TS615IPWT View Datasheet(PDF) - STMicroelectronics

Part Name

Description

Manufacturer

TS615IPWT

DUAL WIDE BAND OPERATIONAL AMPLIFIER WITH HIGH OUTPUT CURRENT

STMicroelectronics 

TS615

NOISE MEASUREMENT

Figure 62 : Noise Model

R3

iN+

+

TS615

_

N3 iN- eN

N2

R2

R1

N1

output

HP3577

Input noise:

8nV/√Hz

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 :

AV = g = 1 + R--R---f-g-b--

The six noise sources are :

V2 = iNn × R2

V5 = 4kTR2

V1 = eN × 1 + RR-----21-

V3 = iNp × R3 × 1 + RR-----21-

V4 = –RR-----21- × 4kTR1

V6 = 1 + RR-----21- 4kTR3

Assuming the thermal noise of a resistance R as:

4kTR∆F

with ∆F the specified bandwidth.

On 1Hz bandwidth the thermal noise is reduced to

4kTR

k is the Boltzmann’s constant equals to

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.

eNo = V12 + V22 + V32 + V42 + V52 + V62,(eq1 )

eNo2 = eN2 × g2 + iNn2 × R22 + iNp2 × R32 × g2

…

+





RR-----21- 

2

×

4kTR1

+

4kTR2

+





1

+

RR-----21-

2

×

4kTR3,

(eq2)

The input noise of the instrumentation must be ex-

tracted from the measured noise value. The real

output noise value of the driver is:

eNo = (Measured)2 – (instrumentation )2, (eq3)

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:

2

eNo

=

2

eN

×

2

g

+

2

iNn

×

R22

+

2

iNp

×

R32

×

2

g

… + g × 4kTR2 + 1 + RR-----21- 2 × 4kTR3, (eq4)

Measurement of eN:

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

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

eNo =

2

eN

×

2

g

+

2

iNn

×

R22

+

g

×

4 k T R 2,

(eq5)

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=10Ω, 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=100Ω, 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=100Ω, 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)

21/27

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