ADA4940-1 AD [Analog Devices], ADA4940-1 Datasheet - Page 21

• Manufacturer Part Number: ADA4940-1
• Description: Ultralow Power, Low Distortion
• Manufacturer: AD [Analog Devices]

Data Sheet
THEORY OF OPERATION
The
differential amplifiers fabricated on Analog Devices advanced
dielectrically isolated SiGe bipolar process. They provide two
closely balanced differential outputs in response to either
differential or single-ended input signals. An external feedback
network that is similar to a voltage feedback operational
amplifier sets the differential gain. The output common-mode
voltage is independent of the input common-mode voltage and
is set by an external voltage at the V
input stage allows input common-mode voltages between the
negative supply and 1.2 V below the positive supply. A rail-to-
rail output stage supplies a wide output voltage range.
The DISABLE pin can be used to reduce the supply current of
the amplifier to 13.5 µA.
Figure 62 shows the
The differential feedback loop consists of the differential trans-
conductance G
the R
loop is set up with a voltage divider across the two differential
outputs to create an output voltage midpoint and a common-
mode transconductance, G
+D
–D
+IN
IN
–IN
IN
ADA4940-1/ADA4940-2
F
/R
R
R
G
G
G
Figure 62.
feedback networks. The common-mode feedback
G
DIFF
DIFF
ADA4940-1/ADA4940-2
working through the G
ADA4940-1/ADA4940-2
V
CM
REF
.
are high speed, low power
G
G
OCM
C
C
O
O
C
C
G
CM
Architectural Block
terminal. The PNP
R
R
F
F
O
output buffers and
architecture.
–OUT
V
+OUT
OCM
Rev. B | Page 21 of 32
The differential feedback loop forces the voltages at +IN and −IN
to equal each other. This fact sets the following relationships:
Subtracting the previous equations gives the relationship that
shows R
The common-mode feedback loop drives the output common-
mode voltage that is sampled at the midpoint of the output
voltage divider to equal the voltage at V
following relationships:
Note that the differential amplifier’s summing junction input
voltages, +IN and −IN, are set by both the output voltages and
the input voltages.
(V
V
V
V
V
+
−
+OUT
−OUT
+
−
R
R
D
D
+OUT
IN
IN
G
G
F
IN
IN
and R
=
=
= V
= V
=
=
− V
−
+
−
−
D
D
OCM
OCM
V
V
G
−OUT
IN
IN
−
+
setting the differential gain.
R
R








OUT
OUT
+
−
F
F
R
R
) = (+D
F
F
V
V
R
R
OUT,
OUT,
+
+
F
F
2
2
R
R
dm
dm
G
G
IN








ADA4940-1/ADA4940-2
+
+
– (−D
V
V
−
+
OUT
OUT
IN
)) ×








R
R
OCM
F
F
R
R
+
+
R
R
G
G
. This results in the
G
R
R
F
G
G







