AD8129ARM datasheet(20/28 Pages) AD

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AD8129/AD8130

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THEORY OF OPERATION

The AD8129/AD8130 use an architecture called active feed-

back which differs from that of conventional op amps. The

most obvious differentiating feature is the presence of two sepa-

rate pairs of differential inputs compared to a conventional op

amp’s single pair. Typically for the active-feedback architecture,

one of these input pairs is driven by a differential input signal,

while the other is used for the feedback. This active stage in the

feedback path is where the term “active feedback” is derived.

The active feedback architecture offers several advantages over a

conventional op amp in several types of applications. Among

these are excellent common-mode rejection, wide input common-

mode range and a pair of inputs that are high-impedance and

totally balanced in a typical application. In addition, while an

external feedback network establishes the gain response as

in a conventional op amp, its separate path makes it totally

independent of the signal input. This eliminates any interaction

between the feedback and input circuits, which traditionally

causes problems with CMRR in conventional differential-input

op amp circuits.

Another advantage is the ability to change the polarity of the

gain merely by switching the differential inputs. A high input-

impedance inverting amplifier can be made. Besides a high

input impedance, a unity-gain inverter with the AD8130 will

have a noise gain of unity. This will produce lower output noise

and higher bandwidth than op amps that have noise gain equal

to 2 for a unity gain inverter.

The two differential input stages of the AD8129/AD8130 are each

transconductance stages that are well matched. These stages

convert the respective differential input voltages to internal

currents. The currents are then summed and converted to a

voltage, which is buffered to drive the output. The compensa-

tion capacitor is in the summing circuit.

When the feedback path is closed around the part, the output

will drive the feedback input to that voltage which causes the

internal currents to sum to zero. This occurs when the two

differential inputs are equal and opposite; that is, their algebraic

sum is zero.

In a closed-loop application, a conventional op amp will have its

differential input voltage driven to near zero under nontransient

conditions. The AD8129/AD8130 generally will have differential

input voltages at each of its input pairs, even under equilibrium

conditions. As a practical consideration, it is necessary to inter-

nally limit the differential input voltage with a clamp circuit.

Thus, the input dynamic ranges are limited to about 2.5 V for

the AD8130 and 0.5 V for the AD8129 (see Specification

section for more detail). For this and other reasons, it is not

recommended to reverse the input and feedback stages of the

AD8129/AD8130, even though some apparently normal func-

tionality might be observed under some conditions.

A few simple circuits can illustrate how the active feedback

architecture of the AD8129/AD8130 operates.

Op Amp Configuration

If only one of the input stages of the AD8129/AD8130 is used,

it will function very much like a conventional op amp. (See

Figure 4.) Classical inverting and noninverting op amps circuits

can be created, and the basic governing equations will be the

same as for a conventional op amp. The unused input pins form

the second input and should be shorted together and tied to

ground or some midsupply voltage when they are not used.

–VS

0.1 F10 F

–V

VOUT

VIN

0.1 F

10 F

Figure 4. With both inputs grounded, the feedback stage

functions like an op amp: VOUT = VIN (1 + RF/RG). NOTE: This

circuit is provided to demonstrate device operation. It is

not suggested to use this circuit in place of an op amp.

With the unused pair of inputs shorted, there is no differential

voltage between them. This dictates that the differential input

voltage of the used inputs will also be zero for closed-loop

applications. Since this is the governing principle of conven-

tional op amp circuits, an active feedback amplifier can function

as a conventional op amp under these conditions.

Note that this circuit is presented only for illustration purposes,

to show the similarities of the active feedback architecture func-

tionality to conventional op amp functionality. If it is desired to

design a circuit that can be created from a conventional op amp,

it is recommended to choose a conventional op amp whose

specifications are better suited to that application. These op amp

principles are the basis for offsetting the output as described in

the Output Offset/Level Translator section.

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