Design Note 140: Updated Operational Amplifier Selection Guide for Optimum Noise Performance
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 LT1028 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 figure 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.
Some other simplifications are made to facilitate comparison. For instance, the noise is referred to the input of the circuit so that the effect of the circuit gain, which will vary with application, does not confuse the issue. Also, the calculations assume a temperature of 27°C or 300°K.
The formula used to calculate the spot noise and the schematic of the circuit used are shown in Figure 1. Figures 2 through 4 plot the spot noise of selected op amps vs the equivalent source resistance. The first two plots show precision op amps intended for low frequency applications, whereas the last plot shows high speed voltage-feedback op amps. There are two plots for the low frequency op amps because at very low frequencies (less than about 200Hz) an additional noise mechanism, which is inversely proportional to frequency, becomes important. This is called 1/f or flicker noise. Figure 2 shows slightly higher levels of noise due to this contribution.
Studying the formula and the plots leads to several conclusions. The values of the resistors used should be as small as possible to minimize noise, but since the feedback resistor is a load on the output of the op amp, it must not be too small. For a small equivalent source resistance, the voltage noise dominates. As the resistance increases, the resistor noise becomes most important. When the source resistance is greater than 100k, the current noise dominates because the contribution of the current noise is proportional to Req, whereas the resistor noise is proportional to the √Req.
For low frequency applications and a source resistance greater than 100k, the LT1169 JFET input op amp is the obvious choice. Not only does the LT1169 have an extremely low current noise of 0.8fA/√Hz, it also has a very low voltage noise of 6nV/√Hz. The LT1169 also has excellent DC specifications, with a very low input bias current of 3pA (typical), which is maintained over the input common mode range, and a high gain of 120dB.
High speed op amps, here defined by slew rates greater than 100V/μs, are plotted in Figure 4. These op amps come in a wider range of speeds than the precision op amps plotted in Figures 2 and 3. The faster parts will generally have slightly more spot noise, but because they will most likely be selected on the basis of speed, a selection of parts is plotted. For example, the LT1354–LT1363 (these are single op amps; duals and quads are available) are close in noise performance and consequently cluster close together on the plot, but have a speed range of 12MHz GBW to 70MHz GBW.
The same information is presented in tabular form in Table 1.
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