+V
cc
V
in
R
s
V
s
R
T
6.8碌F
3
7
0.1碌F
CLC425
2
6.8碌F
6
V
out
Total Input Noise vs. Source Resistance
In order to determine maximum signal-to-noise ratios
from the CLC425, an understanding of the interaction
between the amplifier's intrinsic noise sources and the
noise arising from its external resistors is necessary.
Figure 3 describes the noise model for the non-inverting
amplifier configuration showing all noise sources. In
addition to the intrinsic input voltage noise (e
n
) and
current noise (i
n
=i
n+
=i
n-
) sources, there also exists ther-
mal voltage noise (
e
t
=
4
k
TR
) associated with each of
the external resistors. Equation 1 provides the general
form for total equivalent input voltage noise density (e
ni
).
Equation 2 is a simplification of Equation 1 that assumes
4
-V
cc
R
s
eq
= R
s
|| R
T
R
g
A
v
= 1 +
0.1碌F
R
f
R
f
R
g
Figure 1: Non-inverting Amplifier Configuration
Introduction
The CLC425 is a very wide gain-bandwidth, ultra-low
noise voltage feedback operational amplifier which en-
ables application areas such as medical diagnostic ultra-
sound, magnetic tape & disk storage and fiber-optics to
achieve maximum high-frequency signal-to-noise ratios.
The set of characteristic plots located in the "Typical
Performance" section illustrates many of the perfor-
mance trade-offs. The following discussion will enable
the proper selection of external components in order to
achieve optimum device performance.
Bias Current Cancellation
In order to cancel the bias current errors of the non-
inverting configuration, the parallel combination of the
gain-setting (R
g
) and feedback (R
f
) resistors should equal
the equivalent source resistance (R
seq
) as defined in
Figure 1. Combining this constraint with the non-invert-
ing gain equation also seen in Figure 1, allows both R
f
and R
g
to be determined explicitly from the following
equations: R
f
=A
v
R
seq
and R
g
=R
f
/(A
v
-1). When driven from
a 0鈩?source, such as that from the output of an op amp,
the non-inverting input of the CLC425 should be isolated
with at least a 25鈩?series resistor.
As seen in Figure 2, bias current cancellation is accom-
plished for the inverting configuration by placing a resis-
tor (R
b
) on the non-inverting input equal in value to the
resistance seen by the inverting input (R
f
||(R
g
+R
s
)). R
b
is
recommended to be no less than 25鈩?for best CLC425
performance. The additional noise contribution of R
b
can
be minimized through the use of a shunt capacitor.
+V
cc
7
0.1碌F
6.8碌F
e
n
R
s
eq
鈭?kTR
s
eq
i
n
+
CLC425
i
n
-
R
g
R
f
鈭?kTR
f
鈭?kTR
g
4
kT
=
16.4
e
鈭?/div>
21
Joules
@
25
擄
C
Figure 3: Non-inverting Amplifer Noise Model
e
ni
=
2
e
n
+
i
n
+
R
s
(
eq
)
2
+
4
kTR
s
eq
+
i
n
鈭?/div>
R
f
||
R
g
(
(
)
)
2
+
4
kT R
f
||
R
g
(
)
Equation 1: General Noise Equation
R
f
||R
g
= R
seq
for bias current cancellation. Figure 4
illustrates the equivalent noise model using this as-
sumption. Figure 5 is a plot of e
ni
against equivalent
source resistance (R
seq
) with all of the contributing volt-
age noise sources of Equation 2 shown. This plot gives
the expected e
ni
for a given R
seq
which assumes R
f
||R
g
=
R
seq
for bias current cancellation. The total equivalent
output voltage noise (e
no
) is e
ni
鈭桝
v
.
鈭?kT2R
s
eq
2R
s
eq
e
n
A
v
3
R
b
V
out
6
i
n
鈭?
CLC425
2
6.8碌F
4
Figure 4: Noise Model with R
f
||R
g
= R
seq
V
in
R
s
V
s
A
v
= - R
f
R
g
R
g
-V
cc
0.1碌F
R
f
e
ni
=
2
e
n
+
2
i
n
R
s
eq
(
)
2
+
4
kT
2
R
s
eq
(
)
Figure 2: Inverting Amplifier Configuration
Equation 2: Noise Equation with R
f
||R
g
= R
seq
5
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