platforma said:
I tried this topology but I have offset problem . these opamps are working 500Mhz but they have offset such as 4-5mV. but I have 20mV range for every step. 2^6=64 step . so 4mV is large for me when I decreased offset I have settling problem. I didnt find any opamp with low offset and high speed at 0.18tsmc . any suggestion for high speed and low offset opamp . or a new topology.
Lower offset means larger input FETs and hence lower fT.
For other topologies s. e.g. these papers:
Michael Choi and Asad A. Abidi: "A 6-b 1.3-Gsample/s A/D Converter in 0.35-µm CMOS"
IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 36, NO. 12, DECEMBER 2001
Peter C. S. Scholtens, Member and Maarten Vertregt: "A 6-b 1.6-Gsample/s Flash ADC in 0.18-µm CMOS Using Averaging Termination"
IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 37, NO. 12, DECEMBER 2002
Xicheng Jiang, Zhengyu Wang, M. Frank Chang: "A 2GS/s 6b ADC in 0.18μm CMOS"
ISSCC 2003 / SESSION 18 / NYQUIST A/D CONVERTERS / PAPER 18.3
2003 IEEE International Solid-State Circuits Conference 0-7803-7707-9/03
Xicheng Jiang and Mau-Chung Frank Chang: "A 1-GHz Signal Bandwidth 6-bit CMOS ADC With Power-Efficient Averaging" (0.18µm)
IEEE JOURNAL OF SOLID-STATE CIRCUITS, VOL. 40, NO. 2, FEBRUARY 2005