Noise measurements

RHF330

14/22

Doc ID 15576 Rev 3

5

Noise measurements

The noise model is shown in Figure 30.

●

eN: input voltage noise of the amplifier

●

iNn: negative input current noise of the amplifier

●

iNp: positive input current noise of the amplifier

Figure 30.

Noise model

The thermal noise of a resistance R is:

where

ΔF is the specified bandwidth.

On a 1 Hz bandwidth the thermal noise is reduced to:

where k is the Boltzmann's constant, equal to 1,374.E(-23)J/°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

HP3577

Input noise:

8 nV/

√Hz

N1

N2

N3

eN

AM00837

4kTR

ΔF

4kTR

eNo

V1

2

V2

2

V3

2

V4

2

V5

2

V6

2

+++++

=