Noise measurements

RHF310

14/22

Doc ID 15577 Rev 3

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 equivalent input noise because 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 fifth terms of Equation 2, you obtain:

Equation 4

4.1

Measurement of the input voltage noise

eN

Assuming a short-circuit on the non-inverting input (R3=0), from Equation 4 you can derive:

Equation 5

To easily extract the value of eN, the resistance R2 must be as low as possible. On the other

hand, the gain must be high enough.

R3=0, gain: g=100

4.2

Measurement of the negative input current noise

iNn

To measure the negative input current noise iNn, R3 is set to zero and Equation 5 is used.

This time, the gain must be lower in order to decrease the thermal noise contribution.

R3=0, gain: g=10

4.3

Measurement of the positive input current noise

iNp

To extract iNp from Equation 3, a resistance R3 is connected to the non-inverting input. The

value of R3 must be selected so as to keep its thermal noise contribution as low as possible

against the iNp contribution.

R3=100

Ω, gain: g=10

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

×

g4kTR2

1

R2

R1

--------

+

2

4kTR3

×

+

×

+

=

eNo

eN

2

g

2

iNn

2

R2

2

g4kTR2

×

+

×

+

×

=