ADA4940-1 Datasheet, PDF (21/32 Pages) Analog Devices – Ultralow Power, Low Distortion

Data Sheet
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 VOCM 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 GDIFF working through the GO output buffers and
the RF/RG 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, GCM.
R_G
+D_IN
R_F
C_C
G_O
+IN
G_DIFF
–IN
G_CM
V_REF
G_O
–OUT
V_OCM
+OUT
–D_IN
R_G
C_C
R_F
Figure 62. ADA4940-1/ADA4940-2 Architectural Block
ADA4940-1/ADA4940-2
The differential feedback loop forces the voltages at +IN and −IN
to equal each other. This fact sets the following relationships:
+D_{IN = −} V_−OUT
R_G
R_F
−D_{IN = −} V_+OUT
R_G
R_F
Subtracting the previous equations gives the relationship that
shows RF and RG setting the differential gain.
(V+OUT − V−OUT) = (+DIN – (−DIN)) × ^RF
R_G
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 VOCM. This results in the
following relationships:
V = V + +OUT
OCM
V_{OUT, dm}
2
V = V − −OUT
OCM
V_{OUT, dm}
2
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_{+ IN}
=

+D_{IN }
R_F
R_F + R_G


+ V_{−OUT }
R_G
R_F + R_G

V_{− IN}
=
−D_{IN }

R_F
R_F + R_G
 ^{+ V+OUT} 


R_G
R_F + R_G

Rev. B | Page 21 of 32