7/29/2019 Op Amp Selection Guide for Optimum Noise Performance

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Op Amp Selection Guide for Optimum Noise PerformanceDesign Note 355

Glen Brisebois

01/05/355

Introduction

Linear Technology continues to add to its portfolio of lownoise op amps. This is not because the physics of noisehas changed, but because low noise specifications arebeing combined with new features such as rail-to-rail

operation, shutdown, low voltage and low power opera-tion. Op amp noise is dependent on input stage operatingcurrent, device type (bipolar or FET) and input circuitry.This selection guide is intended to help you identify basicnoise tradeoffs and select the best op amps, new or old,for your application.

Quantifying Resistor ThermalNoise and Op Amp Noise

The key to understanding noise tradeoffs is the fact thatresistors have noise. At room temperature, a resistorR has an RMS voltage noise density (or spot noise)

of VR

=

0.13

R noise in nV/

Hz. So a 10k resistor has13nV/Hz and a 1M resistor has 130nV/Hz. Rigor-

ously speaking, the noise density is given by the equationVR=4kTR, where k is Boltzmans constant and T is thetemperature in degrees Kelvin. This dependency on tem-perature explains why some low noise circuits resort tosuper-cooling the resistors. Note that the same resistor canalso be considered to have a noise current of IR=4kT/R ora noise power density PR=4kT=16.6 10

21W/Hz=16.6zeptoWatts/Hz independent of R. Selecting the right ampli-fier is simply finding which one will add the least amountof noise above the resistor noise.

Dont be alarmed by the strange unit /Hz. It arises simplybecause noise power adds with bandwidth (per Hertz), sonoise voltage adds with the square root of the bandwidth(per root Hertz). To make use of the specification, simplymultiply it by the square root of the application bandwidthto calculate the resultant RMS noise within that bandwidth.Peak-to-peak noise, as encountered on an oscilloscope forexample, will be about 6 times the total RMS noise 99%of the time (assuming Gaussian bell curve noise). Do

not rely on the op amp to limit the bandwidth. For bestnoise performance, limit the bandwidth with passive orlow noise active filters.

Op amp input noise specifications are usually given interms of nV/Hz for noise voltage and pA/Hz or fA/Hzfor noise current and are therefore directly comparablewith resistor thermal noise. Due to the fact that noisedensity varies at low frequencies, most op amps alsospecify a typical peak-to-peak noise within a 0.1Hz to10Hz or 0.01Hz to 1Hz bandwidth. For the best ultralowfrequency performance, you may want to consider a zero

drift amplifier like the LTC2050 or LTC2054.

Summing the Noise Sources

Figure 1 shows an idealized op amp and resistors with thenoise sources presented externally. The equation for theinput referred RMS sum of all the noise sources, VN(TOTAL)is also shown. It is this voltage noise density, multipliedby the noise gain of the circuit (NG=1+R1/R2) thatappears at the output.

+

R1

R2

RS

IN

VNVR(EQ)

VN AND IN ARE THE NOISE VOLTAGE AND NOISE CURRENTDENSITIES OF THE OP AMP FROM THE DATASHEET

VR(EQ) = 0.13 REQ nV HzIS RESISTOR THERMAL NOISE IN

nV HzEXPRESS VN, VR(EQ) AND INREQ IN

nV Hz= THE TOTAL INPUT REFERRED NOISE IN

REQ = EQUIVALENT SOURCE RESISTANCE = RS + R1||R2 DN355 F01

VN(TOTAL) = VN2 +VR(EQ)

2 + INREQ( )2

Figure 1. The Op Amp Noise Model. VN and IN Are OpAmp Noise Sources (Correlated Current Noise Is NotShown). VR(EQ) Is the Voltage Noise Due to the Resistors

, LTC and LT are registered trademarks of Linear Technology Corporation.All other trademarks are the property of their respective owners.

