Ultralow-noise chopper amplifier with low input charge injection

A chopper amplifier is presented that achieves an ultralow voltage noise of 0.73 nV/√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 fA/√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 kΩ. 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.