Data Sheet

ADA4927-1/ADA4927-2

Rev. B | Page 21 of 25

circuit with the Thevenin equivalent (closest 1% value used for

loop is shown in Figure 53.

ADA4927

26.7Ω

348Ω

348Ω

348Ω

348Ω

1.06V p-p

26.7Ω

Figure 53. Thevenin Equivalent and Matched Gain Resistors

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

in both loops, lowering the overall closed-loop gain. The

on the output voltage, and for large resistor values in the

feedback loops (~1 kΩ), the effects essentially cancel each

diminished closed-loop gain is not canceled completely by the

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

input circuitry. This is discussed in Step 4.

4.

The feedback resistor value is modified as a final gain

adjustment to obtain the desired output voltage.

calculated using the following formula:



35

06

.

1

Ω

374.7

1

,













p

p

V

p

p

V

V

R

R

V

Desired

R

TH

TS

G

dm

OUT

F

The closest standard 1% values to 353 Ω are 348 Ω and

voltage of 1.01 V p-p. The closed-loop bandwidth is

diminished by a factor of approximately 348/357 from

gain that is characteristic of current feedback amplifiers.

The final circuit is shown in Figure 54.

ADA4927

1.01V p-p

50Ω

348Ω

348Ω

357Ω

357Ω

2V p-p

1V p-p

56.2Ω

26.7Ω

Figure 54. Terminated Single-Ended-to-Differential System with G = 1

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

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 respective load. See Figure 58 for an example that

uses input and output capacitive ac coupling.

SETTING THE OUTPUT COMMON-MODE VOLTAGE

divider comprising two 10 kΩ resistors at a voltage approximately

on the externally applied voltage and 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

level (CML) output of an ADC; however, care must be taken to

ensure that the output has sufficient drive capability. The input

ADA4927 devices share one ADC reference output, a buffer may

be necessary to drive the parallel inputs.