Datasheet

ADA4940-1/ADA4940-2 Data Sheet
Rev. C | Page 22 of 32
THEORY OF OPERATION
The ADA4940-1/ADA4940-2 are high speed, low power
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
OCM
terminal. The PNP
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 ADA4940-1/ADA4940-2 architecture.
The differential feedback loop consists of the differential trans-
conductance G
DIFF
working through the G
O
output buffers and
the R
F
/R
G
feedback networks. The common-mode feedback
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
CM
.
08452-058
G
O
C
C
C
C
R
F
R
G
G
CM
G
DIFF
G
O
R
G
V
REF
–OUT
+OUT
R
F
+D
IN
–D
IN
+IN
–IN
V
OCM
Figure 62. ADA4940-1/ADA4940-2 Architectural Block
The differential feedback loop forces the voltages at +IN and −IN
to equal each other. This fact sets the following relationships:
F
OUT
G
IN
R
V
R
D
−
−=
+
F
OUT
G
IN
R
V
R
D
+
−
=
−
Subtracting the previous equations gives the relationship that
shows R
F
and R
G
setting the differential gain.
(V
+OUT
− V
−OUT
) = (+D
IN
– (−D
IN
)) ×
G
F
R
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
OCM
. This results in the
following relationships:
V
+OUT
= V
OCM
+
2
dmOUT,
V
V
−OUT
= V
OCM
−
2
dmOUT,
V
Note that the differential amplifier’s summing junction input
voltages, +IN and −IN, are set by both the output voltages and
the input voltages.








+
+








+
+=
−
+
G
F
G
OUT
G
F
F
ININ
RR
R
V
RR
R
DV








+
+








+
−=
+
−
G
F
G
OUT
G
F
F
ININ
RR
R
V
RR
R
DV