鈮?/div>
(e
n
/i
n
)
Figure 6 is a plot of NF vs R
s
with R
f
||R
g
= 9.09 (A
v
= +10).
The NF curves for both Unterminated and Terminated
systems are shown. The Terminated curve assumes R
s
= R
T
. The table indicates the NF for various source
resistances including R
s
= R
OPT
.
Figure 5: Voltage Noise Density vs. Source Resistance
If bias current cancellation is not a requirement, then
R
f
||R
g
does not need to equal R
seq
. In this case, according
to Equation 1, R
f
||R
g
should be as low as possible in
order to minimize noise. Results similar to Equation 1
are obtained for the inverting configuration of Figure 2 if
R
seq
is replaced by R
b
and R
g
is replaced by R
g
+R
s
. With
these substitutions, Equation 1 will yield an e
ni
refered to
the non-inverting input. Refering e
ni
to the inverting input
is easily accomplished by multiplying e
ni
by the ratio of
non-inverting to inverting gains.
Noise Figure
Noise Figure (NF) is a measure of the noise degradation
caused by an amplifier.
錚?/div>
e
ni 2
錚?/div>
錚?/div>
S
i
/ N
i
錚?/div>
NF
=
10LOG
錚?/div>
錚?=
10LOG
錚?/div>
2
錚?/div>
錚?/div>
S
o
/ N
o
錚?/div>
錚?/div>
e
t
錚?/div>
The Noise Figure formula is shown in Equation 3. The
addition of a terminating resistor R
T
, reduces the
external thermal noise but increases the resulting NF.
The NF is increased because R
T
reduces the input signal
amplitude thus reducing the input SNR.
2
錚?/div>
2
e
n
+
i
n 2
錚?/div>
R
seq
+
R
f
| | R
g
錚?+
4kTR
seq
+
4kT R
f
| | R
g
錚?/div>
錚?/div>
錚?/div>
NF
=
10LOG
錚?/div>
4kTR
seq
錚?/div>
錚?/div>
Figure 6: Noise Figure vs Source Resistance
Supply Current Adjustment
The CLC425's supply current can be externally adjusted
downward from its nominal value by adding an optional
resistor (R
p
) between pin 8 and the negative supply as
shown in Figure 7. Several of the plots found within the plot
pages demonstrate the CLC425鈥檚 behavior at different
supply currents. The plot labeled 鈥淚
cc
vs. R
p
鈥?provides the
means for selecting R
p
and shows the result of standard IC
process variation which is bounded by the 25擄C curve.
+V
cc
3
2
7
CLC425
4
8
R
p
-V
cc
6
V
out
Figure 7: External Supply Current Adjustment
(
)
(
)
錚?/div>
錚?/div>
錚?/div>
錚?/div>
錚?/div>
R
seq
= R
s
for Unterminated Systems
R
seq
= R
s
II R
T
for Terminated Systems
Equation 3: Noise Figure Equation
http://www.national.com
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
p
for stability. The com-
pensation capacitors are chosen to reduce frequency
response peaking to less than 1dB. The plot in the
"Typical Performance" section labeled 鈥淒ifferential Gain
and Phase鈥?shows the video performance of the CLC425
with this compensation circuitry.
6
5962-9325901 PDF文件相關(guān)型號
CLC425,CLC425A8B,CLC425AJE,CLC425AJM5,CLC425AJP,CLC425ALC,CLC425AMC
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