ada4927 Analog Devices, Inc., ada4927 Datasheet - Page 20

• Manufacturer Part Number: ada4927
• Description: Ultralow Distortion Current Feedback Differential Adc Driver
• Manufacturer: Analog Devices, Inc.

ADA4927-1/ADA4927-2
4.
1.06V p-p
R
1 V p-p, which was obtained with R
circuit with the Thevenin equivalent (closest 1% value used for
R
loop is shown in Figure 53.
Figure 53 presents a tractable circuit with matched
feedback loops that can be easily evaluated.
It is useful to point out two effects that occur with a
terminated input. The first is that the value of R
in both loops, lowering the overall closed-loop gain. The
second is that V
when R
on the output voltage, and for large resistor values in the
feedback loops (~1 kΩ), the effects essentially cancel each
other out. For small R
diminished closed-loop gain is not canceled completely by the
increased V
The desired differential output in this example is 1 V p-p
because the terminated input signal is 1 V p-p and the
closed-loop gain = 1. The actual differential output voltage,
however, is equal to (1.06 V p-p)(348/374.7) = 0.984 V p-p.
To obtain the desired output voltage of 1 V p-p, a final gain
adjustment can be made by increasing R
any of the input circuitry. This is discussed in Step 4.
The feedback resistor value is modified as a final gain
adjustment to obtain the desired output voltage.
calculated using the following formula:
The closest standard 1% values to 353 Ω are 348 Ω and
357 Ω. Choosing 357 Ω for R
voltage of 1.01 V p-p. The closed-loop bandwidth is
diminished by a factor of approximately 348/357 from
what it would be with R
proportional relationship between R
gain that is characteristic of current feedback amplifiers.
The final circuit is shown in Figure 54.
R
To make the output voltage V
TH
(
TS
F
Desired
Figure 53. Thevenin Equivalent and Matched Gain Resistors
V
) of the terminated source and R
=
= R
TH
TH
T
= 50 Ω. These two effects have opposite impacts
26.7Ω
= R
26.7Ω
V
R
R
TH
TH
OUT
TS
V
. This can be seen by evaluating Figure 53.
S
||R
TH
,
dm
TH
348Ω
V
348Ω
T
R
R
OCM
) (
= 26.5 Ω. Note that V
G
G
is a little larger than 1 V p-p, as it is
R
G
F
+
and R
F
R
= 348 Ω due to the inversely
TS
348Ω
348Ω
ADA4927
) (
R
R
G
F
F
, or high gains, however, the
=
F
+V
–V
gives a differential output
OUT
S
S
1
V
TS
= 1 V p-p, R
. 1
p
T
06
in the lower feedback
F
−
= 50 Ω. The modified
and closed-loop
F
V
p
without modifying
TH
)(
p
374.7
is greater than
−
R
L
G
p
is increased
F
V
Ω
OUT, dm
must be
)
=
35
Rev. 0 | Page 20 of 24
INPUT COMMON-MODE VOLTAGE RANGE
The ADA4927 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 operation with ±5 V supplies, the input common-mode
range at the summing nodes of the amplifier is specified as
−3.5 V to +3.5 V and is specified as +1.3 V to +3.7 V with a
single +5 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 ADA4927 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 58 for an example
that uses input and output capacitive ac coupling.
SETTING THE OUTPUT COMMON-MODE VOLTAGE
The V
divider comprising two 10 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 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 presumes
that the V
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
ADA4927 devices share one ADC reference output, a buffer may
be necessary to drive the parallel inputs.
2V p-p
Figure 54. Terminated Single-Ended-to-Differential System with G = 1
V
S
G
OCM
. This ac coupling blocks the flow of the dc common-
OCM
50Ω
pin of the ADA4927 is internally biased with a voltage
R
S
input is driven by a low impedance voltage source.
1V p-p
56.2Ω
26.7Ω
R
R
TS
T
OCM
OCM
348Ω
348Ω
pin is approximately 10 kΩ. If multiple
V
R
R
pin sources and sinks current, depending
OCM
G
G
OCM
357Ω
357Ω
ADA4927
S
R
R
) + (−V
F
F
+V
–V
input to a common-mode
S
S
S
)]/2. Because of this
R
L
V
1.01V p-p
OUT, dm