lmh6629sdx National Semiconductor Corporation, lmh6629sdx Datasheet - Page 16

• Manufacturer Part Number: lmh6629sdx
• Description: Lmh6629 Ultra-low Noise, High Speed Operational Amplifier
• Manufacturer: National Semiconductor Corporation

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FIGURE 9. Voltage Noise Density vs. Source Resistance
If bias current cancellation is not a requirement, then R
need not equal R
R
similar to
tion of
R
referred to the non-inverting input. Referring e
ing input is easily accomplished by multiplying e
of non-inverting to inverting gains (1+R
NOISE FIGURE
Noise Figure (NF) is a measure of the noise degradation
caused by an amplifier.
Looking at the two parts of the NF expression (inside the log
function) yields:
S
N
R
To simplify this, consider N
amplifier (reflected to its input port):
S
N
Substituting these two expressions into the NF expression:
The noise figure expression has simplified to depend only on
the ratio of the noise power added by the amplifier at its input
(considering the source resistor to be in place but noiseless
in getting N
sistor (considering all amplifier elements to be in place but
noiseless in getting N
For a given amplifier with a desired closed loop gain, to min-
imize noise figure:
•
f
g
i
o
S
i
o
/S
/S
||R
/N
/N
, divided by the noise power at the input due to R
Equation 4: Simplified Noise Figure Equation
+ R
Minimize R
Equation 3: General Noise Figure Equation
o
o
g
→
i
→
i
→
→
should be as low as possible to minimize noise. Results
Figure 5
s
Inverse of the power gain provided by the amplifier
Total output noise power, including the contribution of
1/G
. With these substitutions,
G * (N
Equation 1
a
) to the noise power delivered by the source re-
i
+N
f
||R
if R
a
g
seq
) /N
seq
. In this case, according to
are obtained for the inverting configura-
i
is replaced by R
i
).
(where G*(N
a
as the noise power added by the
Equation 1
i
+N
b
g
and R
a
/R
) = N
f
).
g
will yield an e
ni
o
is replaced by
)
ni
to the invert-
30068022
Equation
by the ratio
S
f
|| R
(3)
(4)
1,
ni
g
16
•
R
and is approximated by:
Figure 10
= 240Ω, A
(R
Table 1
R
SINGLE SUPPLY OPERATION
The LMH6629 can be operated with single power supply as
shown in
coupled to set the DC operating point.
OPT
S
T
R
Choose the Optimum R
= R
R
= open) and Terminated systems (R
FIGURE 10. Noise Figure vs. Source Resistance
S
50
is the point at which the NF curve reaches a minimum
OPT
(Ω)
OPT
indicates NF for various source resistances including
Figure
TABLE 1. Noise Figure for Various R
is a plot of NF vs R
V
.
FIGURE 11. Single Supply Operation
= +10V/V). The NF curves for both Unterminated
NF (Terminated)
11. Both the input and output are capacitively
(R
OPT
(dB)
7.96
4.13
= 750Ω)
R
OPT
S
(R
S
≈
OPT
with the circuit of
e
n
/i
)
n
NF (Unterminated)
(R
T
= R
OPT
(dB)
3.18
1.12
S
= 350Ω)
) are shown.
30068079
Figure 4
s
30068026
(R
f