7/29/2019 Op Amp Selection Guide for Optimum Noise Performance

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LINEAR TECHNOLOGY CORPORATION 2005

dn355f LT/TP 0205 305K PRINTED IN THE USA

For applications help,call (408) 432-1900, Ext. 2593

Data Sheet Download

http://www.linear.com

From the equation for VN(TOTAL), we can draw sev-eral conclusions. For the lowest noise, the values of theresistors should be as small as possible, but since R1 isa load on the op amp output, it must not be too small. Insome applications, such as transimpedance amplifiers,R1 is the only resistor in the circuit and is usually large.For low REQ, the op amp voltage noise dominates (as VN

is the remaining term); for very high REQ, the op ampcurrent noise dominates (as IN is the coefficient of thehighest order REQ term). At middle values of REQ, theresistor noise dominates and the op amp contributes littlesignificant noise. This is the ROPTIMUM of the amplifierand can be found by taking the quotient of the op ampsnoise specs: VN/IN = ROPT.

Selecting the Best Op Amps

Figure 2 shows plots of voltage noise density of the sourceresistance and of various op amps at three different fre-quencies. Each point labeled by an op amp part numberis that parts voltage noise density plotted at its ROPT.

Use the graph with the most applicable frequency ofinterest. Find your source resistance on the horizontal axisand mark that resistance at the point where it crosses theresistor noise line. This is the source resistance point.The best noise performance op amps are under that point,the further down the better.

For all candidate op amps, draw a horizontal line from yoursource resistance point all the way to the right hand sideof the plot. Op amps beneath that line will give good noiseperformance, again the lower the better. Draw another linefrom the source resistance point down and to the left at

one decade per decade. Op amps below that line are alsogood candidates.

If you still cant find any candidates, then you have a verylow source impedance and should use op amps that areclosest to the bottom. In such cases, paralleling of lownoise op amps is also an option.

Conclusion

Noise analysis can be a daunting task at first and is anunfamiliar territory for many design engineers. The great-est influence on overall noise perfomance is the sourceimpedance associated with the signal. This selection guide

helps the designer, whether novice or veteran, to choosethe best op amps for any given source impedance.

VOLTAGEN

OISE

DENSITY(

)

nV

Hz

REQ FOR RESISTOR NOISE; ROPT FOR OP AMPS ()

10M10

1k

0.1100 1k 10k 100k 1M

10

1

100

LT1494

LTC1992LT1490

LT1113

LT1012

LT1881LT1112LT1097

LT6010LT1880

LT1028

LT1792

LT1211LT1468LT1213

LT6013LT1001LT1124

LT1677

LT1884

LT1169LT1793

LT1122

LT1022, LT1055

LT1007

LT6233

RESISTOR NOISE

f = 10Hz

VOLTAGE

NOISE

DENSITY(

)

nV

Hz

REQ FOR RESISTOR NOISE; ROPT FOR OP AMPS ()

10M10

1k

0.1100 1k 10k 100k 1M

10

1

100

LT6013LT1169

LT1113

LT1793

LTC1992

LT1122LT1022,55

LT1490

LT1222

LT1806LT1722

LT1815,8LT1800

LT6220LT1357

LT1215

LT1211LT1001

LT6010LT1880

LT1881LT1097LT1112

LT1012

LT1567LT1028

LT6233

LT6202LT1007

LT1884

LT1468

LT1213

LT1677LT1124

LT6200LT1226

LT6230

RESISTOR NOISE

f = 1kHz

VOLTAGE

NOISE

DENSITY(

)

nV

Hz

REQ FOR RESISTOR NOISE; ROPT FOR OP AMPS ()

1M10

1k

0.1

10

1

100

100 1k 10k 100k

LT1028LT6230LT6200

LT1222LT1226

LT1806, LT1722

LT1815,LT1818

LT1468

LT1363LT1360

LT6220LT1812LT1800

LT1215

LT1567 LT1677

LT6233 LT6202

LT1792

LT1357

LT1211LT1213LT1793

RESISTOR NOISE

f = 100kHz

DN355 F02

SHADED AREA ENCLOSESCANDIDATE OP AMPS FORLOW NOISE AT REQ = 100k

LT1677

202

LT1792

LT1213LT1793

TOR NOISE

SHADED AREA ECANDIDATE OPLOW NOISE AT R

Figure 2. Use these Three Plots to Find the Best Low NoiseOp Amps for Your Application

Linear Technology Corporation1630 McCarthy Blvd., Milpitas, CA 95035-7417(408) 432-1900 FAX: (408) 434-0507 www.linear.com