AN-940 View Datasheet(PDF) - Analog Devices
Part Name
Description
Manufacturer
AN-940 Datasheet PDF : 12 Pages
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AN-940
Application Note
Similar types of graph can be constructed for a chosen frequency
from the data in the op amp data sheet (see Figure 8). For example,
the AD8599 has an input-referred voltage noise of 1.07 nV/√Hz
and an input-referred current noise of 2.3 pA/√Hz at 1 kHz. The
RS, OP is about ~465 Ω at 1 kHz. In addition, note the following:
The Johnson noise associated with this device is equivalent
to a source resistor of about 69.6 Ωsee Figure 6).
For a source resistance above ~465 Ω, the noise voltage
produced by the amplifier’s current noise exceeds that
contributed by the source resistance; the amplifier’s
current noise becomes the dominant noise source.
To use the graph (see Figure 7), follow Step 1 through Step 4.
1. Typically, the source resistances are known (such as sensor
impedances). If the resistances are not known, calculate them
from the surrounding or preceding circuit components.
2. Locate the given source resistance, such as 1 kΩ, on the
Johnson noise line.
3. Create a horizontal line from the point located in Step 2 to
the right of the plot.
4. Create a line down and to the left from the point located in
Step 2) by decreasing one decade of voltage noise per one
decade of resistance.
Any amplifiers below and to the right of the lines are good low
noise op amps for the design as highlighted in the shade of gray
in Figure 7.
For the example shown in Figure 7, the following devices are
good candidates for the design: AD8597, AD8599, AD797,
ADA4075-2, ADA4004, OP270, OP27/OP37, AD743/AD745,
and OP184.
100
JOHNSON NOISE LINE
LOW NOISE BOUNDRY
IDEAL OP AMPS FOR A
LOW NOISE APPLICATION
10
f = 1kHz
OP285
OP467
OP271
AD8622/AD8624
OPx177
AD8610/
AD8620
STEP 2
OP275
OP213
STEP 3
OP270
OPx84
OP27/OP37
ADA4004
AD743/AD745
ADA4075-2
1
AD8597/AD8599
AD797
STEP 4
0.1
10
100
1k
10k
100k
1M
SOURCE RESISTANCE (Ω)
Figure 7. Selecting Op Amp for Low Noise Design
Rev. D | Page 8 of 12
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