AD8143

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THEORY OF OPERATION

The AD8143 amplifiers use an architecture called active

feedback, which differs from that of conventional op amps. The

most obvious differentiating feature is the presence of two

separate 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

completely 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 entirely

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 of active feedback is the ability to change the

polarity of the gain merely by switching the differential inputs.

A high input impedance inverting amplifier can therefore be

made. Besides high input impedance, a unity-gain inverter with

the AD8143 has noise gain of unity, producing 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 AD8143 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 compensation

capacitor is included in the summing circuit. When the

feedback path is closed around the part, the output drives

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 has its

differential input voltage driven to near zero under non-

transient conditions. The AD8143 generally has differential

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

conditions. As a practical consideration, it is necessary to

internally limit the differential input voltage with a clamp

circuit. Thus, the input dynamic ranges are limited to about

2.5 V for the AD8143 (see Specifications section for more

detail). For this and other reasons, it is not recommended to

reverse the input and feedback stages of the AD8143, even

though some apparently normal functionality may be observed

under some conditions.