LIMITER WITH SWITCHED CAPACITOR HYSTERESIS
Original Publication Date: 1992-Dec-01
Included in the Prior Art Database: 2002-Jan-18
A well-known way of adding hysteresis to a compa- rator is the Schmitt trigger conftguration which uses pos- itive feedback through a resistor. However, there are draw- backs to creating hysteresis using integrated resistors. First, the resistor ratio which creates the hysteresis level is not well controlled when using integrated resistors. High resistivity well diffusions are typically used to cre- ate large valued integrated resistors, but the thickness of each well resistor, and thus its resistance value, is voltage- dependent due to the reverse bias between the well and the substrate. Well diisions are also hard to match because of nonuniform out-diffusion. Second, large sili- con area is used because of the large resistor values needed to minimiied current drain. Third, if hysteresis programmability is desired, adding transmission gates in series with resistors increases inaccuracy and/or sili- con area used due to the extremely large size transmis- sion gates needed for low "on" resistance. Finally, auto- zeroing to eliminate hysteresis inaccuracy due to comparator input offset is not possible with a continu- ous time circuit.