Data Sheet ADA4939-1/ADA4939-2
Rev. A | Page 17 of 24
THEORY OF OPERATION
The ADA4939-1/ADA4939-2 differ from conventional op amps
in that they have two outputs whose voltages move in opposite
directions and an additional input, VOCM. Like op amps, they
rely on high open-loop gain and negative feedback to force
these outputs to the desired voltages. The ADA4939-1/
ADA4939-2 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 ADA4939-1/ADA4939-2 have high
input impedance and low output impedance. Because they use
voltage feedback, the ADA4939-1/ADA4939-2 manifest a
nominally constant gain-bandwidth product.
Two feedback loops are employed to control the differential and
common-mode output voltages. The differential feedback, set
with external resistors, controls only the differential output voltage.
The common-mode feedback controls only 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 by the internal
common-mode feedback loop to be equal to the voltage applied
to the VOCM input.
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 ADA4939-1/ADA4939-2 use high open-loop gain and
negative feedback to force their 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 +IN and −IN (see Figure 42). For most purposes, this
voltage is zero. Similarly, the difference between the actual
output common-mode voltage and the voltage applied to VOCM
is also 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 can be
determined by
G
F
dmIN
dmOUT
R
R
V
V
,
,
This presumes that the input resistors (RG) and feedback resistors
(RF) on each side are equal.
STABLE FOR GAINS ≥2
The ADA4939-1/ADA4939-2 frequency response exhibits
excessive peaking for differential gains <2; therefore, operate the
devioce with differential gains ≥2.
ESTIMATING THE OUTPUT NOISE VOLTAGE
To estimate the differential output noise of the ADA4939-1/
ADA4939-2 use the noise model shown in Figure 43. The input-
referred noise voltage density, vnIN, is modeled as a differential
input, and the noise currents, inIN− and inIN+, appear between
each input and ground. The output voltage due to vnIN is obtained
by multiplying vnIN by the noise gain, GN (defined in the GN
equation that follows). 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/VOCMx
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. 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 RFx/RGx. Table 11 summarizes the input noise sources, the
multiplication factors, and the output-referred noise density
terms.
+
R
F2
V
nOD
V
nCM
V
OCM
V
nIN
R
F1
R
G2
R
G1
nRF1
V
nRF2
nRG1
V
nRG2
i
nIN+
i
nIN–
07429-050
ADA4939-1/
ADA4939-2
Figure 43. Noise Model