ada4930-1 Analog Devices, Inc., ada4930-1 Datasheet - Page 17

• Manufacturer Part Number: ada4930-1
• Description: Ultralow Noise Drivers For Low Voltage Adcs Ada4930-1/ada4930-2
• Manufacturer: Analog Devices, Inc.

THEORY OF OPERATION
The ADA4930-1/ADA4930-2 differ from conventional op amps
in that they have two outputs whose voltages move in opposite
directions and an additional input, V
on high open-loop gain and negative feedback to force these
outputs to the desired voltages. The ADA4930-1/ADA4930-2
behave much like standard voltage feedback op amps and facilitate
single-ended-to-differential conversions, common-mode level
shifting, and amplifications of differential signals. Like op amps,
the ADA4930-1/ADA4930-2 have high input impedance and low
output impedance.
Two feedback loops control the differential and common-mode
output voltages. The differential feedback, set with external
resistors, controls the differential output voltage. The common-
mode feedback controls the common-mode output voltage. This
architecture makes it easy to set the output common-mode level
to any arbitrary value within the specified limits. The output
common-mode voltage is forced to be equal to the voltage applied
to the V
The internal common-mode feedback loop produces outputs
that are highly balanced over a wide frequency range without
requiring tightly matched external components. This results
in differential outputs that are very close to the ideal of being
identical in amplitude and are exactly 180°◀apart in phase.
ANALYZING AN APPLICATION CIRCUIT
The ADA4930-1/ADA4930-2 use high open-loop gain and
negative feedback to force their differential and common-mode
output voltages 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 42). 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
be assumed 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 42 is
determined by
where the gain and feedback resistors, R
are equal.
V
V
OUT
OCM
IN
,
,
dm
dm
input by the internal common-mode feedback loop.
=
R
R
G
F
OCM
. Like an op amp, they rely
G
and R
F
, on each side
OCM
can also
Rev. A | Page 17 of 28
ESTIMATING THE OUTPUT NOISE VOLTAGE
The differential output noise of the ADA4930-1/ADA4930-2 can
be estimated using the noise model in Figure 43. The input-referred
noise voltage density, v
currents, i
Similar to the case of conventional op amps, the output noise
voltage densities can be estimated by multiplying the input-
referred terms at +IN and −IN by an appropriate output factor.
The output voltage due to v
the noise gain, G
The circuit noise gain is
where the feedback factors are
When the feedback factors are matched, R
β1 = β2 = β, and the noise gain becomes
The noise 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
feedback networks have the same feedback factor, as in most
cases, the output noise due to v
output noise from V
Each of the four resistors contributes (4kTR
from the feedback resistors appears directly at the output, and
the noise from the gain resistors appears at the output multiplied
by R
The total differential output noise density, v
square of the individual output noise terms.
F
G
/R
v
nOD
N
G
.
=
nIN−
=
(
β
V
V
nRG1
nRG2
1
and i
∑
i
+
=
2
8
1
(
β
N
v
2
.
R
R
nODi
)
nIN+
i
i
G1
G2
nIN+
nIN–
OCM
)
, appear between each input and ground.
Figure 43. Noise Model
nIN
2
is zero.
, is modeled as differential. The noise
V
nIN
nIN
ADA4930-1/ADA4930-2
R
R
is obtained by multiplying v
β
F1
F2
1
ADA4930
nCM
+
=
R
OCM
V
V
V
is common-mode and the
F1
nRF1
OCM
nRF2
R
+
pin is v
G1
R
G1
G
F1
N
/R
nOD
xx
and
V
=
nCM
)
nOD
V
G1
1/2
, is the root-sum-
nCM
1
β
. The noise
. When the
= R
β
=
2
1
=
F2
+
R
/R
R
R
F2
nIN
G2
G
F
R
+
,
G2
by
.
R
G2
.