Download DN355 - Op Amp Selection Guide for Optimum Noise Performance

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Op Amp Selection Guide for Optimum Noise Performance
Design Note 355
Glen Brisebois
Introduction
Linear Technology continues to add to its portfolio of low
noise op amps. This is not because the physics of noise
has changed, but because low noise specifications are
being combined with new features such as rail-to-rail
operation, shutdown, low voltage and low power operation. Op amp noise is dependent on input stage operating
current, device type (bipolar or FET) and input circuitry.
This selection guide is intended to help you identify basic
noise tradeoffs and select the best op amps, new or old,
for your application.
Quantifying Resistor Thermal
Noise and Op Amp Noise
The key to understanding noise tradeoffs is the fact that
resistors have noise. At room temperature, a resistor
R has an RMS voltage noise density (or “spot noise”)
of VR = 0.13√⎯R noise in nV/√⎯H⎯z. So a 10k resistor has
13nV/√⎯H⎯z and a 1M resistor has 130nV/√⎯H⎯z. Rigorously speaking, the noise density is given by the equation
VR = √⎯4⎯k⎯T⎯R, where k is Boltzman’s constant and T is the
temperature in degrees Kelvin. This dependency on temperature explains why some low noise circuits resort to
super-cooling the resistors. Note that the same resistor can
also be considered to have a noise current of IR = √⎯4k⎯ T⎯ /R
⎯ or
a noise power density PR = 4kT = 16.6 • 10–21W/Hz = 16.6
zeptoWatts/Hz independent of R. Selecting the right amplifier is simply finding which one will add the least amount
of noise above the resistor noise.
Don’t be alarmed by the strange unit “/√⎯Hz⎯ .” It arises simply
because noise power adds with bandwidth (per Hertz), so
noise voltage adds with the square root of the bandwidth
(per root Hertz). To make use of the specification, simply
multiply it by the square root of the application bandwidth
to calculate the resultant RMS noise within that bandwidth.
Peak-to-peak noise, as encountered on an oscilloscope for
example, will be about 6 times the total RMS noise 99%
of the time (assuming Gaussian “bell curve” noise). Do
RS
VR(EQ)
VN
+
IN
–
R1
VN AND IN ARE THE NOISE VOLTAGE AND NOISE CURRENT
DENSITIES OF THE OP AMP FROM THE DATASHEET
REQ = EQUIVALENT SOURCE RESISTANCE = RS + R1||R2
VR(EQ) = 0.13 REQ IS RESISTOR THERMAL NOISE IN nV
EXPRESS VN, VR(EQ) AND IN•REQ IN nV
(
VN(TOTAL) = VN2 + VR(EQ)2 + IN •REQ
)
R2
DN355 F01
Hz
Hz
2
= THE TOTAL INPUT REFERRED NOISE IN nV
Hz
Figure 1. The Op Amp Noise Model. VN and IN Are Op
Amp Noise Sources (Correlated Current Noise Is Not
Shown). VR(EQ) Is the Voltage Noise Due to the Resistors
not rely on the op amp to limit the bandwidth. For best
noise performance, limit the bandwidth with passive or
low noise active filters.
Op amp input noise specifications are usually given in
terms of nV/√⎯H⎯z for noise voltage and pA/√⎯H⎯z or fA/√⎯H⎯z
for noise current and are therefore directly comparable
with resistor thermal noise. Due to the fact that noise
density varies at low frequencies, most op amps also
specify a typical peak-to-peak noise within a “0.1Hz to
10Hz” or “0.01Hz to 1Hz” bandwidth. For the best ultralow
frequency performance, you may want to consider a zero
drift amplifier like the LTC®2050 or LTC2054.
Summing the Noise Sources
Figure 1 shows an idealized op amp and resistors with the
noise sources presented externally. The equation for the
input referred RMS sum of all the noise sources, VN(TOTAL)
is also shown. It is this voltage noise density, multiplied
by the noise gain of the circuit (NG = 1 + R1/R2) that
appears at the output.
, LTC and LT are registered trademarks of Linear Technology Corporation.
All other trademarks are the property of their respective owners.
