AD8129/AD8130
THEORY OF OPERATION
The AD8129/AD8130 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 with 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.
Therefore, the input dynamic ranges are limited to about
2.5 V for the AD8130 and 0.5 V for the AD8129 (see the
AD8129/AD8130 Specifications 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 functionality may be observed under
some conditions. A few simple circuits can illustrate how the
active feedback architecture of the AD8129/AD8130 operates.
The active feedback architecture offers several advantages over a
conventional op amp in many 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 completely 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.
OP AMP CONFIGURATION
If only one of the input stages of the AD8129/AD8130 is used, it
functions very much like a conventional op amp (see Figure 131).
Classical inverting and noninverting op amps circuits can be
created, and the basic governing equations are 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 a
midsupply voltage when they are not used.
+V
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 has
a noise gain of unity. This produces lower output noise and
higher bandwidth than op amps that have noise gain equal
to 2 for a unity-gain inverter.
10μF
0.1μF
3
7
1
8
+
+
PD +V
S
6
V
OUT
4
5
V
IN
–V
S
2
R
F
R
G
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 compensation
capacitor is in the summing circuit.
10μF
0.1μF
–V
NOTES
1. THIS CIRCUIT IS PROVIDED TO DEMONSTRATE
DEVICE OPERATION. IT IS NOT RECOMMENDED
TO USE THIS CIRCUIT IN PLACE OF AN OP AMP.
Figure 131. With Both Inputs Grounded, the Feedback Stage Functions like
an Op Amp: VOUT = VIN (1 + RF/RG).
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 0. This occurs when the two differential
inputs are equal and opposite; that is, their algebraic sum is 0.
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 is also 0 for closed-loop applications.
Because this is the governing principle of conventional op amp
circuits, an active feedback amplifier can function as a
conventional op amp under these conditions.
In a closed-loop application, a conventional op amp has its
differential input voltage driven to near 0 under nontransient
conditions. The AD8129/AD8130 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.
Note that this circuit is presented only for illustration purposes
to show the similarities of the active feedback architecture
functionality to conventional op amp functionality. If it is
desired to design a circuit that can be created from a conven-
tional op amp, it is recommended to choose a conventional
op amp with specifications that 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.
Rev. C | Page 32 of 40
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