The input noise voltage of a quiet op amp is 1nv/sqrt(Hz) but there are plenty available with 20nV/sqrt(Hz).
英
美
Op amps with bipolar front-ends have lower voltage noise and higher current noise than those with FET front-ends (低噪音運放的輸入噪聲電壓可以達到1nv/sqrt(Hz) ,但是我們大量用到的運放具有20nV/sqrt(Hz)的輸入噪音。