Ultra-low noise chopper amplifier with low input charge injection

A chopper amplifier is presented which achieves an ultra-low voltage noise of 0.73 nV/√Hz down to a few mHz. 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 kO. Special care was taken to compensate the effect of switching spikes in the input chopper, resulting in low charge injection into the signal source.