Fully-Integrated, Digitally-Programmable Rising/Decaying Exponential Current Waveform Generator

This paper reports on an innovative fully-integrated exponential pulse generator. The circuit is capable of producing both rising and decaying exponential current pulses, which are digitally-programmable in time constant and peak amplitude. Intended for implantable neural stimulators, the width of generated pulses is determined using a timing pulse on the order of milliseconds. The circuit employs the exponential law of subthreshold-biased MOS transistors their gate-source voltages linearly swept using a ramp voltage. The ramp generator circuit designed for this purpose uses the difference between two digitally-programmable currents to linearly charge or discharge an on-chip capacitor. The circuit benefits from an embedded error-zeroing mechanism that compensates the destructive effect of device mismatches on the slope of the voltage ramps. This work is designed and microfabricated in a 180-nm standard CMOS process and occupies a total area of 0.022 mm<sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sup> (with a total on-chip capacitance of 1.5pF). Operated under a supply voltage of 1.6V, the circuit consumes a total power of about 4.6μW.