ADA4939-2YCPZ-R2 datasheet(21/24 Pages) AD

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Data Sheet

ADA4939-1/ADA4939-2

Rev. A | Page 21 of 24

INPUT COMMON-MODE VOLTAGE RANGE

The ADA4939-1/ADA4939-2 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 ADA4937-1/

ADA4937-2. 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 ADA4939-1/ADA4939-2

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 ADA4939-1/ADA4939-2, 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/VOCMx pin of the ADA4939-1/ADA4939-2 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/VOCMx 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/VOCMx pin is approximately 10 kΩ. If

multiple ADA4939-1/ADA4939-2 devices share one reference

output, it is recommended that a buffer be used.

ADA4939-2YCPZ-R2 Datasheet(PDF) 21 Page - Analog Devices

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