ADA4950-2YCPZ-R7 datasheet(22/26 Pages) AD

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ADA4950-1/ADA4950-2

Data Sheet

Rev. B | Page 22 of 26

INPUT COMMON-MODE VOLTAGE RANGE

The ADA4950-x input common-mode voltage range is shifted

down by approximately one VBE, in contrast to other ADC

drivers with centered input ranges such as the ADA4939-x. The

downward-shifted input common-mode range is especially

suited to dc-coupled, single-ended-to-differential, and single-

supply applications.

For ±5 V operation, the input common-mode voltage range at the

summing nodes of the amplifier is specified as −4.8 V to +3.2 V.

With a 5 V supply, the input common-mode voltage range at the

summing nodes of the amplifier is specified as +0.2 V to +3.2 V.

To avoid nonlinearities, the voltage swing at the +INx and −INx

terminals must be confined to these ranges.

INPUT AND OUTPUT CAPACITIVE AC COUPLING

Although the ADA4950-x is well suited to dc-coupled applica-

tions, it is nonetheless possible to use it in ac-coupled circuits.

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 ADA4950-x dc input

common-mode voltage to equal the dc output common-mode

voltage. The 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.

INPUT SIGNAL SWING CONSIDERATIONS

The input terminals of fully differential amplifiers with external

gain and feedback resistors connect directly to the amplifier

summing nodes; the common-mode voltage swing at these

terminals is generally smaller than the input and output swings.

In most linear applications, the summing node voltages do not

approach levels that result in the forward-biasing of the internal

ESD protection diodes on the amplifier inputs.

Signals at the inputs of the ADA4950-x are applied to the input

side of the gain resistors, and, if caution is not exercised, these

signals can be large enough to forward-bias the ESD protection

diodes. The four inputs that make up the differential signal paths

each have four ESD diodes in series to the negative supply and

one diode to the positive supply; the VOCM input has one ESD

diode to each supply. Figure 60 illustrates the ESD protection

circuitry.

ADA4950-x

+VS

–VS

500Ω

250Ω

VOCM

250Ω

×1

×4

×1

×4

×1

×4

×1

×4

Figure 60. Input ESD Protection Circuitry

SETTING THE OUTPUT COMMON-MODE VOLTAGE

The VOCM pin of the ADA4950-x is internally biased with a vol-

tage divider comprising two 50 kΩ resistors across the supplies,

with a tap 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 approximately 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. If an external voltage divider consisting of equal

resistor values is used to set VOCM to midsupply with greater

accuracy than produced internally, higher values can be used

because the external resistors are placed in parallel with the

internal resistors. The input VOCM 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 to a voltage

of nominally midsupply. If multiple ADA4950-x devices share

one ADC reference output, a buffer may be necessary to drive

the parallel inputs.

ADA4950-2YCPZ-R7 Datasheet(PDF) 22 Page - Analog Devices

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