Ultralow-Noise Chopper Amplifier With Low Input Charge Injection

摘要

A chopper amplifier is presented that achieves an ultralow voltage noise of 0.73 $hbox{nV}/sqrt{hbox{Hz}}$ down to a few millihertz. Excess white and $1/f$ current noise was observed, which increases with the chopping frequency. At the lowest chopping frequency of 570 Hz, a noise level of 40 $hbox{fA}/sqrt{hbox{Hz}}$ with a $1/f$ corner at 3 mHz is obtained, corresponding to a noise temperature around 1 K at the optimum source impedance of 18 $hbox{k}Omega$. Special care was taken to compensate the effect of switching spikes in the input chopper, resulting in a low charge injection into the signal source. The amplifier has a fixed gain of 1000. Postamplification and postfiltering with a wide range of settings, as well as optical output isolation based on a highly linear voltage-to-frequency converter, make the instrument well suited for demanding applications in metrology.