参数资料
型号: AD8552ARZ-REEL7
厂商: Analog Devices Inc
文件页数: 6/24页
文件大小: 0K
描述: IC OPAMP CHOPPER R-R DUAL 8SOIC
产品培训模块: Op Amp Basics
标准包装: 1,000
放大器类型: 断路器(零漂移)
电路数: 2
输出类型: 满摆幅
转换速率: 0.4 V/µs
增益带宽积: 1.5MHz
电流 - 输入偏压: 10pA
电压 - 输入偏移: 1000µV
电流 - 电源: 850µA
电流 - 输出 / 通道: 30mA
电压 - 电源,单路/双路(±): 2.7 V ~ 5.5 V
工作温度: -40°C ~ 125°C
安装类型: 表面贴装
封装/外壳: 8-SOIC(0.154",3.90mm 宽)
供应商设备封装: 8-SO
包装: 带卷 (TR)
AD8551/AD8552/AD8554
Data Sheet
Rev. E | Page 14 of 24
FUNCTIONAL DESCRIPTION
The AD855x family of amplifiers are high precision, rail-to-rail
operational amplifiers that can be run from a single-supply voltage.
Their typical offset voltage of less than 1 μV allows these amplifiers
to be easily configured for high gains without risk of excessive
output voltage errors. 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, making the AD855x
amplifiers ideal for a variety of sensitive measurement applications
in harsh operating environments, such as underhood and
braking/suspension systems in automobiles.
The AD855x family are CMOS amplifiers and achieve their
high degree of precision through auto-zero stabilization. This
autocorrection topology allows the AD855x to maintain its low
offset voltage over a wide temperature range and over its
operating lifetime.
AMPLIFIER ARCHITECTURE
Each AD855x op amp consists of two amplifiers, a main ampli-
fier 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 rDS of these transistors increases, raising
the voltage drop across these transistors. Simply put, the output
voltage does not swing as close to the rail under heavy output
current conditions as it does with light output current. This is a
characteristic of all rail-to-rail output amplifiers. Figure 12 and
Figure 13 show how close the output voltage can get to the rails
with a given output current. The output of the AD855x is short-
circuit protected to approximately 50 mA of current.
The AD855x amplifiers have exceptional gain, yielding greater than
120 dB of open-loop gain with a load of 2 kΩ. Because the output
transistors are configured in a common-source configuration,
the gain of the output stage, and thus the open-loop gain of the
amplifier, is dependent on the load resistance. Open-loop gain
decreases with smaller load resistances. This is another
characteristic of rail-to-rail output amplifiers.
BASIC AUTO-ZERO AMPLIFIER THEORY
Autocorrection amplifiers are not a new technology. Various IC
implementations have been available for more than 15 years with
some improvements made over time. The AD855x design offers
a number of significant performance improvements over previous
versions while attaining a very substantial reduction in device
cost. This section offers a simplified explanation of how the
AD855x is able to offer extremely low offset voltages and high
open-loop gains.
As noted in the Amplifier Architecture section, each AD855x
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 Figure 50 and Figure 51 these are labeled
as VOSX, where x denotes the amplifier associated with the offset:
A for the nulling amplifier and B for the primary amplifier. The
open-loop gain for the +IN and IN inputs of each amplifier is
given as AX. Both amplifiers also have a third voltage input with
an associated open-loop gain of BX.
There are two modes of operation determined by the action of
two sets of switches in the amplifier: an auto-zero phase and an
amplification phase.
Auto-Zero Phase
In this phase, all φA switches are closed and all φB 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 VOSA, inherent in the nulling amplifier
which maintains a potential difference between the +IN and
IN inputs. The nulling amplifier feedback loop is closed through
φB2 and VOSA appears at the output of the nulling amp and on
CM1, an internal capacitor in the AD855x. Mathematically, this
is expressed in the time domain as
VOA[t] = AAVOSA[t] BAVOA[t]
(1)
which can be expressed as
[ ]
A
OSA
A
OA
B
t
V
A
t
V
+
=
1
(2)
This demonstrates that the offset voltage of the nulling amplifier
times a gain factor appears at the output of the nulling amplifier
and, thus, on the CM1 capacitor.
+
AB
BB
CM2
VIN+
VNB
CM1
VOA
–BA
VNA
ФB
ФA
AA
VOSA
ФB
ФA
VOUT
VIN–
01101-
050
Figure 50. Auto-Zero Phase of the AD855x
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