ada4950-1 Analog Devices, Inc., ada4950-1 Datasheet - Page 20

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

ADA4950-1/ADA4950-2
Table 12. Differential Input, DC-Coupled
Nominal Linear Gain
1
2
3
Table 13. Single-Ended, Ground-Referenced Input, DC-Coupled, R
Nominal Linear Gain
1
2
3
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 +INx and −INx by the appropriate output
factor, where:
When the feedback factors are matched, R
β1 = β2 = β, and the noise gain becomes
Note that the output noise from V
total differential output noise density, v
square of the individual output noise terms.
Table 12 and Table 13 list the three available gain settings,
associated resistor values, input impedance, and output noise
density for both balanced and unbalanced input configurations.
CALCULATING THE INPUT IMPEDANCE FOR AN
APPLICATION CIRCUIT
The effective input impedance of a circuit depends on whether
the amplifier is being driven by a single-ended or differential
signal source. For balanced differential input signals, as shown
in Figure 54, the input impedance (R
The value of R
G
β
R
1
G2
N
=
= R
=
R
G
v
R
IN, dm
nOD
G1
(
F1
N
β
R
+ (R
1
=
+
G1
+
=
2
R
= (R
1
β
S
||R
β
G1
=
2
∑
i
T
8
=
G
).
)
1
1
G
and
v
depends on the selected gain.
+
is the circuit noise gain.
+ R
nOi
2
R
R
G
F
G
β
) = 2 × R
2
=
R
F2
R
+
R
500
500
500
G2
R
500
500
500
G
F
R
F
(Ω)
G2
(Ω)
OCM
are the feedback factors.
IN, dm
goes to 0 in this case. The
nOD
R
500
250
250||500 61.9
) is
G1
, is the root-sum-
F1
/R
(Ω)
G1
= R
R
53.6
57.6
R
500
250
250||500
F2
T
G
(Ω) (Std 1%) R
/R
(Ω)
G2
,
Rev. 0 | Page 20 of 28
667
375
267
R
1000
500
333
IN, se
IN, dm
S
= 50 Ω
(Ω)
For an unbalanced, single-ended input signal (see Figure 55),
the input impedance is
The input impedance of the circuit is effectively higher than it
is for a conventional op amp connected as an inverter because a
fraction of the differential output voltage appears at the inputs
as a common-mode signal, partially bootstrapping the voltage
across the input resistor, R
amplifier input terminals can be easily determined by noting
that the voltage at the inverting input is equal to the noninverting
output voltage divided down by the voltage divider that is formed
by R
input terminals due to negative voltage feedback and is in phase
with the input signal, thus reducing the effective voltage across
R
(Ω)
G
in the upper loop and partially bootstrapping R
Figure 54. ADA4950-x Configured for Balanced (Differential) Inputs
F
R
and R
IN
Figure 55. ADA4950-x with Unbalanced (Single-Ended) Input
R
526
277
194
Differential Output Noise Density (nV/√Hz)
9.25
12.9
16.6
,
R
G2
se
IN, se
(Ω)
=
G
V
IN, dm
in the lower loop. This voltage is present at both
⎛
⎜
⎜
⎜
⎜
⎝
1
1
−
V
R
R
OCM
G
G
R
R
2
Differential Output Noise Density (nV/√Hz)
9.07
12.2
15.0
G
G
×
(
R
V
R
G
OCM
R
G
+IN
–IN
F
ADA4950-x
+
G
. The common-mode voltage at the
R
R
R
ADA4950-x
F
F
+V
–V
F
+V
–V
R
R
)
S
S
F
S
S
F
⎞
⎟
⎟
⎟
⎟
⎠
R
L
V
V
OUT, dm
OUT, dm
G
.