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 LT®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 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. 10/96/140_conv R2 VTR2 Vn + In – R3 VTR3 R1 VTR1 DN140 F01 WHERE: V TR1, V TR2 and V TR3 ARE THERMAL NOISE FROM RESISTORS Req = R2 + (R1)(R3) R1 + R3 4kT = (16.56)(10)– 21 J AND Vn IS THE VOLTAGE SPOT NOISE AND In IS THE CURRENT SPOT NOISE OF THE OP AMP AS GIVEN ON THE DATA SHEET. V = √(4kT)Req + Vn2 + In2(Req2) IS THE INPUT REFERRED SPOT NOISE IN A 1Hz BANDWIDTH. Figure 1 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 L, LT, LTC, LTM, Linear Technology and the Linear logo are registered trademarks of Linear Technology Corporation. All other trademarks are the property of their respective owners. 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. 1000 300 LT1028 LT1128 LT1124 100 LT1007 LT1037 30 LT1169 10 3 RESISTOR NOISE ONLY 1 0.3 10 100k 1M 10M 100 1k 10k EQUIVALENT SOURCE RESISTANCE (Ω) DN140 F03 Figure 3. 1kHz Spot Noise vs Equivalent Source Resistance 1000 LT1124 LT1028 LT1128 100 LT1001 LT1169 LT1007 LT1037 30 10 3 LT1220 LT1221 LT1222 LT1224 LT1225 LT1226 100 30 10 LT1351 3 RESISTOR NOISE ONLY 1 LT1334 LT1357 LT1360 LT1363 300 SPOT NOISE (nV/√Hz) 300 SPOT NOISE (nV/√Hz) 1000 SPOT NOISE (nV/√Hz) current of 3pA (typical), which is maintained over the input common mode range, and a high gain of 120dB. 1 0.3 RESISTOR NOISE ONLY 0.3 10 100k 1M 10M 100 1k 10k EQUIVALENT SOURCE RESISTANCE (Ω) 10 100k 1M 100 1k 10k EQUIVALENT SOURCE RESISTANCE (Ω) DN140 F02 DN140 F04 Figure 2. 10kHz Spot Noise vs Equivalent Source Resistance Figure 4. 10kHz Spot Noise vs Equivalent Source Resistance (High Speed Ampliﬁers) Table 1. Best Op Amp for Lowest Noise vs Source Resistance BEST OP AMP SOURCE R (Req) 10Hz PRECISION 1000Hz PRECISION 10kHz HIGH SPEED 0Ω to 500Ω LT1028, LT1115, LT1128 LT1028, LT1115, LT1128 LT1220/21/22/24/25/26 500Ω to 1.5k LT1007, LT1037 LT1028, LT1115, LT1128 LT1220/21/22/24/25/26 1.5k to 3k LT1124/25/26/27 LT1028, LT1115, LT1128 LT1220/21/22/24/25/26 3k to 5k LT1124/25/26/27 LT1007, LT1037 LT1220/21/22/24/25/26 5k to 10k LT1124/25/26/27 LT1124/25/26/27 LT1354/57/60/63 10k to 20k LT1001/02 LT1113, LT1124/25/26/27 LT1354/57/60/63 20k to 100k LT1001/02 LT1055/56/57/58, LT1113, LT1169 LT1351 100k to 1M LT1022, LT1055/56/57/58, LT1113, LT1122, LT1169 LT1022, LT1055/56/57/58, LT1113 LT1122, LT1169, LT1457 LT1351 1M to 10M LT1022, LT1055/56/57/58, LT1113, LT1122, LT1169 LT1022, LT1055/56/57/58, LT1113 LT1122, LT1169, LT1457 Data Sheet Download www.linear.com Linear Technology Corporation For applications help, call (408) 432-1900 dn140f_conv LT/GP 1096 155K • PRINTED IN THE USA 1630 McCarthy Blvd., Milpitas, CA 95035-7417 (408) 432-1900 ● FAX: (408) 434-0507 ● www.linear.com © LINEAR TECHNOLOGY CORPORATION 1996

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