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AD8551/AD8552/AD8554

Data Sheet

Rev. F | Page 14 of 24

FUNCTIONAL DESCRIPTION

The AD8551/AD8552/AD8554 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

AD8551/AD8552/AD8554 amplifiers ideal for a variety of

sensitive measurement applications in harsh operating

environments, such as underhood and braking/suspension

systems in automobiles.

The AD8551/AD8552/AD8554 are CMOS amplifiers and

achieve their high degree of precision through auto-zero

stabilization. This autocorrection topology allows the

AD8551/AD8552/AD8554 to maintain its low offset voltage

over a wide temperature range and over its operating lifetime.

AMPLIFIER ARCHITECTURE

Each AD8551/AD8552/AD8554 op amps consist 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 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 AD8551/AD8552/

AD8554 is short-circuit protected to approximately 50 mA of

current.

The AD8551/AD8552/AD8554 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 AD8551/AD8552/

AD8554 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 AD8551/AD8552/AD8554 are able to

offer extremely low offset voltages and high open-loop gains.

As noted in the Amplifier Architecture section, each AD8551/

AD8552/AD8554 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 AD8551/AD8552/AD8554.

Mathematically, this is expressed in the time domain as

VOA[t] = AAVOSA[t] − BAVOA[t]

(1)

which can be expressed as



 

OSA





(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.

CM2

VIN+

VNB

CM1

VOA

–BA

VNA

ФB

ФA

VOSA

ФB

ФA

VOUT

VIN–

Figure 50. Auto-Zero Phase of the AD8551/AD8552/AD8554

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