10
100 1k 10k 100k
R
SEQ
(:)
0.1
1
10
100
VOLTAGE NOISE DENSITY (nV/
Hz)
e
ni
e
n
e
t
i
n
LMH6624
,
LMH6626
www.ti.com
SNOSA42G –NOVEMBER 2002–REVISED DECEMBER 2014
Feature Description (continued)
Figure 50. Noise Model with R
f
||R
g
= R
seq
(4)
As seen in Figure 51, e
ni
is dominated by the intrinsic voltage noise (e
n
) of the amplifier for equivalent source
resistances below 26 Ω. Between 26 Ω and 3.1 kΩ, e
ni
is dominated by the thermal noise (e
t
= √(4kT(2R
seq
)) of
the equivalent source resistance R
seq
. Above 3.1 kΩ, e
ni
is dominated by the amplifier’s current noise (i
n
= √2
i
n
R
seq
). When R
seq
= 283 Ω (that is, R
seq
= e
n
/√2 i
n
) the contribution from voltage noise and current noise of
LMH6624 and LMH6626 is equal. For example, configured with a gain of +20V/V giving a −3 dB of 90 MHz and
driven from R
seq
= Rf || Rg = 25 Ω (e
ni
= 1.3 nV√Hz from Figure 51), the LMH6624 produces a total output noise
voltage (e
ni
× 20 V/V × √(1.57 × 90 MHz)) of 309 μVrms.
Figure 51. 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 48 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.
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