AD8551ARM-R2 datasheet(14/24 Pages) AD

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AD8551/AD8552/AD8554

Rev. C | 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] − B

VOA[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.

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