TS615

Noise Measurements

24/36

On a 1Hz bandwidth the thermal noise is reduced to

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

The input noise of the instrumentation must be extracted from the measured noise value. The real output

noise value of the driver is:

Equation 3

The input noise is called the Equivalent Input Noise as it is not directly measured but is evaluated from the

measurement of the output divided by the closed loop gain (eNo/g).

After simplification of the fourth and the fifth term of Equation 2 we obtain:

Equation 4

7.1 Measurement of eN

If we assume a short-circuit on the non-inverting input (R3=0), Equation 4 becomes:

Equation 5

In order to easily extract the value of eN, the resistance R2 will be chosen as low as possible. On the

other hand, the gain must be large enough:

Ω, R2=910Ω, R3=0, Gain=92

√Hz

√Hz

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

×

++

×

+

eNo

Measured

()

2

instrumentation

()

2

–

=

eNo

2

eN

2

g

2

iNn

2

R2

2

iNp

2

+

×

+

×

R3

2

×

g

2

×

1

R2

R1

--------

+





2

4kTR 3

×

+

×

+

=

eNo

eN

2

g

2

iNn

2

R2

2

g4kTR2

×

+

×

+

×

=