Noise Measurements

TSH300

15/18

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

real output noise value of the driver is:

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:

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:

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

–

(Equation 3)

=

eNo

2

eN

2

g

2

iNn

2

R2

2

iNp

2

+

×

+

×

R3

2

×

g

2

×

g4kTR2

g

2

4kTR3

×

+

×

(Equation 4)

+

=

eNo

eN

2

g

2

iNn

2

R2

2

g4kTR2

×

+

×

+

×

(Equation 5)

=