Datasheet
1 10 100 1k 10k 100k
R
SEQ
(:)
Voltage Noise Density (nV/ Hz)
1000
100
10
1
0.1
i
n
e
ni
e
t
e
n
R
SEQ
= R
F
|| R
G
LMH6629
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SNOSB18G –APRIL 2010–REVISED MARCH 2013
Figure 59 illustrates the equivalent noise model using this assumption. Figure 60 is a plot of e
ni
against
equivalent source resistance (R
seq
) with all of the contributing voltage noise source of Equation 4. 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
.
Figure 59. Noise Model with R
f
||R
g
= R
seq
As seen in Figure 60, e
ni
is dominated by the intrinsic voltage noise (e
n
) of the amplifier for equivalent source
resistances below 15Ω. Between 15Ω and 2.5 kΩ, e
ni
is dominated by the thermal noise (e
t
= √(4kT(2R
seq
)) of
the equivalent source resistance R
seq
; incidentally, this is the range of R
seq
values where the LMH6629 has the
best (lowest) Noise Figure (NF) for the case where R
seq
= R
f
|| R
g
.
Above 2.5 kΩ, e
ni
is dominated by the amplifier’s current noise (i
n
= √(2) i
n
R
seq
). When R
seq
= 190Ω (i.e., R
seq
=
e
n
/√(2) i
n
), the contribution from voltage noise and current noise of LMH6629 is equal. For example, configured
with a gain of +10V/V giving a −3dB of 825 MHz and driven from R
seq
= R
f
|| R
g
= 20Ω (e
ni
= 1.07 nV√Hz from
Figure 60), the LMH6629 produces a total equivalent output noise voltage (e
ni
* 10 V/V * √(1.57 * 825 MHz)) of
385 μV
rms
.
Figure 60. Voltage Noise Density vs. Source Resistance
If bias current cancellation is not a requirement, then R
f
|| R
g
need not equal R
seq
. In this case, according to
Equation 3, R
f
|| R
g
should be as low as possible to minimize noise. Results similar to Equation 3 are obtained
for the inverting configuration of Figure 56 if R
seq
is replaced by R
b
and R
g
is replaced by R
g
+ R
s
. With these
substitutions, Equation 3 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 (1+R
g
/ R
f
).
NOISE FIGURE
Noise Figure (NF) is a measure of the noise degradation caused by an amplifier.
Equation 5: General Noise Figure Equation (5)
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