Datasheet

ADA4937-1/ADA4937-2 Data Sheet
Rev. D | Page 18 of 28
THEORY OF OPERATION
The ADA4937-x differs from conventional op amps in that it
has two outputs whose voltages move in opposite directions.
Like an op amp, it relies on open-loop gain and negative feed-
back to force these outputs to the desired voltages. The ADA4937-
x behaves much like a standard voltage feedback op amp, which
makes it easier to perform single-ended-to-differential conversions,
common-mode level shifting, and amplifications of differential
signals. Also like an op amp, the ADA4937-x has high input
impedance and low output impedance.
Two feedback loops control the differential and common-mode
output voltages. The differential feedback loop, set with external
resistors, controls only the differential output voltage. The
common-mode feedback loop controls only the common-mode
output voltage. This architecture makes it easy to set the output
common-mode level to any arbitrary value. It is forced, by
internal common-mode feedback, to be equal to the voltage
applied to the V
OCM
input without affecting the differential
output voltage.
The ADA4937-x architecture results in outputs that are highly
balanced over a wide frequency range without requiring tightly
matched external components. The common-mode feedback
loop forces the signal component of the output common-mode
voltage to zero. This results in nearly perfectly balanced differential
outputs that are identical in amplitude and are exactly 180° apart
in phase.
ANALYZING AN APPLICATION CIRCUIT
The ADA4937-x uses open-loop gain and negative feedback to
force its 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 52). 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
OCM
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 52 can be
determined by
G
F
dmIN
dmOUT
R
R
V
V
=
,
,
This assumes that the input resistors (R
G
) and feedback resistors
(R
F
) on each side are equal.
ESTIMATING THE OUTPUT NOISE VOLTAGE
The differential output noise of the ADA4937-x can be esti-
mated using the noise model in Figure 53. The input-referred
noise voltage density, v
nIN
, is modeled as a differential input, and
the noise currents, i
nIN−
and i
nIN+
, appear between each input and
ground. The noise currents are assumed to be equal and produce
a voltage across the parallel combination of the gain and feedback
resistances. v
n, cm
is the noise voltage density at the V
OCM
pin. Each
of the four resistors contributes (4kTR
x
)
1/2
.
Table 9 summarizes the input noise sources, the multiplication
factors, and the output-referred noise density terms.
ADA4937
+
R
F2
V
nOD
V
nCM
V
OCM
V
nIN
R
F1
R
G2
R
G1
V
nRF1
V
nRF2
V
nRG1
V
nRG2
i
nIN+
i
nIN–
06591-050
Figure 53. ADA4937-x Noise Model
Table 9. Output Noise Voltage Density Calculations
Input Noise Contribution Input Noise Term
Input Noise
Voltage Density
Output
Multiplication Factor
Output Noise
Voltage Density Term
Differential Input v
nIN
v
nIN
G
N
v
nO1
= G
N
(v
nIN
)
Inverting Input i
nIN−
i
nIN−
× (R
G2
||R
F2
) G
N
v
nO2
= G
N
[i
nIN−
× (R
G2
||R
F2
)]
Noninverting Input i
nIN+
i
nIN+
× (R
G1
||R
F1
) G
N
v
nO3
= G
N
[i
nIN+
× (R
G1
||R
F1
)]
V
OCM
Input
v
n, cm
v
n, cm
G
N
(β
1
− β
2
)
v
nO4
= G
N
(β
1
− β
2
)(v
n, cm
)
Gain Resistor R
G1
v
nRG1
(4kTR
G1
)
1/2
G
N
(1 − β
1
) v
nO5
= G
N
(1 − β
1
)(4kTR
G1
)
1/2
Gain Resistor R
G2
v
nRG2
(4kTR
G2
)
1/2
G
N
(1 − β
2
) v
nO6
= G
N
(1 − β
2
)(4kTR
G2
)
1/2
Feedback Resistor R
F1
v
nRF1
(4kTR
F1
)
1/2
1 v
nO7
= (4kTR
F1
)
1/2
Feedback Resistor R
F2
v
nRF2
(4kTR
F2
)
1/2
1 v
nO8
= (4kTR
F2
)
1/2