ADA4927-1/ADA4927-2
R
F
RTS = RTH = RS||RT = 26.5 ꢁ. Note that VTH is greater than
1 V p-p, which was obtained with RT = 50 ꢁ. The modified
circuit with the Thevenin equivalent (closest 1% value used for
RTH) of the terminated source and RTS in the lower feedback
loop is shown in Figure 53.
357Ω
+V
1V p-p
S
R
R
S
G
50Ω
348Ω
R
T
V
S
56.2Ω
V
OUT, dm
1.01V p-p
V
OCM
ADA4927
R
2V p-p
L
R
F
R
G
348Ω
+V
348Ω
R
TS
26.7Ω
S
R
R
–V
S
TH
G
R
F
26.7Ω
348Ω
V
TH
1.06V p-p
357Ω
V
OCM
V
OUT, dm
ADA4927
R
L
Figure 54. Terminated Single-Ended-to-Differential System with G = 1
R
G
348Ω
R
TS
26.7Ω
INPUT COMMON-MODE VOLTAGE RANGE
–V
S
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.
R
F
348Ω
Figure 53. Thevenin Equivalent and Matched Gain Resistors
Figure 53 presents a tractable circuit with matched
feedback loops that can be easily evaluated.
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.
It is useful to point out two effects that occur with a
terminated input. The first is that the value of RG is increased
in both loops, lowering the overall closed-loop gain. The
second is that VTH is a little larger than 1 V p-p, as it is
when RT = 50 ꢁ. These two effects have opposite impacts
on the output voltage, and for large resistor values in the
feedback loops (~1 kꢁ), the effects essentially cancel each
other out. For small RF and RG, or high gains, however, the
diminished closed-loop gain is not canceled completely by the
increased VTH. This can be seen by evaluating Figure 53.
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 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.
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 RF without modifying
any of the input circuitry. This is discussed in Step 4.
ꢀutput 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 VꢀCM pin of the ADA4927 is internally biased with a voltage
divider comprising two 10 kꢁ resistors at a voltage approximately
equal to the midsupply point, [(+VS) + (−VS)]/2. Because of this
internal divider, the VꢀCM 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 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 ꢁ. The output
common-mode offset listed in the Specifications section presumes
that the VꢀCM input is driven by a low impedance voltage source.
It is also possible to connect the VꢀCM 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 VꢀCM pin is approximately 10 kꢁ. If multiple
ADA4927 devices share one ADC reference output, a buffer may
be necessary to drive the parallel inputs.
4. The feedback resistor value is modified as a final gain
adjustment to obtain the desired output voltage.
To make the output voltage VꢀUT = 1 V p-p, RF must be
calculated using the following formula:
RF =
(
Desired VOUT,dm
)
(
RG + RTS
)
(
)(
)
= 35
1V p − p 374.7Ω
=
VTH
1.06V p − p
The closest standard 1% values to 353 ꢁ are 348 ꢁ and
357 ꢁ. Choosing 357 ꢁ for RF gives a differential output
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 RF = 348 ꢁ due to the inversely
proportional relationship between RF and closed-loop
gain that is characteristic of current feedback amplifiers.
The final circuit is shown in Figure 54.
Rev. 0 | Page 20 of 24
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