Wide common mode input operational amplifier with serially programmable output offset

Abstract

Operational amplifiers continue to develop to meet modern demands on performance. This document describes an operational amplifier designed for a highly specific telecommunications application - to serve as the buffer between an I/Q demodulator and Gm-C filter. Requirements set forth by this application call for a wide common mode input range, stretching from ground to the positive supply, and a programmable output offset, offering up to ±25mV differentially from the output common mode. This programmable offset is implemented using two complementary current mode digital-to-analog converters (DACs) directly driving the input pins of the operational amplifier. The output offset is serially programmed with a three pin SPI interface. In addition to programming the offset DACs, the SPI interface is also designed to be able to program four additional parameters necessary for the function of the programmable Gm-C filter, into which the opamp will be integrated.