Data Sheet

ADA4927-1/ADA4927-2

Rev. B | Page 17 of 25

THEORY OF OPERATION

The ADA4927 differs from conventional operational amplifiers

in that it has two outputs whose voltages move in opposite

uses a current feedback architecture. Like a traditional current

feedback operational amplifier, the ADA4927 relies on high

open-loop trans-impedance, T(s), and negative current feedback to

force the outputs to the desired voltages. The ADA4927 behaves

much like a standard current feedback operational amplifier and

facilitates single-ended-to-differential conversions, common-mode

level shifting, and amplifications of differential signals. Also, like

a current feedback operational amplifier, the ADA4927 has low

input impedance summing nodes, which are actually emitter-

follower outputs. The ADA4927 outputs are low impedance,

and the closed-loop output impedances are equal to the open-loop

output impedances divided by a factor of 1 + loop gain. Because

it uses current feedback, the ADA4927 manifests a nominally

constant feed-back resistance, bandwidth product. In other

words, the closed-loop bandwidth and stability of the ADA4927

depend primarily on the feedback resistor value. The closed-

loop gain equations for typical configurations are the same as

those of comparable voltage feedback differential amplifiers.

The chief difference is that the ADA4927 dynamic performance

depends on the feed-back resistor value rather than on the noise

gain. Because of this, the elements used in the feedback loops

must be resistive with values that ensure stability and sufficient

bandwidth.

Two feedback loops are employed to control the differential and

common-mode output voltages. The differential feedback loops

use a current feedback architecture with external resistors and

control only the differential output voltage. The common-mode

feedback loop is internal, uses voltage feedback, and controls only

the common-mode output voltage. This architecture makes it

easy to set the output common-mode level to any arbitrary

value within the specified limits. The output common-mode

voltage is forced, by the internal common-mode loop, to be

The internal common-mode feedback loop produces outputs

that are highly balanced over a wide frequency range without

requiring tightly matched external components. This results

in differential outputs that are very close to the ideal of being

identical in amplitude and are exactly 180° apart in phase.

DEFINITION OF TERMS

+IN

–IN

+OUT

–OUT

–FB

+FB

ADA4927

Figure 46. Circuit Definitions

Differential Voltage

Differential voltage refers to the difference between two

node voltages. For example, the output differential voltage (or

equivalently, output differential-mode voltage) is defined as

−OUT terminals with respect to a common ground reference.

Similarly, the differential input voltage is defined as

Common-Mode Voltage

Common-mode voltage refers to the average of two node voltages

with respect to the local ground reference. The output

common-mode voltage is defined as

Balance

Output balance is a measure of how close the differential signals

are to being equal in amplitude and opposite in phase. Output

balance is most easily determined by placing a well-matched

resistor divider between the differential voltage nodes and

comparing the magnitude of the signal at the divider midpoint

with the magnitude of the differential signal (see Figure 44). By

this definition, output balance is the magnitude of the output

common-mode voltage divided by the magnitude of the output

differential mode voltage.

dm

OUT

cm

OUT

V

V

Error

Balance

Output

,

,





