Noise Measurements

TSH350

13/21

5 Noise Measurements

The noise model is shown in Figure 32, where:

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:

Equation 1

Equation 2

Figure 32: 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

++

+++

=

eNo

2

eN

2

g

2

iNn

2

R2

2

iNp

2

+

×

+

×

R3

2

×

g

2

×

R2

R1

--------

2

4kTR1

4kTR2

1

R2

R1

--------

+

2

4kTR3

×

++

×

+

=