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參數資料
| 型號： | AD8571ARM |
| 廠商： | ANALOG DEVICES INC |
| 元件分類： | 運動控制電子 |
| 英文描述： | Zero-Drift, Single-Supply, Rail-to-Rail Input/Output Operational Amplifiers |
| 中文描述： | OP-AMP, 10 uV OFFSET-MAX, 1 MHz BAND WIDTH, PDSO8 |
| 封裝： | MSOP-8 |
| 文件頁數： | 10/19頁 |
| 文件大小： | 247K |
| 代理商： | AD8571ARM |
AD8571/AD8572/AD8574
–10–
REV. 0
FUNCTIONAL DESCRIPTION
The AD857x family are CMOS amplifiers that achieve their
high degree of precision through random frequency autozero
stabilization. The autocorrection topology allows the AD857x
to maintain its low offset voltage over a wide temperature range,
and the randomized autozero clock eliminates any intermodulation
distortion (IMD) errors at the amplifier’s output.
The AD857x can be run from a single supply voltage as low as
2.7 V. The extremely low offset voltage of 1
μ
V and no IMD
products allows the amplifier to be easily configured for high
gains without risk of excessive output voltage errors. This makes
the AD857x an ideal amplifier for applications requiring both dc
precision and low distortion for ac signals. The extremely small
temperature drift of 5 nV/
°
C ensures a minimum of offset voltage
error over its entire temperature range of –40
°
C to +125
°
C. These
combined features make the AD857x an excellent choice for a
variety of sensitive measurement and automotive applications.
Amplifier Architecture
Each AD857x op amp consists of two amplifiers, a main amplifier
and a secondary amplifier, used to correct the offset voltage of the
main amplifier. Both consist of a rail-to-rail input stage, allowing
the input common-mode voltage range to reach both supply rails.
The input stage consists of an NMOS differential pair operating
concurrently with a parallel PMOS differential pair. The outputs
from the differential input stages are combined in another gain
stage whose output is used to drive a rail-to-rail output stage.
The wide voltage swing of the amplifier is achieved by using two
output transistors in a common-source configuration. The output
voltage range is limited by the drain-to-source resistance of these
transistors. As the amplifier is required to source or sink more
output current, the voltage drop across these transistors increases
due to their rds. Simply put, the output voltage will not swing as
close to the rail under heavy output current conditions as it will
with light output current. This is a characteristic of all rail-to-rail
output amplifiers. Figures 6 and 7 show how close the output
voltage can get to the rails with a given output current. The out-
put of the AD857x is short circuit protected to approximately
50 mA of current.
The AD857x amplifiers have exceptional gain, yielding greater
than 120 dB of open-loop gain with a load of 2 k
. Because the
output transistors are configured in a common-source configu-
ration, the gain of the output stage, and thus the open-loop gain
of the amplifier, is dependent on the load resistance. Open-loop
gain will decrease with smaller load resistances. This is another
characteristic of rail-to-rail output amplifiers.
Basic Autozero Amplifier Theory
Autocorrection amplifiers are not a new technology. Various IC
implementations have been available for over 15 years and some
improvements have been made over time. The AD857x design
offers a number of significant performance improvements over
older versions while attaining a very substantial reduction in
device cost. This section offers a simplified explanation of how
the AD857x is able to offer extremely low offset voltages and
high open-loop gains.
As noted in the previous section on amplifier architecture, each
AD857x op amp contains two internal amplifiers. One is used as
the primary amplifier, the other as an autocorrection, or nulling,
amplifier. Each amplifier has an associated input offset voltage
that can be modeled as a dc voltage source in series with the
noninverting input. In Figures 44 and 45 these are labeled as
V
OSX
, where x denotes the amplifier associated with the offset; A
for the nulling amplifier, B for the primary amplifier. The open-
loop gain for the +IN and –IN inputs of each amplifier is given
as A
X
. Both amplifiers also have a third voltage input with an
associated open-loop gain of B
X
.
There are two modes of operation determined by the action of
two sets of switches in the amplifier: An autozero phase and an
amplification phase.
Autozero Phase
In this phase, all
φ
A switches are closed and all
φ
B switches are
opened. Here, the nulling amplifier is taken out of the gain loop
by shorting its two inputs together. Of course, there is a degree of
offset voltage, shown as
V
OSA
, inherent in the nulling amplifier,
which maintains a potential difference between the +IN and –IN
inputs. The nulling amplifier feedback loop is closed through
φ
A
2
and V
OSA
appears at the output of the nulling amp and on C
M1
,
an internal capacitor in the AD857x. Mathematically, we can
express this in the time domain as:
[ ]
=
which can be expressed as,
V
t
A V
t
B V
t
OA
[ ]
[ ]
(1)
V
t
A V
+
1
t
B
OA
A
[ ]
=
[ ]
(2)
This shows us that the offset voltage of the nulling amplifier
times a gain factor appears at the output of the nulling amplifier
and thus on the C
M1
capacitor.
V
IN+
V
IN
2
V
OUT
A
B
A
A
F
A
F
B
V
OSA
+
B
B
C
M2
C
M1
F
A
F
B
V
NB
V
NA
2
B
A
V
OA
Figure 44. Autozero Phase of the AD857x
Amplification Phase
When the
φ
B switches close and the
φ
A switches open for the
amplification phase, this offset voltage remains on C
M1
and
essentially corrects any error from the nulling amplifier. The
voltage across C
M1
is designated as V
NA
. Let us also designate
V
IN
as the potential difference between the two inputs to the
primary amplifier, or V
IN
= (V
IN+
– V
IN–
). Now the output of the
nulling amplifier can be expressed as:
[ ]
=
V
t
A V
t
V
t
B V
t
OA
IN
OSA
[ ]
[ ]
)
[ ]
(3)
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