ada4950-1 Analog Devices, Inc., ada4950-1 Datasheet - Page 19

• Manufacturer Part Number: ada4950-1
• Description: Low Power, Selectable Gain Differential Adc Driver, G = 1, 2, 3
• Manufacturer: Analog Devices, Inc.

APPLICATIONS INFORMATION
ANALYZING AN APPLICATION CIRCUIT
The ADA4950-x uses high open-loop gain and negative feedback
to force its 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 +INx and −INx
(see Figure 52). For most purposes, this voltage can be assumed
to be 0. Similarly, the difference between the actual output
common-mode voltage and the voltage applied to V
also be assumed to be 0. Starting from these principles, any
application circuit can be analyzed.
SELECTING THE CLOSED-LOOP GAIN
Using the approach described in the Analyzing an Application
Circuit section, the differential gain of the circuit in Figure 52
can be determined by
where the input resistors ( R
each side are equal.
For G = 1, the +INA and −INA inputs are used, and the +INB
and −INB inputs are left floating. The differential gain in this
case is calculated as follows:
For G = 2, the +INB and −INB inputs are used, and the +INA
and −INA inputs are left floating. The differential gain in this
case is calculated as follows:
Table 11. Output Noise Voltage Density Calculations for Matched Feedback Networks
Input Noise Contribution
Differential Input
Inverting Input
Noninverting Input
V
Gain Resistor, R
Gain Resistor, R
Feedback Resistor, R
Feedback Resistor, R
OCM
Input
G
G
V
V
=
=
OUT
IN
R
R
R
R
,
,
dm
G
G
F
F
dm
=
=
G1
G2
=
500
500
500
250
R
R
Ω
Ω
Ω
Ω
G
F
F1
F2
=
=
1
2
G
) and the feedback resistors ( R
Input Noise Term
v
i
i
v
v
v
v
v
nIN−
nIN+
nIN
nCM
nRG1
nRG2
nRF1
nRF2
OCM
can
Input Noise
Voltage Density
v
i
i
v
(4kTR
(4kTR
(4kTR
(4kTR
nIN−
nIN+
F
nIN
nCM
) on
Rev. 0 | Page 19 of 28
× (R
× (R
G1
G2
F1
F2
)
)
)
)
1/2
1/2
F2
F1
1/2
1/2
)
)
For G = 3, the +INA and +INB inputs are connected together,
and the −INA and −INB inputs are connected together. The
differential gain in this case is calculated as follows:
ESTIMATING THE OUTPUT NOISE VOLTAGE
The differential output noise of the ADA4950-x can be estimated
using the noise model in Figure 53. The values of R
the selected gain. The input-referred noise voltage density, v
is modeled as a differential input, and the noise currents, i
i
due to v
(defined in the G
currents are uncorrelated with the same mean-square value,
and each produces an output voltage that is equal to the noise
current multiplied by the associated feedback resistance. The
noise voltage density at the V
networks have the same feedback factor, as is true in most cases,
the output noise due to v
resistors contributes (4kTR
resistors appears directly at the output, and the noise from the
gain resistors appears at the output multiplied by R
summarizes the input noise sources, the multiplication factors,
and the output-referred noise density terms.
nIN+
, appear between each input and ground. The output voltage
G
Output
Multiplication Factor
G
1
1
0
R
R
1
1
=
nIN
F1
F2
N
R
R
/R
/R
F
G
is obtained by multiplying v
G1
G2
v
v
=
nRG1
nRG2
500
N
R
R
500
Ω
equation that follows Table 13). The noise
i
i
G1
G2
nIN+
nIN–
||
250
Ω
Figure 53. Noise Model
nCM
v
Ω
nIN
xx
ADA4950-1/ADA4950-2
R
R
=
is common mode. Each of the four
)
OCM
1/2
F1
F2
3
ADA4950-x
. The noise from the feedback
+
pin is v
v
v
V
nRF1
OCM
nRF2
Differential Output Noise
Voltage Density Term
v
v
v
v
v
v
v
v
nO1
nO2
nO3
nO4
nO5
nO6
nO7
nO8
nIN
= G
= (i
= (i
= 0 V
= (R
= (R
= (4kTR
= (4kTR
nCM
by the noise gain, G
nIN−
nIN+
N
. When the feedback
F1
F2
v
(v
/R
/R
nOD
v
)(R
)(R
nIN
nCM
F1
F2
G1
G2
)
F2
F1
)
)
)(4kTR
)(4kTR
F
1/2
1/2
/R
)
)
G
G
depend on
. Table 11
G1
G2
nIN−
)
)
1/2
1/2
nIN
and
N
,