ada4950-1 Analog Devices, Inc., ada4950-1 Datasheet - Page 22

• Manufacturer Part Number: ada4950-1
• Description: Low Power, Selectable Gain Differential Adc Driver, G = 1, 2, 3
• Manufacturer: Analog Devices, Inc.

ADA4950-1/ADA4950-2
INPUT COMMON-MODE VOLTAGE RANGE
The ADA4950-x input common-mode voltage range is shifted
down by approximately one V
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 R
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 V
diode to each supply. Figure 60 illustrates the ESD protection
circuitry.
G
. This ac coupling blocks the flow of the dc common-
BE
, in contrast to other ADC
OCM
input has one ESD
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SETTING THE OUTPUT COMMON-MODE VOLTAGE
The V
tage divider comprising two 50 kΩ resistors across the supplies,
with a tap at a voltage approximately equal to the midsupply
point, [(+V
V
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 V
accuracy than produced internally, higher values can be used
because the external resistors are placed in parallel with the
internal resistors. The input V
section assumes that the V
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
of nominally midsupply. If multiple ADA4950-x devices share
one ADC reference output, a buffer may be necessary to drive
the parallel inputs.
OCM
pin sources and sinks current, depending on the externally
OCM
pin of the ADA4950-x is internally biased with a vol-
S
) + (−V
Figure 60. Input ESD Protection Circuitry
×1
×1
OCM
×1
×4
S
)]/2. Because of this internal divider, the
×1
×4
pin is approximately 10 kΩ to a voltage
OCM
×1
×4
×1
×4
OCM
input is driven by a low impedance
OCM
500Ω
500Ω
250Ω
250Ω
offset listed in the Specifications
V
to midsupply with greater
OCM
OCM
500Ω
input to a common-mode
500Ω
ADA4950-x
+V
–V
S
S