### DN140 - Updated Operational Amplifier Selection Guide for Optimum Noise Performance

```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