Noise Measurements

TSH300

14/18

5

Noise Measurements

The noise model is shown in

Figure 45, where:

●

eN: input voltage noise of the amplifier

●

iNn: negative input current noise of the amplifier

●

iNp: positive input current noise of the amplifier

The thermal noise of a resistance R is:

where

∆F is the specified bandwidth.

On a 1Hz bandwidth the thermal noise is reduced to

where k is the Boltzmann's constant, equal to 1,374.10-23J/°K. T is the temperature (°K).

The output noise eNo is calculated using the Superposition Theorem. However eNo is not the

simple sum of all noise sources, but rather the square root of the sum of the square of each

noise source, as shown in Equation 1:

Figure 45. Noise model

+

_

R3

R1

output

R2

iN-

iN+

HP3577

Input noise:

8nV/

√Hz

N1

N2

N3

eN

+

_

R3

R1

output

R2

iN-

iN+

HP3577

Input noise:

8nV/

√Hz

N1

N2

N3

eN

4kTR

4kTR

eNo

V1

2

V2

2

V3

2

V4

2

V5

2

V6

2

++

+++

(Equation 1)

=

eNo

2

eN

2

g

2

iNn

2

R2

2

iNp

2

+

×

+

×

R3

2

×

g

2

×

R2

R1

--------

()

2

4kTR1

4kTR2

g

2

4kTR3

(Equation 2)

×

++

×

+

=