ADA4939-1YCPZ-R7 datasheet(21/24 Pages) AD

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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 RG. 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 RG VALUE OF 50 Ω

Due to the wide bandwidth of the ADA4939, the value of RG must

be greater than or equal to 50 Ω to provide sufficient damping in

the amplifier front end. In the terminated case, RG includes the

Thevenin resistance of the source and load terminations.

SETTING THE OUTPUT COMMON-MODE VOLTAGE

The VOCM 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, [(+VS) + (−VS)]/2. Because of this

internal divider, the VOCM 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 VOCM input is driven

by a low impedance voltage source.

It is also possible to connect the VOCM 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 VOCM pin is approximately 10 kΩ. If multiple

ADA4939 devices share one reference output, it is recommended

that a buffer be used.

ADA4939-1YCPZ-R7 Datasheet(PDF) 21 Page - Analog Devices

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