In this chapter, CMOS comparator structures have been discussed. It is shown that an OTA can only be used for low frequency applications. If a fast response time is required, latched comparators must be employed. It is shown that the resolution of a latched comparator is a function of the clock feed-through and the transistor mismatches. To overcome those problems the advantages of the OTA-latch structure are analyzed. The effect of clock feed-through and transistor mismatch can now be reduced by increasing Gm. This can best be achieved by designing the input stage at the boundary of strong inversion (Vgs-Vt ≈ 200mV). As a result the accuracy of the comparator is reduced approximately to the offset voltage of a differential pair. The speed of this comparator is approximately equal to a loaded inverter. Hence accurate high speed comparators can be obtained.
High performance operational amplifiers and comparators, Page 1 of 2
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