TS615 ST Microelectronics, Inc., TS615 Datasheet - Page 21

• Manufacturer Part Number: TS615
• Description: Dual Wide Band Operational Amplifier
• Manufacturer: ST Microelectronics, Inc.

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
k
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
is
=
R3
R3
N3
N3
the
R1
R1
V1
2
iN+
iN+
iN-
iN-
N1
N1
Boltzmann’s
V3
+
V6
N2
N2
V4
V2
V1
A
=
V
=
2
V2
V5
eN
eN
iNp
=
=
+
=
4kTR F
1
–
V3
=
eN
=
4kTR
R2
------ -
R1
+
+
g
_
_
+
iNn
TS615
TS615
2
R3
R2
------ -
R1
=
4 kTR2
+
R2
R2
1
1
V4
constant
+
4kTR1
+
R 2
4kTR3
1
R
--------- -
2
R
R2
------ -
R1
fb
+
+
g
R2
------ -
R1
V5
2
output
output
+
equals
V6
HP3577
HP3577
Input noise:
Input noise:
8nV/ Hz
8nV/ Hz
2
,
eq1
to
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
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
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
Equivalent Input Noise: 3.93nV/ Hz
Positive Input Current Noise: iNp=15pA/ Hz
Conditions: frequency=100kHz, V
Instrumentation: Spectrum Analyzer HP3585A
(input noise of the HP3585A: 8nV/ Hz)
eNo
+
eNo
eNo
eNo
R2
------ -
R1
=
2
2
=
+
=
2
=
Measured
R2=910
g
eN
eN
eN
R2=910
R2=910
4kTR1
2
2
4kTR2
2
g
g
g
2
2
2
+
+
+
+
+
4kTR2
iNn
iNn
2
iNn
R3=0, Gain=92
1
R3=0, Gain=10.1
R3=100
–
2
+
2
2
instrumentation
R2
------ -
R1
CC
+
R2
R2
R2
=±2.5V
1
2
2
2
2
+
+
+
+
R 2
------ -
R 1
4kTR3
iNp
g
Gain=10.1
iNp
2
4kTR2
2
2
4kTR3
R3
R3
eq4
2
2
2
,
TS615
eq5
eq3
g
21/27
g
2
2
eq2