AD8145

Rev. A | Page 14 of 24

THEORY OF OPERATION

The AD8145 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 AD8145

has an internal feedback resistor from each amplifier output to

the negative input of its feedback input stage. This limits the

possible closed-loop gain configurations for the AD8145.

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 AD8145 has a 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 AD8145 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 the voltage that 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 AD8145 generally has differential

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

conditions. As a practical consideration, it is necessary to limit

the differential input voltage internally with a clamp circuit.

Therefore, the input dynamic ranges are limited to approximately

2.5 V (see the Specifications section for more details). For this

reason, it is not recommended to reverse the input and feedback

stages of the AD8145, even though some normal functionality

may be observed under some conditions.