Noise Measurements

TSH310

13/19

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

Measurement of the input voltage noise eN

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

Equation 5

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

other hand, the gain must be large enough:

R3=0, gain: g=100

Measurement of the negative input current noise iNn

To measure the negative input current noise iNn, we set R3=0 and use Equation 5. This time the gain

must be lower in order to decrease the thermal noise contribution:

R3=0, gain: g=10

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 chosen in order to keep its thermal noise contribution as low as possible against the iNp

contribution:

R3=100

Ω, gain: g=10

eNo

Measured

()

2

instrumentation

()

2

–

=

eNo

2

eN

2

g

2

iNn

2

R2

2

iNp

2

+

×

+

×

R3

2

×

g

2

×

g

4kTR21

R2

R1

-------

+

2

4kTR3

×

+

×

+

=

eNo

eN

2

g

2

iNn

2

R 2

2

g

4kTR 2

×

+

×

+

×

=