Analysis and Design of a High Speed Continuous-time Modulator Using the Assisted Opamp Technique Ankesh Jain, Muthusubramaniam V enkatesan

Abstract

We apply the “assisted opamp technique” to the design of a 1 GS/s single-bit continuous-time modulator (CTDSM) that achieves 10 bit resolution in 15.625 MHz band- width. The enhanced linearity and speed of th e first integrator of the modulator, necessitated by sin gle-bit operation, are obtained in a power ef ficient manner using opam p assistance. However, timing-skew between the feedback and assi stant DAC currents can be a potential problem at high speeds. We analyze and give intuition for the effects of timing mismatch in such CTDSMs, and show that opamp assistance is quite robust to timing errors. Measurement results from an implementation in a 0.13 mC M O S process show that the modulator achieves a dynamic range of 67 dB in 15.625 MHz bandwidth while c onsuming 4 mW.