Datasheet

REDUCED INPUT VOLTAGE NOISE =
e
n1
+e
n2
+
2 2

+e
nN
2
=
1
N
1
N
Ne
n
2
=
N
N
e
n
=
1
e
n
N
10
1k 100k 10M
R
S
(:)
0.1
1
100
1000
1M
10k
100
10
e
ni
e
n
e
i
VOLTAGE NOISE DENSITY
(nV/
Hz)
e
t
LMP2234
SNOSAW4D –SEPTEMBER 2007–REVISED MARCH 2013 www.ti.com
Figure 47. Total Input Noise
VOLTAGE NOISE REDUCTION
The LMP2234 has an input voltage noise of 60 nV/√Hz . While this value is very low for micropower amplifiers,
this input voltage noise can be further reduced by placing multiple amplifiers in parallel as shown in Figure 48.
The total voltage noise on the output of this circuit is divided by the square root of the number of amplifiers used
in this parallel combination. This is because each individual amplifier acts as an independent noise source, and
the average noise of independent sources is the quadrature sum of the independent sources divided by the
number of sources. For N identical amplifiers, this means:
(2)
Figure 48 shows a schematic of this input voltage noise reduction circuit using the LMP2234. Typical resistor
values are: R
G
= 10Ω, R
F
= 1 kΩ, and R
O
= 1 kΩ.
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