An automatic calibration technique for the OPAMP and its former stage output offset

An automatic calibration technique is developed to trim the differential OPAMP's input-referred and its former output stage offset voltages. The offset voltage is reduced by digitally adjusting the resulting offset current at the input and its first output stage. Without using complex circuits, the technique just need a comparator, a simple control logical and a current DAC (I-DAC). The proposed circuits have been designed in a 0.25 mum digital CMOS process. The OPAMP can calibrate a maximum offset of plusmn6 mV. The measured input referred offset voltage is less than 100 muV