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Browse Prior Art Database

LIMITER WITH SWITCHED CAPACITOR HYSTERESIS

IP.com Disclosure Number: IPCOM000006650D
Original Publication Date: 1992-Dec-01
Included in the Prior Art Database: 2002-Jan-18
Document File: 1 page(s) / 66K

Publishing Venue

Motorola

Related People

Virgilio A. Fernandez: AUTHOR [+2]

Abstract

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.

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MOTOROLA INC. Technical Developments Volume 17 December 1992

LIMITER WITH SWITCHED CAPACITOR HYSTERESIS

by Virgilio A. Fernandez and Norman Todd Rollins

  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.

In contrast, the limiter circuit shown uses switched-

capacitor techniques to create hysteresis. Switched- capacitor techniques have several advantages for inte- gration. Fist, capaci...