Data Sheet
Application Information
Basic Operation
Figures 1 and 2 illustrate typical circuit configurations for
non-inverting, inverting, and unity gain topologies for dual
supply applications. They show the recommended bypass
capacitor values and overall closed loop gain equations.
+V
s
6.8μF
versus R
f
and R
g
. As the value of R
f
increases, the total
input referred noise also increases.
3
2.75
Input Referred Noise (nV/rtHz)
2.5
2.25
2
1.75
1.5
1.25
1
0.75
G = +21
G = +11
G = +5
C
omlinear
CLC1002
Ultra-Low Noise Amplifier
Input
+
-
0.1μF
Output
0.1μF
R
L
R
f
G = 1 + (R
f
/R
g
)
0.5
100
1000
Rf (Ohms)
Figure 3: Input Referred Voltage Noise vs. R
f
and R
g
R
g
-V
s
6.8μF
The noise caused by a resistor is modeled with either a
voltage source in series with the resistance:
4kTR
Figure 1. Typical Non-Inverting Gain Circuit
+V
s
6.8μF
Or a current source in parallel with it:
R
1
Input
R
g
+
-
0.1μF
6.8μF
-V
s
R
f
G = - (R
f
/R
g
)
For optimum input offset
voltage set R
1
= R
f
|| R
g
0.1μF
Output
R
L
i
R
=
4kT
R
Rev 1D
Op amp noise is modeled with three noise sources, e
n
, i
n
and i
i
. These three sources are analogous to the DC input
voltage and current errors V
os
, I
bn
and I
bi
.
Figure 2. Typical Inverting Gain Circuit
The noise models must be analyzed in-circuit to deter-
mine the effect on the op amp output noise.
Achieving Low Noise in an Application
Making full use of the low noise of the CLC1002 requires
careful consideration of resistor values. The feedback and
gain set resistors (R
f
and R
g
) and the non-inverting source
impedance (R
source
) all contribute noise to the circuit and
can easily dominate the overall noise if their values are
too high. The datasheet is specified with an R
g
of 25Ω, at
which point the noise from R
f
and R
g
is about equal to the
noise from the CLC1002. Lower value resistors could be
used at the expense of more distortion.
Figure 3 shows total input voltage noise (amp+resistors)
Since noise is statistical in nature rather than a continuous
signal, the set of noise sources in circuit add in an RMS
(root mean square) fashion rather than in a linear fashion.
For uncorrelated noise sources, this means you add the
squares of the noise voltages. A typical non-inverting ap-
plication (see figure 1) results in the following noise at the
output of the op amp:
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www.cadeka.com
13
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