ada4939-1 Analog Devices, Inc., ada4939-1 Datasheet - Page 21

• Manufacturer Part Number: ada4939-1
• Description: Ultralow Distortion Differential Adc Driver
• Manufacturer: Analog Devices, Inc.

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
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
Due to the wide bandwidth of the ADA4939, the value of R
be greater than or equal to 50 Ω to provide sufficient damping in
the amplifier front end. In the terminated case, R
Thevenin resistance of the source and load terminations.
G
. This ac coupling blocks the flow of the dc common-
G
VALUE OF 50 Ω
G
includes the
G
must
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SETTING THE OUTPUT COMMON-MODE VOLTAGE
The V
divider comprising two 20 kΩ resistors at a voltage approximately
equal to the midsupply point, [(+V
internal divider, the V
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
by a low impedance voltage source.
It is also possible to connect the V
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
ADA4939 devices share one reference output, it is recommended
that a buffer be used.
OCM
pin of the ADA4939 is internally biased with a voltage
OCM
OCM
pin is approximately 10 kΩ. If multiple
pin sources and sinks current, depending
ADA4939-1/ADA4939-2
OCM
S
) + (−V
input to a common-mode
OCM
S
input is driven
)]/2. Because of this