ADA4930-1/ADA4930-2
Rev. A | Page 17 of 28
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, VOCM. Like an op amp, they rely
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
common-mode voltage is forced to be equal to the voltage applied
to the VOCM input by the internal common-mode feedback loop.
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 VOCM can also
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
G
F
dm
IN
dm
OUT
R
V
=
,
where the gain and feedback resistors,
RG and RF, on each side
are equal.
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, vnIN, is modeled as differential. The noise
currents, inIN and inIN+, appear between each input and ground.
ADA4930
+
RF2
VnOD
VnCM
VOCM
VnIN
RF1
RG2
RG1
VnRF1
VnRF2
VnRG1
VnRG2
inIN+
inIN–
09
20
9-
0
50
Figure 43. Noise Model
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 vnIN is obtained by multiplying vnIN by
the noise gain, GN.
The circuit noise gain is
()
2
1
N
β
G
+
=
2
where the feedback factors are
G1
F1
G1
1
R
β
+
=
and
G2
F2
G2
2
R
β
+
=
.
When the feedback factors are matched, RF1/RG1 = RF2/RG2,
β1 = β2 = β, and the noise gain becomes
G
F
N
R
β
G
+
=
1
.
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 VOCM pin is vnCM. When the
feedback networks have the same feedback factor, as in most
cases, the output noise due to vnCM is common-mode and the
output noise from VOCM is zero.
Each of the four resistors contributes (4kTRxx)1/2. The noise
from the feedback resistors appears directly at the output, and
the noise from the gain resistors appears at the output multiplied
by RF/RG.
The total differential output noise density, vnOD, is the root-sum-
square of the individual output noise terms.
∑
=
8
1
i
2
)
(
nODi
nOD
v