Operational Amplifier Selection Guide for
Optimum Noise Performance – Design Note 140
Frank Cox
Eight years ago, George Erdi wrote a very useful Design
Note (DN6) that presented information to aid in the
selection of op amps for optimum noise performance,
in both graphical and tabular form. Design Note 140 is
an update of DN6. It covers new low noise op amps as
well as some high speed op amps. Although a great deal
has changed in eight years, especially in electronics,
noise is still a critical issue in op amp circuit design
and the LTВ®1028 is still the lowest noise op amp for
low source impedance applications.
The amount of noise an op amp circuit will produce
is determined by the device used, the total resistance
in the circuit, the bandwidth of the measurement, the
temperature of the circuit and the gain of the circuit. A
convenient п¬Ѓgure of merit for the noise performance
of an op amp is the spectral density or spot noise. This
is obtained by normalizing the measurement to a unit
of bandwidth. Here the unit is 1Hz and the noise is reported as “nV/√Hz.” The noise in a particular application
bandwidth can be calculated by multiplying the spot
noise by the square root of the application bandwidth. …