clc425 National Semiconductor Corporation, clc425 Datasheet - Page 6

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

As seen in Figure 5, e
voltage noise (e
resistances below 33.5 . Between 33.5
e
the external resistors. Above 6.43k , e
the amplifier's current noise ( 2i R
which the CLC425's voltage noise and current noise
contribute equally occurs for R
As an example, configured with a gain of +20V/V giving
a -3dB of 90MHz and driven from an R
CLC425 produces a total equivalent input noise voltage
( e
If bias current cancellation is not a requirement, then
R
to Equation 1, R
order to minimize noise. Results similar to Equation 1
are obtained for the inverting configuration of Figure 2 if
R
these substitutions, Equation 1 will yield an e
the non-inverting input. Refering e
is easily accomplished by multiplying e
non-inverting to inverting gains.
Noise Figure
Noise Figure (NF) is a measure of the noise degradation
caused by an amplifier.
The Noise Figure formula is shown in Equation 3. The
addition of a terminating resistor R
external thermal noise but increases the resulting NF.
The NF is increased because R
amplitude thus reducing the input SNR.
http://www.national.com
NF
ni
f
s eq
Figure 5: Voltage Noise Density vs. Source Resistance
||R
ni
is dominated by the thermal noise ( e
is replaced by R
10LOG
g
1.57 90MHz
does not need to equal R
NF
Equation 3: Noise Figure Equation
e
n
2
10LOG
n
i
R
R
n
) of the amplifier for equivalent source
f
2
||R
seq
seq
) of 16.5 V
R
g
b
seq
= R
= R
and R
should be as low as possible in
S / N
S / N
s
s
ni
o
i
for Unterminated Systems
II R
R | | R
is dominated by the intrinsic
f
T
g
i
o
for Terminated Systems
is replaced by R
4kTR
s eq
g
rms
T
s eq
2
reduces the input signal
. In this case, according
.
10LOG
=464
ni
seq
n
to the inverting input
4kTR
seq
ni
T
,
seq
). The point at
ni
is dominated by
t
by the ratio of
(i.e. e
e
e
reduces the
s eq
and 6.43k ,
ni
4kT R | | R
t
4 TR
2
ni
2
=25 , the
g
k
refered to
+R
n
/
f
s
seq
. With
2i
) of
g
n
).
6
The noise figure is related to the equivalent source
resistance (R
R
recommended:
R
minimum and is approximated by:
Figure 6 is a plot of NF vs R
The NF curves for both Unterminated and Terminated
systems are shown. The Terminated curve assumes R
= R
resistances including R
Supply Current Adjustment
The CLC425's supply current can be externally adjusted
downward from its nominal value by adding an optional
resistor (R
shown in Figure 7. Several of the plots found within the plot
pages demonstrate the CLC425’s behavior at different
supply currents. The plot labeled “I
means for selecting R
process variation which is bounded by the 25°C curve.
Non-Inverting Gains Less Than 10V/V
Using the CLC425 at lower non-inverting gains requires
external compensation such as the shunt compensation
as shown in Figure 8. The quiescent supply current must
also be reduced to 5mA with R
pensation capacitors are chosen to reduce frequency
response peaking to less than 1dB. The plot in the
"Typical Performance" section labeled “Differential Gain
and Phase” shows the video performance of the CLC425
with this compensation circuitry.
g
OPT
. To minimize noise figure, the following steps are
T
. The table indicates the NF for various source
Figure 7: External Supply Current Adjustment
is the point at which the NF curve reaches a
Figure 6: Noise Figure vs Source Resistance
•
•
p
Minimize R
Choose the optimum R
) between pin 8 and the negative supply as
s eq
) and the parallel combination of R
3
2
p
R
CLC425
and shows the result of standard IC
f
||R
OPT
+V
s
-V
= R
g
cc
cc
7
4
s
with R
(e
8
OPT
6
n
R
/i
.
p
p
n
)
for stability. The com-
f
cc
||R
s
(R
V
vs. R
out
g
OPT
= 9.09 (A
)
p
” provides the
v
= +10).
f
and
s