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Data Sheet

AD8571/AD8572/AD8574

Rev. F | Page 15 of 28

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

AD8571/AD8572/AD8574 to maintain its low offset voltage over

a wide temperature range, and the randomized auto-zero clock

eliminates any inter-modulation distortion (IMD) errors at the

amplifier output.

The AD8571/AD8572/AD8574 can run from a single-supply

voltage as low as 2.7 V. The extremely low offset voltage of 1 µV

and no IMD products allow the amplifier to be easily configured

for high gains without risk of excessive output voltage errors, which

makes the AD8571/AD8572/AD8574 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 −40°C to

+125°C temperature range. These combined features make the

AD8571/AD8572/AD8574 an excellent choice for a variety of

sensitive measurement and automotive applications.

AMPLIFIER ARCHITECTURE

Each AD8571/AD8572/AD8574 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 AD8571/

AD8572/AD8574 is short-circuit protected to approximately

50 mA of current.

The AD8571/AD8572/AD8574 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, which 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

AD8571/AD8572/AD8574 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 AD8571/AD8572/AD8574

are able to offer extremely low offset voltages and high open-

loop gains.

As noted in the Amplifier Architecture section, each

AD8571/AD8572/AD8574 op amp contains two internal

amplifiers. One is used as the primary amplifier, and 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 VOSA and VOSB, where A

denotes the nulling amplifier and B denotes 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.

VIN+

VIN–

VOUT

ΦA1

ΦB

VOSA

VOSB

CM2

CM1

ΦA2

VNB

VNA

–BA

VOA

ΦB

Figure 50. Auto-Zero Phase of the Amplifier

VIN+

VIN–

VOUT

ΦA

ΦB

VOSA

VOSB

CM2

CM1

ΦA

VNB

VNA

–BA

VOA

ΦB

Figure 51. Output Phase of the Amplifier

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.

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