TS615 datasheet(21/27 Pages) STMICROELECTRONICS

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TS615

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NOISE MEASUREMENT

Figure 62 : Noise Model

eN : input voltage noise of the amplifier

iNn : negative input current noise of the amplifier

iNp : positive input current noise of the amplifier

The closed loop gain is :

The six noise sources are :

Assuming the thermal noise of a resistance R as:

with

∆F the specified bandwidth.

On 1Hz bandwidth the thermal noise is reduced to

the

Boltzmann’s

constant

equals

1,374.10-23J/°K. T is the temperature (°K).

The output noise eNo is calculated using the Su-

perposition Theorem. But it is not the sum of all

noise sources. The output noise is the square root

of the sum of the square of each noise source.

The input noise of the instrumentation must be ex-

tracted 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 it is evalu-

ated from the measurement of the output divided

by the closed loop gain (eNo/g).

After simplification of the fourth and the fifth term

of (eq2) we obtain:

Measurement of eN:

We assume a short-circuit on the non-inverting in-

put (R3=0). (eq4) comes:

In order to easily extract the value of eN, the resis-

tance R2 will be chosen as low as possible. In the

other hand, the gain must be large enough.

R1=10

Ω, R2=910Ω, R3=0, Gain=92

Equivalent Input Noise: 2.57nV/

√Hz

Input Voltage Noise: eN=2.5nV/

√Hz

Measurement of iNn:

R3=0 and the output noise equation is still the

(eq5). This time the gain must be decreased to de-

crease the thermal noise contribution.

R1=100

Ω, R2=910Ω, R3=0, Gain=10.1

Equivalent Input Noise: 3.40nV/

√Hz

Negative Input Current Noise: iNn =21pA/

√Hz

Measurement of iNp:

To extract iNp from (eq3), a resistance R3 is con-

nected 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.

R1=100

Ω, R2=910Ω, R3=100Ω, Gain=10.1

Equivalent Input Noise: 3.93nV/

√Hz

Positive Input Current Noise: iNp=15pA/

√Hz

Conditions: frequency=100kHz, VCC=±2.5V

Instrumentation: Spectrum Analyzer HP3585A

(input noise of the HP3585A: 8nV/

√Hz)

output

iN-

iN+

HP3577

Input noise:

8nV/

√Hz

TS615

output

iN-

iN+

HP3577

Input noise:

8nV/

√Hz

TS615

----------

-------





iNn

R 2

iNp

R31

-------





-------

4kTR1

–

V54 kTR2

V61

-------



 4kTR3

4kTR F

∆

4kTR

eNo

+++

eq1

(

)

eNo

iNn

iNp

R 3

…

-------





4kTR14kTR21

R 2

R 1

-------





4kTR3

eq2

(

)

eNo

Measured

()

instrumentation

()

–

eq3

()

eNo

iNn

iNp

… g 4kTR21

-------





4kTR3

eq 4

()

eNo

iNn

4kTR2

eq5

()

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