Investigating the linearity of MOSFET-only switched-capacitor ΔΣ modulators under low-voltage condition

Abstract

Effect of asymmetric operating condition of depletion-mode MOS capacitors on the linearity of MOSFET-only switched-capacitor ΔΣ modulators is investigated. In very low-voltage switched-capacitor circuits the opamp input and output common-mode levels are separated to provide maximum overdrive voltage for MOS switches. This causes an asymmetric voltage drop on capacitors during operation. On the other hand, the depletion-mode MOS capacitors are most linear when operated under a symmetric condition. The effect of this increased nonlinearity on a ΔΣ modulator which is operated under an asymmetric condition is simulated and the results are compared with a ΔΣ modulator which is operated under a symmetric condition.