AD8571_06 datasheet(14/24 Pages) AD

Part #	AD8571

Description	Zero-Drift, Single-Supply, Rail-to-Rail Input/Output Operational Amplifiers

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Manufacturer	AD [Analog Devices]

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AD8571/AD8572/AD8574

Rev. B | Page 14 of 24

FUNCTIONAL DESCRIPTION

The AD8571/AD8572/AD8574 are CMOS amplifiers that

achieve their high degree of precision through random

frequency auto-zero stabilization. The autocorrection topology

allows the AD857x to maintain its low offset voltage over a wide

temperature range, and the randomized auto-zero clock

eliminates any intermodulation distortion (IMD) errors at the

amplifier output.

The AD857x can be run from a single-supply voltage as low as

2.7 V. The extremely low offset voltage of 1 μV 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 that is 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 on resistance (rds). 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 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 kΩ. Because the

output transistors are configured in a common-source configura-

tion, 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 and

some improvements have been made over time. The AD857x

design offers a number of significant performance improve-

ments 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 Amplifier Architecture section, 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 Figure 50 and Figure 51, these are

labeled as VOSX, 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 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

that maintains a potential difference between the +IN and −IN

inputs. The nulling amplifier feedback loop is closed through

φA2 and VOSA appears at the output of the nulling amp and on

CM1, an internal capacitor in the AD857x. Mathematically, we

can express this in the time domain as

]

[

]

[

]

[

t

V

B

t

V

A

t

V

OA

A

OSA

A

OA

−

=

(1)

this also can be expressed as

[]

[ ]

A

OSA

A

OA

B

t

V

A

t

V

+

=

1

(2)

This shows 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.

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