Datasheet

ADA4939-1/ADA4939-2
Rev. 0 | Page 21 of 24
INPUT COMMON-MODE VOLTAGE RANGE
The ADA4939 input common-mode range is centered between the
two supply rails, in contrast to other ADC drivers with level-shifted
input ranges, such as the
ADA4937. The centered input common-
mode range is best suited to ac-coupled, differential-to-differential
and dual supply applications.
For 5 V single-supply operation, the input common-mode
range at the summing nodes of the amplifier is specified as
1.1 V to 3.9 V and is specified as 0.9 V to 2.4 V with a 3.3 V
supply. To avoid nonlinearities, the voltage swing at the +IN
and −IN terminals must be confined to these ranges.
INPUT AND OUTPUT CAPACITIVE AC COUPLING
Input ac coupling capacitors can be inserted between the source
and R
G
. This ac coupling blocks the flow of the dc common-
mode feedback current and causes the ADA4939 dc input
common-mode voltage to equal the dc output common-mode
voltage. These ac coupling capacitors must be placed in both
loops to keep the feedback factors matched.
Output ac coupling capacitors can be placed in series between
each output and its respective load. See
Figure 54 for an
example that uses input and output capacitive ac coupling.
MINIMUM R
G
VALUE OF 50 Ω
Due to the wide bandwidth of the ADA4939, the value of R
G
must
be greater than or equal to 50 Ω to provide sufficient damping in
the amplifier front end. In the terminated case, R
G
includes the
Thevenin resistance of the source and load terminations.
SETTING THE OUTPUT COMMON-MODE VOLTAGE
The V
OCM
pin of the ADA4939 is internally biased with a voltage
divider comprising two 20 kΩ resistors at a voltage approximately
equal to the midsupply point, [(+V
S
) + (−V
S
)]/2. Because of this
internal divider, the V
OCM
pin sources and sinks current, depending
on the externally applied voltage and its associated source
resistance. Relying on the internal bias results in an output
common-mode voltage that is within about 100 mV of the
expected value.
In cases where more accurate control of the output common-
mode level is required, it is recommended that an external
source or resistor divider be used with source resistance less
than 100 Ω. The output common-mode offset listed in the
Specifications section assumes that the V
OCM
input is driven
by a low impedance voltage source.
It is also possible to connect the V
OCM
input to a common-mode
level (CML) output of an ADC. However, care must be taken to
ensure that the output has sufficient drive capability. The input
impedance of the V
OCM
pin is approximately 10 kΩ. If multiple
ADA4939 devices share one reference output, it is recommended
that a buffer be used.