National Semiconductor LMH6624 Manual - page 14
Application Section
(Continued)
R
f
||R
g
= R
seq
for bias current cancellation. Figure 4 illus-
trates the equivalent noise model using this assumption.
Figure 5 is a plot of e
ni
against equivalent source resistance
(R
seq
) with all of the contributing voltage noise source of
Equation 2. 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
*
A
V
.
(2)
As seen in Figure 5, e
ni
is dominated by the intrinsic voltage
noise (e
n
) of the amplifier for equivalent source resistances
below 33.5
Ω. Between 33.5Ω and 6.43kΩ, e
ni
is dominated
by the thermal noise (e
t
=
√
(4kT(2R
seq
)) of the external
resistor. Above 6.43k
Ω, e
ni
is dominated by the amplifier’s
current noise (i
n
=
√
(2) i
n
R
seq
). The point at which the
LMH6624’s voltage noise and current noise contribute
equally occurs for R
seq
= 464
Ω (ie., e
n
/
√
(2)i
n
). For example,
configured with a gain of +20V/V giving a −3dB of 90MHz
and driven from R
seq
= 25
Ω, the LMH6624 produces a total
equivalent input noise voltage (e
ni
x
1.57
*
90MHz) of
16.5µV
rms
.
If bias current cancellation is not a requirement, then R
f
||R
g
need not equal R
seq
. In this case, according to Equation 1,
R
f
||R
g
should be as low as possible 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
referred to the non-inverting input. Referring 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.
(3)
The Noise Figure formula is shown in Equation 3. The addi-
tion of a terminating resistor R
T
, reduces the external ther-
mal noise but increases the resulting NF. The NF is in-
creased because R
T
reduces the input signal amplitude thus
reducing the input SNR.
(4)
The noise figure is related to the equivalent source resis-
tance (R
seq
) and the parallel combination of R
f
and R
g
. To
minimize noise figure.
•
Minimize R
f
||R
g
•
Choose the Optimum R
S
(R
OPT
)
R
OPT
is the point at which the NF curve reaches a minimum
and is approximated by:
R
OPT
≈ e
n
/i
n
NON-INVERTING GAINS LESS THAN 10V/V
Using the LMH6624 at lower non-inverting gains requires
external compensation such as the shunt compensation as
shown in Figure 6. The compensation capacitors are chosen
to reduce frequency response peaking to less than 1dB.
INVERTING GAINS LESS THAN 10V/V
The lag compensation of Figure 7 will achieve stability for
lower gains. Placing the network between the two input
terminals does not affect the closed-loop nor noise gain, but
is best used for the inverting configuration because of its
affect on the non-inverting input impedance.
20058921
FIGURE 4. Noise Model with R
f
||R
g
= R
seq
20058922
FIGURE 5. Voltage Noise Density vs. Source
Resistance
20058924
FIGURE 6. External Shunt Compensation
LMH6624
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