The input noise voltage of a quiet op amp is 1nv/sqrt(Hz) but there are plenty available with 20nV/sqrt(Hz).

• Op amps with bipolar front-ends have lower voltage noise and higher current noise than those with FET front-ends （低噪音运放的输入噪声电压可以达到1nv/sqrt(Hz) ，但是我们大量用到的运放具有20nV/sqrt(Hz)的输入噪音。