Dynamic MOS sigmoid array folding analog-to-digital conversion

A dynamic, saturating difference circuit for large-scale parallel folding analog-to-digital conversion is presented. The circuit comprises a subthreshold nMOS transistor source-coupled to a capacitor, implementing a log-domain integrator. The output current is a logistic sigmoidal function of the change in voltage on the gate. Offset and gain of the differential sigmoid are controlled by timing of global clock signals and are independent of transistor mismatch. Folding operation for analog-to-digital conversion is obtained by differentially combining and integrating currents from a bank of sigmoid units. A 128-channel parallel bank of 4-bit Gray-code folding analog-to-digital converters measures 0.75 mm×2 mm in 0.5 μm CMOS and delivers 768 Msps at 82-mW power dissipation.