AD8132
THEORY OF OPERATION
The AD8132 differs from conventional op amps by the external
presence of an additional input and output. The additional input,
VOCM, controls the output common-mode voltage. The additional
output is the analog complement of the single output of a conven-
tional op amp. For its operation, the AD8132 uses two feedback
loops as compared to the single loop of conventional op amps.
Although this provides significant freedom to create various
novel circuits, basic op amp theory can still be used to analyze
the operation.
For each feedback network, a feedback factor can be defined as
the fraction of the output signal that is fed back to the opposite
sign input. These terms are
β1 = RG1
β2 = RG2
RG1 + RF1
RG2 + RF2
The feedback factor, β1, is for the side that is driven, and the
feedback factor, β2, is for the side that is tied to a reference voltage
(ground). Note that each feedback factor can vary anywhere
between 0 and 1.
One of the feedback loops controls the output common-mode
voltage, VOUT, cm. Its input is VOCM (Pin 2) and the output is the
common-mode, or average voltage, of the two differential outputs
(+OUT and −OUT). The gain of this circuit is internally set to
unity. When the AD8132 is operating in its linear region, this
A single-ended-to-differential gain equation can be derived
(this is true for all values of β1 and β2):
2
1− β1
)
G =
establishes one of the operational constraints: VOUT, cm = VOCM
.
(
β1 + β2
The second feedback loop controls the differential operation.
Similar to an op amp, the gain and gain shaping of the transfer
function can be controlled by adding passive feedback networks.
However, only one feedback network is required to close the
loop and fully constrain the operation, but depending on the
function desired, two feedback networks can be used. This is
possible because there are two outputs that are each inverted
with respect to the differential inputs.
This expression is not very intuitive, but some further examples can
provide better understanding of its implications. One observation
that can be made immediately is that a tolerance error in β1 does
not have the same effect on gain as the same tolerance error in β2.
DIFFERENTIAL AMPLIFIER WITHOUT RESISTORS
(HIGH INPUT IMPEDANCE INVERTING AMPLIFIER)
The simplest closed-loop circuit that can be made does not
require any resistors and is shown in Figure 70. In this circuit,
β1 is equal to 0, and β2 is equal to 1. The gain is equal to 2.
GENERAL USAGE OF THE AD8132
Several assumptions are made here for a first-order analysis; they
are the typical assumptions used for the analysis of op amps:
A more intuitive method to figure the gain is by simple inspection.
+OUT is connected to −IN, whose voltage is equal to the voltage at
+IN under equilibrium conditions. Thus, +VOUT is equal to VIN,
and there is unity gain in this path. Because −OUT has to swing
in the opposite direction from +OUT due to the common-mode
constraint, its effect doubles the output signal and produces a
gain of 2.
•
The input bias currents are sufficiently small so they can
be neglected.
•
•
The output impedances are arbitrarily low.
The open-loop gain is arbitrarily large, and drives the
amplifier to a state where the input differential voltage is
effectively 0.
One useful function that this circuit provides is a high input
impedance inverter. If +OUT is ignored, there is a unity-gain,
high input impedance amplifier formed from +IN to −OUT.
Most traditional op amp inverters have relatively low input
impedances, unless they are buffered with another amplifier.
•
Offset voltages are assumed to be 0.
Though it is possible to operate the AD8132 with a purely
differential input, many of its applications call for a circuit
that has a single-ended input with a differential output.
VOCM is assumed to be at midsupply. Because there is still the
constraint that +VOUT must equal VIN, changing the VOCM voltage
does not change +VOUT (equal to VIN). Therefore, the effect of
changing VOCM must show up at −OUT.
For a single-ended-to-differential circuit, the RG of the input
that is not driven is tied to a reference voltage. This is ground.
Other conditions are discussed in the following sections. In
addition, the voltage at VOCM, and therefore VOUT, cm, is assumed
to be ground. Figure 67 shows a generalized schematic of such a
circuit using an AD8132 with two feedback paths.
For example, if VOCM is raised by 1 V, then −VOUT must increase
by 2 V. This makes VOUT, cm also increase by 1 V, because it is defined
as the average of the two differential output voltages. This means
that the gain from VOCM to the differential output is 2.
Rev. F | Page 21 of 32
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