ADA4940-1 AD [Analog Devices], ADA4940-1 Datasheet - Page 22

• Manufacturer Part Number: ADA4940-1
• Description: Ultralow Power, Low Distortion
• Manufacturer: AD [Analog Devices]

ADA4940-1/ADA4940-2
APPLICATIONS INFORMATION
ANALYZING AN APPLICATION CIRCUIT
The
feedback to force their differential and common-mode output
voltages in such a way as to minimize the differential and common-
mode error voltages. The differential error voltage is defined as
the voltage between the differential inputs labeled +IN and −IN (see
Figure 61). For most purposes, this voltage can be assumed to be
zero. Similarly, the difference between the actual output common-
mode voltage and the voltage applied to V
to be zero. Starting from these two assumptions, any application
circuit can be analyzed.
SETTING THE CLOSED-LOOP GAIN
The differential mode gain of the circuit in Figure 61 can be
determined by
This assumes that the input resistors (R
(R
ESTIMATING THE OUTPUT NOISE VOLTAGE
The differential output noise of the
be estimated using the noise model in Figure 63. The input-referred
noise voltage density, v
the noise currents, i
ground. The noise currents are assumed to be equal and produce
a voltage across the parallel combination of the gain and feedback
resistances. v
of the four resistors contributes (4kTR
the input noise sources, the multiplication factors, and the
output-referred noise density terms. For more noise calculation
information, go to the Analog Devices Differential Amplifier
Calculator (DiffAmpCalc™), click ADIDiffAmpCalculator.zip
and follow the on-screen prompts.
Table 14. Output Noise Voltage Density Calculations
Input Noise Contribution
Differential Input
Inverting Input
Noninverting Input
V
Gain Resistor R
Gain Resistor R
Feedback Resistor R
Feedback Resistor R
OCM
F
) on each side are equal.
ADA4940-1/ADA4940-2
Input
V
V
OUT
IN
,
,
dm
dm
nCM
=
G1
G2
is the noise voltage density at the V
R
R
G
F
F1
F2
nIN−
nIN
and i
, is modeled as a differential input, and
nIN+
use open-loop gain and negative
, appear between each input and
ADA4940-1/ADA4940-2
Input Noise Term
v
i
i
v
v
v
v
v
nIN−
nIN+
nIN
nCM
nRG1
nRG2
nRF1
nRF2
x
)
G
OCM
1/2
) and feedback resistors
. Table 14 summarizes
can also be assumed
OCM
pin. Each
Input Noise
Voltage Density
v
i
i
v
(4kTR
(4kTR
(4kTR
(4kTR
nIN−
nIN+
nIN
nCM
can
Rev. B | Page 22 of 32
× (R
× (R
G1
G2
F1
F2
)
)
)
)
1/2
1/2
1/2
1/2
G2
G1
||R
||R
F2
F1
)
)
As with conventional op amp, the output noise voltage densities
can be estimated by multiplying the input-referred terms at +IN
and −IN by the appropriate output factor,
where:
When R
becomes
Note that the output noise from V
The total differential output noise density, v
square of the individual output noise terms.
G
β
1
N
=
=
G
v
R
nOD
(
F1
N
Output
Multiplication Factor
G
G
G
G
G
G
1
1
β
R
1
F1
N
N
N
N
N
N
=
+
G1
+
/R
2
(β
(1 − β
(1 − β
V
V
=
R
1
β
nRG1
nRG2
β
1
G1
G1
Figure 63.
2
− β
=
∑
i
)
= R
=
8
1
1
2
1
and
is the circuit noise gain.
R
R
v
)
)
2
+
i
i
)
G1
G2
nOi
nIN+
nIN–
2
F2
R
R
/R
G
F
β
ADA4940-1/ADA4940-2
2
G2
=
V
, then β1 = β2 = β, and the noise gain
nIN
R
R
R
F2
F1
F2
R
ADA4940-1/
ADA4940-2
+
+
G2
R
G2
OCM
are the feedback factors.
Output-Referred Noise
Voltage Density Term
v
v
v
v
v
v
v
v
nO1
nO2
nO3
nO4
nO5
nO6
nO7
nO8
V
V
V
nRF1
OCM
nRF2
goes to zero in this case.
= G
= G
= G
= G
= G
= G
= (4kTR
= (4kTR
Noise Model
N
N
N
N
N
N
nOD
(v
[i
[i
(β
(1 − β
(1 − β
nIN−
nIN+
Data Sheet
V
, is the root-sum-
nIN
1
F1
F2
nOD
− β
V
)
)
)
nCM
1/2
1/2
× (R
× (R
2
1
)(4kTR
)(4kTR
2
)(v
G2
G1
nCM
||R
||R
G1
G2
F2
F1
)
)]
)]
)
)
1/2
1/2