AD8145 Data Sheet
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.
Rev. B | Page 14 of 21
