Application Section (Continued)
(1)
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
)ise
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 33.5Ω. Between 33.5Ωand 6.43kΩ,e
ni
is dominated
by the thermal noise (e
t
=√(4kT(2R
seq
)) of the external
resistor. Above 6.43kΩ,e
ni
is dominated by the amplifier’s
current noise (i
n
=√(2) i
n
R
seq
). When R
seq
= 464Ω(ie.,
e
n
/√(2) i
n
) the contribution from voltage noise and current
noise of LMH6624/LMH6626 is equal.. For example, config-
ured with a gain of +20V/V giving a −3dB of 90MHz and
driven from R
seq
=25Ω, 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/LMH6626 at lower non-inverting gains
requires external compensation such as the shunt compen-
sation 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. It 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/LMH6626
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