p are the dimensions of transistors, VDD is the power supply
L voltage, and Vin is the input signal. The bulk of NMOS and PMOS transistors are connected to the highest potential, VDD and the lowest potential, VSS respectively. The limitations are that the maximum voltage an NMOS and the minimum voltage a PMOS can pass are [VDD − Vthn ] and |Vthp | respectively.
This drawback can be overcome using CMOS transmission gate (TG) which exhibits lower overall transconductance. Another advantage of using TG based switch is that it passes good ‘1’ (or logic level high) and good ‘0’ (logic level low) without any threshold voltage drop, or in other words, TG gate switch allows rail-to-rail swings. The transconductance of TG gate switch is expressed by the following equation: The on resistance RsON of the switch is the reciprocal of its transconductance, gdsON and given by

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