Datasheet
NF = 10 LOG
e
n
+ i
n
(R
Seq
+ (R
f
||R
g
) ) + 4KT (R
Seq
+ (R
f
||R
g
))
2
4KT (R
Seq
+ (R
f
||R
g
))
2
2
2
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
www.ti.com
SNOSA42F –NOVEMBER 2002–REVISED MARCH 2013
(3)
As seen in Figure 54, e
ni
is dominated by the intrinsic voltage noise (e
n
) of the amplifier for equivalent source
resistances below 33.5Ω. Between 33.5Ω and 6.43kΩ, e
ni
is dominated by the thermal noise (e
t
= √(4kT(2R
seq
))
of the external resistor. Above 6.43kΩ, 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, configured 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
× √Hz1.57*90MHz) of 16.5μV
rms
.
Figure 54. 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 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 51 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.
(4)
The Noise Figure formula is shown in Equation 4. The addition of a terminating resistor R
T
, reduces the external
thermal noise but increases the resulting NF. The NF is increased because R
T
reduces the input signal amplitude
thus reducing the input SNR.
(5)
The noise figure is related to the equivalent source resistance (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
(6)
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