REDUCED INPUT VOLTAGE NOISE =

en1+en2+

2

2

+enN

2

=

1

N

1

N

Nen

2

=

N

N

en

=

1

en

N

10

1k

100k

10M

RS (:)

0.1

1

100

1000

1M

10k

100

10

eni

en

ei

et

LMP2231

SNOSB01E – JANUARY 2008 – REVISED MARCH 2013

www.ti.com

Figure 48. Total Input Noise

VOLTAGE NOISE REDUCTION

The LMP2231 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 N amplifiers in parallel as shown in Figure 49. 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:

Figure 49 shows a schematic of this input voltage noise reduction circuit. Typical resistor values are: RG = 10Ω,

RF = 1 kΩ, and RO = 1 kΩ.

18

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