Method and Apparatus for an Active Negative-Capacitor Circuit to Cancel the Input Capacitance of Comparators

摘要

The differential output of a Programmable Gain Amplifier (PGA) is loaded by the input differential gate capacitance of a plurality of Analog to Digital convertors (ADC) comparators and the differential metal layer traces to interconnect these comparators to the PGA. The differential capacitive load presented to the PGA is quite large and reduces the bandwidth of this interconnect between the PGA and ADC. To overcome the performance degradation due to the differential capacitive load, an active negative-capacitor circuit cancels the effect of the large input capacitance of the ADC comparators. This cancelation extends the gain characteristics of the interconnect between the PGA's output and the inputs of the first stage of the comparators. The active negative-capacitance is comprised of a cross pair NMOS with a capacitor connecting their sources where each NMOS is biased by a current source.

主权项

1. A method of improving a bandwidth response of a programmable gain amplifier (PGA) loaded by a plurality of analog to digital convertors (ADC's) comprising the steps of:
coupling a differential output of said PGA to a differential interconnect; coupling a differential input of each said plurality of ADC's to said differential interconnect; and coupling a negative-capacitance circuit to said differential interconnect, wherein said negative-capacitance circuit comprises:
a first transistor cross-coupled to a second transistor with a capacitor coupling their sources together;a first current source coupled to said first transistor; anda second current source coupled to said second transistor.