Updated Operational Amplifier Selection Guide for

1 Advertisement1096/140 Updated Operational Amplifier Selection Guide forOptimum Noise Performance Design Note 140 Frank CoxEight years ago, George Erdi wrote a very useful DesignNote (DN6) that presented information to aid in the selec-tion of op amps for optimum noise performance, in bothgraphical and tabular form. Design Note 140 is an update ofDN6. It covers new low noise op amps as well as some highspeed op amps. Although a great deal has changed in eightyears, especially in electronics, noise is still a critical issuein op amp circuit design and the LT 1028 is still the lowestnoise op amp for low source impedance amount of noise an op amp circuit will produce isdetermined by the device used, the total resistance in thecircuit, the bandwidth of the measurement, the tempera-ture of the circuit and the gain of the circuit.

2 A convenientfigure of merit for the noise performance of an op amp is thespectral density or spot noise. This is obtained by normaliz-ing the measurement to a unit of bandwidth. Here the unit is1Hz and the noise is reported as nV/ Hz. The noise in aparticular application bandwidth can be calculated by mul-tiplying the spot noise by the square root of the other simplifications are made to facilitate compari-son. For instance, the noise is referred to the input of thecircuit so that the effect of the circuit gain, which will varywith application, does not confuse the issue.

3 Also, thecalculations assume a temperature of 27 C or 300 formula used to calculate the spot noise and theschematic of the circuit used are shown in Figure 1. Figures2 through 4 plot the spot noise of selected op amps vs theequivalent source resistance. The first two plots showprecision op amps intended for low frequency applica-tions, whereas the last plot shows high speed voltage-feedback op amps. There are two plots for the lowfrequency op amps because at very low frequencies (lessthan about 200Hz) an additional noise mechanism, whichis inversely proportional to frequency, becomes is called 1/f or flicker noise.

4 Figure 2 shows slightlyhigher levels of noise due to this contribution., LTC and LT are registered trademarks of Linear Technology Corporation. WHERE: VTR1, VTR2 AND VTR3 ARE THERMAL NOISE FROM RESISTORSReq =4kT = ( )(10) 21 JR2 +(R1)(R3) R1 + R3))AND Vn IS THE VOLTAGE SPOT NOISE AND In IS THE CURRENT SPOT NOISE OF THEOP AMP AS GIVEN ON THE DATA = (4kT)Req + Vn2 + In2(Req2)IS THE INPUT REFERRED SPOT NOISE IN A 1Hz 1 Studying the formula and the plots leads to several conclu-sions. The values of the resistors used should be as small aspossible to minimize noise, but since the feedback resistoris a load on the output of the op amp, it must not be too a small equivalent source resistance, the voltage noisedominates.

5 As the resistance increases, the resistor noisebecomes most important. When the source resistance isgreater than 100k, the current noise dominates because thecontribution of the current noise is proportional to Req,whereas the resistor noise is proportional to the low frequency applications and a source resistancegreater than 100k, the LT1169 JFET input op amp is theobvious choice. Not only does the LT1169 have an ex-tremely low current noise of Hz , it also has a verylow voltage noise of 6nV/ Hz. The LT1169 also has excel-lent DC specifications, with a very low input bias current of3pA (typical), which is maintained over the input commonmode range, and a high gain of 120dB.

6 +InVnVTR2R2 VTR3R3 VTR1DN140 LINEAR TECHNOLOGY CORPORATION 1996LT/GP 1096 155K PRINTED IN THE USAL inear Technology Corporation1630 McCarthy Blvd., Milpitas, CA 95035-7417(408) 432-1900 l FAX: (408) 434-0507 l TELEX: 499-3977 For literature on our Operational Amplifiers,call 1-800-4-LINEAR. For applications help,call (408) 432-1900, Ext. 2593 Table 1. Best Op Amp for Lowest Noise vs Source Resistance BEST OP AMPSOURCE R (Req)10Hz PRECISION1000Hz PRECISION10kHz HIGH SPEED0 to 500 LT1028, LT1115, LT1128LT1028, LT1115, LT1128LT1220/21/22/24/25/26500 to , LT1037LT1028, LT1115, LT1128LT1220/21/22/24/25 to 3kLT1124/25/26/27LT1028, LT1115, LT1128LT1220/21/22/24/25/263k to 5kLT1124/25/26/27LT1007, LT1037LT1220/21/22/24/25/265k to 10kLT1124/25/26/27LT1124/25/26/27LT1354/ 57/60/6310k to 20kLT1001/02LT1113, LT1124/25/26/27LT1354/57/60/6320k to 100kLT1001/02LT1055/56/57/58, LT1113, LT1169LT1351100k to 1 MLT1022, LT1055/56/57/58,LT1022, LT1055/56/57/58, LT1113LT1351LT1113, LT1122.

7 LT1169LT1122, LT1169, LT14571M to 10 MLT1022, LT1055/56/57/58,LT1022, LT1055/56/57/58, LT1113LT1113, LT1122, LT1169LT1122, LT1169, LT1457 High speed op amps, here defined by slew rates greater than100V/ s, are plotted in Figure 4. These op amps come in awider range of speeds than the precision op amps plotted inFigures 2 and 3. The faster parts will generally have slightlymore spot noise, but because they will most likely beselected on the basis of speed, a Selection of parts is example, the LT1354 LT1363 (these are single opamps; duals and quads are available) are close in noiseperformance and consequently cluster close together on theplot, but have a speed range of 12 MHz GBW to 70 MHz same information is presented in tabular form inTable SOURCE RESISTANCE ( )1330300 SPOT NOISE (nV/ Hz)100100010100k10 MDN140 LT1128LT1124LT1169 RESISTOR NOISE ONLYLT1007 LT1037 EQUIVALENT SOURCE RESISTANCE ( )1330300 SPOT NOISE (nV/ Hz)100100010100k10 MDN140 LT1128LT1001LT1124LT1169LT1007 LT1037 RESISTOR NOISE ONLYF igure 4.

8 10kHz Spot Noise vs Equivalent Source Resistance(High Speed Amplifiers)EQUIVALENT SOURCE RESISTANCE ( )1330300 SPOT NOISE (nV/ Hz)100100010100k1 MDN140 NOISE ONLYLT1220 LT1221 LT1222 LT1224 LT1225 LT1226LT1334 LT1357 LT1360 LT1363 Figure 3. 1kHz Spot Noise vs Equivalent Source ResistanceFigure 2. 10Hz Spot Noise vs Equivalent Source