5.1 A 60V auto-zero and chopper operational amplifier with 800kHz interleaved clocks and input bias-current trimming

Abstract

Precision operational amplifiers (opamp) with 30V supply operation have been widely used to support industrial, instrumentation, and other applications [1]. Most of them have been realized with BJT or JFET processes [1] to offer voltage noise PSD better than 10nV/√Hz and offset voltage drift better than 1μV/°C. Recently, opamps with similar specifications have become available using CMOS based processes [2-4], which can offer a cheaper wafer price. Auto-zeroing and/or chopping are used as essential techniques to reduce offset voltage drift and 1/f noise associated with CMOS input differential pairs. The switching action of those techniques, however, results in unwanted output ripples and glitches, which requires a post-filter and limits usable signal bandwidth. Increasing the switching frequency can extend the usable signal bandwidth, though it introduces DC errors such as offset voltage drift and input bias current. Maximum offset voltage drift of 0.02μV/°C and an input bias current of 600pA have been achieved [3], although the switching frequency at 60kHz limits the usable signal bandwidth. A high switching frequency of 333kHz has been achieved [2], while the maximum offset voltage drift and input bias current are 0.085μV/°C and 850pA, respectively.