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參數資料
| 型號： | AD8132ARM-REEL |
| 廠商： | ANALOG DEVICES INC |
| 元件分類： | 音頻／視頻放大 |
| 英文描述： | Low-Cost, High-Speed Differential Amplifier |
| 中文描述： | 1 CHANNEL, VIDEO AMPLIFIER, PDSO8 |
| 封裝： | MO-187AA, MSOP-8 |
| 文件頁數： | 14/20頁 |
| 文件大小： | 391K |
| 代理商： | AD8132ARM-REEL |
REV. 0
AD8132
–14–
When unequal feedback ratios are used, the two gains associated
with V
OUT,dm
become nonzero. This significantly complicates
the mathematical analysis along with any intuitive understand-
ing of how the part operates. Some of these configurations will
be in another section.
THEORY OF OPERATION
The AD8132 differs from conventional op amps by the external
presence of an additional input and output. The additional
input, V
OCM
, controls the output common-mode voltage. The
additional output is the analog complement of the single output
of a conventional op amp. For its operation, the AD8132 makes
use of two feedback loops as compared to the single loop of
conventional op amps. While this provides significant freedom
to create various novel circuits, basic op amp theory can still be
used to analyze the operation.
One of the feedback loops controls the output common-mode
voltage, V
OUT,cm
. Its input is V
OCM
(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
establishes one of the operational constraints: V
OUT,cm
= V
OCM
.
The second feedback loop controls the differential operation.
Similar to an op amp, the gain and gain-shaping of the transfer
function is controllable by adding passive feedback networks. How-
ever, 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 as
a result of having two outputs that are each inverted with respect
to the differential inputs.
General Usage of the AD8132
Several assumptions are made here for a first-order analysis, which
are the typical assumptions used for the analysis of op amps:
The input impedances are arbitrarily large and their loading
effect can be ignored.
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, which drives the
amplifier to a state where the input differential voltage is
effectively zero.
Offset voltages are assumed to be zero.
While it is possible to operate the AD8132 with a purely differ-
ential input, many of its applications call for a circuit that has a
single-ended input with a differential output.
For a single-ended-to-differential circuit, the R
G
of the undriven
input will be tied to a reference voltage. For now this is ground,
and other conditions will be discussed later. Also, the voltage at
V
OCM
, and hence V
OUT,cm
will be assumed to be ground for now.
Figure 4 shows a generalized schematic of such a circuit using
an AD8132 with two feedback paths.
For each feedback network, a feedback factor can be defined,
which is the fraction of the output signal that is fed back to the
opposite-sign input. These terms are:
β
1 = R
G1
/(R
G1
+ R
F1
)
β
2 = R
G2
/(R
G2
+ R
F2
)
The feedback factor
β
1 is for the side that is driven, while the
feedback factor
β
2 is for the side that is tied to a reference volt-
age, (ground for now). Note also that each feedback factor can
vary anywhere between 0 and 1.
A single-ended-to-differential gain equation can be derived
which is true for all values of
β
1 and
β
2:
G
= 2
×
(1–
β
1)/(
β
1 +
β
2)
This expression is not very intuitive, but some further examples
can provide better understanding of its implications. One obser-
vation that can be made right away is that a tolerance error in
β
1
does not have the same effect on gain as the same tolerance
error in
β
2.
Resistorless Differential Amplifier (High Input Impedance
Inverting Amplifier)
The simplest closed-loop circuit that can be made does not require
any resistors and is shown in Figure 7. In this circuit,
β
1 is equal
to zero, and
β
2 is equal to one. The gain is equal to two.
A more intuitive means to figure the gain is by simple inspec-
tion. +OUT is connected to –IN, whose voltage is equal to the
voltage at +IN under equilibrium conditions. Thus, +V
OUT
is
equal to V
IN
, and there is unity gain in this path. Since –OUT
has to swing in the opposite direction from +OUT due to the
common-mode constraint, its effect will double the output
signal and produce a gain of two.
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.
V
OCM
has been assumed to be at midsupply. Since there is
still the constraint from the above discussion that +V
OUT
must
equal V
IN
, changing the V
OCM
voltage will not change +V
OUT
(= V
IN
). Therefore, all of the effect of changing V
OCM
must
show up at –OUT.
For example, if V
OCM
is raised by 1 V, then –V
OUT
must go up
by 2 V. This makes V
OUT,cm
also go up by 1 V, since it is defined
as the average of the two differential output voltages. This means
that the gain from V
OCM
to the differential output is two.
Other 2 = 1 Circuits
The above simple configuration with
β
2 = 1 and its gain-of-two
is the highest gain circuit that can be made under this condition.
Since
β
1 was equal to zero, only higher
β
1 values are possible.
All of these circuits with higher values of
β
1 will have gains lower
than two. However, circuits with
β
1 equal to one are not practical,
because they have no effective input, and result in a gain of 0.
To increase
β
1 from zero, it is necessary to add two resistors in
a feedback network. A generalized circuit that has
β
1 with a
value higher than zero is shown in Figure 6. A couple of differ-
ent convenient gains that can be created are a gain of 1, when
β
1 is equal to 1/3, and a gain of 0.5 when
β
1 equals 0.6.
In all of these circuits with
β
2 equal to 1, V
OCM
serves as the
reference voltage from which to measure the input voltage and
the individual output voltages. In general, when V
OCM
is varied
in these circuits, a differential output signal will be generated in
addition to V
OUT,cm
changing the same amount as the voltage
change of V
OCM
.
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