Many auto-zero or chopper operational amplifiers have been reported with low offset and low-offset drift. The resulting baseband noise can also be a significant error source, and thus reducing the total error down to sub μV levels at DC and low frequencies has been targeted. Chopping is more suitable to lower the baseband noise PSD, as it is primarily determined by the broad-band thermal noise floor and not aliased by the high frequency noise [1]. Reducing the thermal noise floor requires higher value of input transconductance [1]. Consequently, more capacitance or higher output transconductance is required to stabilize the overall feedback loop with regular Miller compensation. A 6.5nV/√Hz conditionally stable chopper operational amplifier has been reported, using multi-pole feedforward compensation techniques [2]. Unconditionally stable chopper operational amplifiers presented so far exhibit more than 10nV/√Hz noise PSD [1,3–5]. This paper reports a 5.9nV/√Hz unconditionally stable chopper operational amplifier with 1.47mA supply current and 1.26mm 2  die area, achieved by phase compensation using current attenuation. In addition, adaptive clock level shift and backgate biasing for the input chopping allows optimization for noise and offset, realizing 0.78μV maximum offset with a worst-case 28.3nV/°C drift.
['Yoshinori Kusuda']
A 5.9nV/√Hz chopper operational amplifier with 0.78μV maximum offset and 28.3nV/°C offset drift