RHF350 datasheet(13/23 Pages) STMICROELECTRONICS

13 / 23 page

RHF350

Noise measurements

Doc ID 15604 Rev 4

13/23

Equation 4

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

The real output noise value of the driver is:

Equation 5

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 the fifth term of

Equation 4 we obtain:

Equation 6

4.1

Measurement of the input voltage noise

If we assume a short-circuit on the non-inverting input (R3 = 0), from

Equation 6 we can

derive:

Equation 7

In order to easily extract the value of

eN, the resistance R2 will be chosen to be as low as

possible. On the other hand, the gain must be large enough.

R3 = 0, gain: g = 100

4.2

Measurement of the negative input current noise

iNn

To measure the negative input current noise

iNn, we set R3 = 0 and use Equation 7. 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 5, 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 W, gain: g = 10

eNo

iNn

iNp

--------

4kTR1

4kTR2

--------

4kTR3

eNo

Measured

()

instrumentation

()

–

eNo

iNn

iNp

g4kTR2

--------

4kTR3

eNo

iNn

g4kTR2

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