Operational Amplifiers - Linear Technology

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Updated 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 selec-

tion 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 tempera-

ture 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 normaliz-

ing 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 mul-

tiplying the spot noise by the square root of the application

bandwidth.

Some other simplifications are made to facilitate compari-

son. 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 applica-

tions, 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.

, LTC and LT are registered trademarks of Linear Technology Corporation.

WHERE: V

TR1

, V

TR2

AND V

TR3

ARE THERMAL NOISE FROM RESISTORS

4kT = (16.56)(10)

–21

R2 + (R1)(R3)

R1 + R3

)

AND V

IS THE VOLTAGE SPOT NOISE AND I

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 conclu-

sions. 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 R

whereas the resistor noise is proportional to the √R

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 ex-

tremely low current noise of 0.8fA/√Hz , it also has a very

low voltage noise of 6nV/√Hz. The LT1169 also has excel-

lent 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.

–

TR2

TR3

TR1

DN140 F01

 LINEAR TECHNOLOGY CORPORATION 1996

LT/GP 1096 155K • PRINTED IN THE USA

Linear Technology Corporation

1630 McCarthy Blvd., Milpitas, CA 95035-7417

(408) 432-1900

●

FAX

: (408) 434-0507

●

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 AMP

SOURCE R (R

) 10Hz PRECISION 1000Hz PRECISION 10kHz HIGH SPEED

0Ω to 500ΩLT1028, 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, LT1022, LT1055/56/57/58, LT1113 LT1351

LT1113, LT1122, LT1169 LT1122, LT1169, LT1457

1M to 10M LT1022, LT1055/56/57/58, LT1022, LT1055/56/57/58, LT1113

LT1113, LT1122, LT1169 LT1122, LT1169, LT1457

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.

EQUIVALENT SOURCE RESISTANCE (Ω)

300

SPOT NOISE (nV/√Hz)

100

1000

10 100k 10M

DN140 F03

0.3 1k 10k 1M100

LT1028

LT1128 LT1124

LT1169

RESISTOR NOISE ONLY

LT1007

LT1037

EQUIVALENT SOURCE RESISTANCE (Ω)

300

SPOT NOISE (nV/√Hz)

100

1000

10 100k 10M

DN140 F02

0.3 1k 10k 1M100

LT1028

LT1128 LT1001

LT1124

LT1169

LT1007

LT1037

RESISTOR NOISE ONLY

Figure 4. 10kHz Spot Noise vs Equivalent Source Resistance

(High Speed Amplifiers)

EQUIVALENT SOURCE RESISTANCE (Ω)

300

SPOT NOISE (nV/√Hz)

100

1000

10 100k 1M

DN140 F04

0.3 1k 10k100

LT1351

RESISTOR NOISE ONLY

LT1220

LT1221

LT1222

LT1224

LT1225

LT1226

LT1334

LT1357

LT1360

LT1363

Figure 3. 1kHz Spot Noise vs Equivalent Source Resistance

Figure 2. 10Hz Spot Noise vs Equivalent Source Resistance