Noise Measurements

TSH330

13/19

5 Noise Measurements

The noise model is shown in

Figure 33, 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:

Equation 1

Equation 2

Figure 33. 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

++

++

+

=

eN o

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

×

++

×

+

=