clc425 National Semiconductor Corporation, clc425 Datasheet - Page 5

• Manufacturer Part Number: clc425
• Description: Ultra Low Noise Wideband Op Amp
• Manufacturer: National Semiconductor Corporation

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
the equivalent source resistance (R
Figure 1. Combining this constraint with the non-invert-
ing gain equation also seen in Figure 1, allows both R
and R
equations: R
a 0 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 series resistor.
As seen in Figure 2, bias current cancellation is accom-
plished for the inverting configuration by placing a resis-
tor (R
resistance seen by the inverting input (R
recommended to be no less than 25 for best CLC425
performance. The additional noise contribution of R
be minimized through the use of a shunt capacitor.
V
V
Figure 1: Non-inverting Amplifier Configuration
s
s
b
g
) on the non-inverting input equal in value to the
Figure 2: Inverting Amplifier Configuration
to be determined explicitly from the following
R
R
R
s
s
s
eq
f
=A
= R
g
) and feedback (R
v
s
R
R
|| R
R
V
T
s eq
V
in
b
in
T
and R
R
A
g
v
= - R
2
3
g
=R
+V
R
R
-V
2
3
CLC425
g
f
g
cc
cc
4
+V
7
-V
f
CLC425
/(A
cc
R
cc
4
7
f
6.8 F
0.1 F
6.8 F
) resistors should equal
0.1 F
f
v
-1). When driven from
R
6.8 F
0.1 F
6.8 F
0.1 F
f
A
6
v
s eq
= 1 +
) as defined in
f
||(R
V
6
out
R
R
g
f
g
+R
s
V
)). R
out
b
can
b
is
f
5
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
current noise (i
mal voltage noise ( e
the external resistors. Equation 1 provides the general
form for total equivalent input voltage noise density (e
Equation 2 is a simplification of Equation 1 that assumes
R
illustrates the equivalent noise model using this as-
sumption. Figure 5 is a plot of e
source resistance (R
age noise sources of Equation 2 shown. This plot gives
the expected e
R
output voltage noise (e
e
ni
f
s eq
||R
4kTR
for bias current cancellation. The total equivalent
g
Figure 3: Non-inverting Amplifer Noise Model
e
= R
R
n
2
Equation 2: Noise Equation with R
s
Figure 4: Noise Model with R
s
eq
eq
e
Equation 1: General Noise Equation
ni
i
s eq
n
4kT2R
R
4
for bias current cancellation. Figure 4
s
ni
n
eq
kT
=i
for a given R
e
s
2
n+
n
eq
2
=i
2R
16 4
t
4
s eq
n-
s
.
2
kTR
eq
) sources, there also exists ther-
) with all of the contributing volt-
e
no
i R
4 TR
n
) is e
i
i
s
k
n
n
21
eq
-
+
s
i
eq
n
e
2
e
Joules
n
s eq
n
ni
i
) associated with each of
2
n
R
which assumes R
A
g
4kTR
R
v
A
4
CLC425
.
@
ni
f
R
v
kT
||
25
f
against equivalent
g
R
f
||R
http://www.national.com
g
2
C
4kTR
R
g
2
f
||R
= R
s
eq
4
g
f
s eq
kT R
= R
n
s eq
) and
f
f
||R
||
R
g
ni
g
).
=