01/05/355
www.BDTIC.com/Linear
For all candidate op amps, draw a horizontal line from your
source resistance point all the way to the right hand side
of the plot. Op amps beneath that line will give good noise
performance, again the lower the better. Draw another line
from the source resistance point down and to the left at
one decade per decade. Op amps below that line are also
good candidates.
If you still can’t find any candidates, then you have a very
low source impedance and should use op amps that are
closest to the bottom. In such cases, paralleling of low
noise op amps is also an option.
Conclusion
Noise analysis can be a daunting task at first and is an
unfamiliar territory for many design engineers. The greatest influence on overall noise perfomance is the source
impedance associated with the signal. This selection guide
helps the designer, whether novice or veteran, to choose
the best op amps for any given source impedance.
Hz )
VOLTAGE NOISE DENSITY (nV
LT1112
LT1881 LT1097
10
LT1211
LT1468
LT1213
LT6233
LT6010
LT1880
LT1494
LT1490
LTC1992
LT1122
LT1022, LT1055
LT1012
LT1169
LT1113 LT1793
LT1792
LT1677
LT1124
LT1007
LT1001 LT1884 LT6013
LT1028
1
RESISTOR NOISE
0.1
1M
1k
10k 100k
100
10
10M
REQ FOR RESISTOR NOISE; ROPT FOR OP AMPS (Ω)
f = 1kHz
Hz )
1k
VOLTAGE NOISE DENSITY (nV
Use the graph with the most applicable frequency of
interest. Find your source resistance on the horizontal axis
and mark that resistance at the point where it crosses the
resistor noise line. This is the “source resistance point.”
The best noise performance op amps are under that point,
the further down the better.
f = 10Hz
100
LT6220
LT1357
LT1211
LT1001
LT1215
100
LT6010
LT1880
LT1800
LT1815,8
10
LT1722
LT1806
LT1222
1
0.1
LT1468
LT1677
LT1124
LT6230
LT1028
LT1567 LT6200
LT1226
LT1881 LT1490
LT1097
LT1112
LTC1992
LT1012
LT1122
LT1022,55
LT6013 LT1793
LT1169
LT1113
LT1884
LT1007
LT6202
LT6233
LT1213
RESISTOR NOISE
1M
1k
10k 100k
100
10
10M
REQ FOR RESISTOR NOISE; ROPT FOR OP AMPS (Ω)
1k
Hz )
Selecting the Best Op Amps
Figure 2 shows plots of voltage noise density of the source
resistance and of various op amps at three different frequencies. Each point labeled by an op amp part number
is that part’s voltage noise density plotted at its ROPT.
1k
VOLTAGE NOISE DENSITY (nV
From the equation for VN(TOTAL), we can draw several conclusions. For the lowest noise, the values of the
resistors should be as small as possible, but since R1 is
a load on the op amp output, it must not be too small. In
some 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 amp
current noise dominates (as IN is the coefficient of the
highest order REQ term). At middle values of REQ, the
resistor noise dominates and the op amp contributes little
significant noise. This is the ROPTIMUM of the amplifier
and can be found by taking the quotient of the op amp’s
noise specs: VN/IN = ROPT.
100
10
1
f = 100kHz
LT1363
LT1360
LT6220
LT1215
LT1812
LT1800
LT1468
LT1815,LT1818
LT1211
LT1213
LT1806, LT1722
LT1357 LT1793
LT1222
LT1226
LT1792
LT6233 LT6202
T6202
LT1567
1567
LT1677
LT1028
LT6230
SHADED AREA EENCLOSES
LT62000
CANDIDATE OP A
AMPS FOR
RESISTOR
STOR NOISE LOW NOISE AT REQ = 100k
0.1
100
1k
10k
100k
10
1M
REQ FOR RESISTOR NOISE; ROPT FOR OP AMPS (Ω)
DN355 F02
Figure 2. Use these Three Plots to Find the Best Low Noise
Op Amps for Your Application
Data Sheet Download
For applications help,
call (408) 432-1900, Ext. 2593
http://www.linear.com
Linear Technology Corporation
dn355f LT/TP 0205 305K • PRINTED IN THE USA
1630 McCarthy Blvd., Milpitas, CA 95035-7417
www.BDTIC.com/Linear
(408) 432-1900 ● FAX: (408) 434-0507
●
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© LINEAR TECHNOLOGY CORPORATION 2005