Noise measurements

TSH350

14/22

5

Noise measurements

The noise model is shown in Figure 32:

●

eN is the input voltage noise of the amplifier

●

iNn is the negative input current noise of the amplifier

●

iNp is the positive input current noise of the amplifier

Figure 32.

Noise model

The thermal noise of a resistance R is

where

ΔF 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

+

_

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

ΔF

4kTR

eNo

V1

2

V2

2

V3

2

V4

2

V5

2

V6

2

+++++

=