ada4939-1 Analog Devices, Inc., ada4939-1 Datasheet - Page 18

• Manufacturer Part Number: ada4939-1
• Description: Ultralow Distortion Differential Adc Driver
• Manufacturer: Analog Devices, Inc.

ADA4939-1/ADA4939-2
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
Table 12. Differential Input, DC-Coupled
Nominal Gain (dB)
6
10
14
Table 13. Single-Ended Ground-Referenced Input, DC-Coupled, R
Nominal Gain (dB)
6
10
14
1
Similar to the case of a 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 the feedback factors are matched, R
β2 = β, and the noise gain becomes
Note that the output noise from V
The total differential output noise density, v
square of the individual output noise terms.
Table 12 and Table 13 list several common gain settings,
associated resistor values, input impedance, and output noise
density for both balanced and unbalanced input configurations.
G
β
R
OCM
1
G2
N
=
= R
=
Input
G
v
R
(
nOD
G1
F1
N
β
R
1
+ (R
=
+
G1
+
2
=
R
1
β
S
β
||R
G1
2
=
∑
)
i
T
=
G1
G2
8
).
1
1
and
is the circuit noise gain.
v
+
nOi
2
R
R
F1
F2
G
F
β
2
=
R
R
402
402
402
R
402
402
402
F2
F
F
R
(Ω)
+
(Ω)
G2
R
G2
Input Noise Term
v
i
i
v
v
v
v
v
OCM
nIN
nIN
nIN
nCM
nRG1
nRG2
nRF1
nRF2
R
200
127
80.6
are the feedback factors.
R
200
127
80.6
G1
G
goes to zero in this case.
(Ω)
(Ω)
F1
/R
nOD
R
400
254
161
R
60.4
66.5
76.8
G1
, is the root-sum-
IN, dm
T
= R
(Ω)
(Ω)
F2
/R
R
301
205
138
G2
IN, cm
, β1 =
Input Noise
Voltage Density
v
i
i
v
(4kTR
(4kTR
(4kTR
(4kTR
nIN
nIN
Differential Output Noise Density (nV/√Hz)
9.7
12.4
16.6
nIN
nCM
Rev. 0 | Page 18 of 24
× (R
× (R
(Ω)
G1
G2
F1
F2
)
)
F2
F1
)
)
1/2
1/2
1/2
1/2
)
)
R
228
155
111
S
G2
= 50 Ω
(Ω)
IMPACT OF MISMATCHES IN THE FEEDBACK
NETWORKS
As previously mentioned, even if the external feedback networks
(R
loop still forces the outputs to remain balanced. The amplitudes
of the signals at each output remain equal and 180° out of phase.
The input-to-output differential mode gain varies proportionately
to the feedback mismatch, but the output balance is unaffected.
The gain from the V
When β1 = β2, this term goes to zero and there is no differential
output voltage due to the voltage on the V
noise). The extreme case occurs when one loop is open and the
other has 100% feedback; in this case, the gain from V
to V
feedback loops are nominally matched to within 1% in most
applications, and the output noise and offsets due to the V
input are negligible. If the loops are intentionally mismatched by a
large amount, it is necessary to include the gain term from V
to V
and β2 = 0.25, the gain from V
is set to 2.5 V, a differential offset voltage is present at the output of
(2.5 V)(0.67) = 1.67 V. The differential output noise contribution is
(7.5 nV/√Hz)(0.67) = 5 nV/√Hz. Both of these results are
undesirable in most applications; therefore, it is best to use
nominally matched feedback factors.
1
F
/R
O, dm
O, dm
2(β1 − β2)/(β1 + β2)
G
Differential Output Noise Density (nV/√Hz)
9.1
11.1
13.5
) are mismatched, the internal common-mode feedback
Output
Multiplication Factor
G
1
1
0
R
R
1
1
is either +2 or −2, depending on which loop is closed. The
F1
F2
N
and account for the extra noise. For example, if β1 = 0.5
/R
/R
G1
G2
OCM
pin to V
OCM
O, dm
to V
Differential Output Noise
Voltage Density Term
v
v
v
v
v
v
v
v
nO1
nO2
nO3
nO4
nO5
nO6
nO7
nO8
is equal to
O, dm
= G
= (i
= (i
= 0
= (R
= (R
= (4kTR
= (4kTR
is 0.67. If the V
OCM
nIN
nIN
N
F1
F2
(v
)(R
)(R
/R
/R
input (including
nIN
G1
G2
F1
F2
F2
F1
)
)
)
)(4kTR
)(4kTR
)
)
1/2
1/2
OCM
G1
G2
)
)
OCM
1/2
1/2
input
OCM
OCM
pin