AD8551/AD8552/AD8554
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.
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.
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.
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
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.
C
M1, an internal capacitor in the AD855x. Mathematically, this
is expressed in the time domain as
V
OA[t] = AAVOSA[t] − BAVOA[t]
which can be expressed as
AAVOSA
(1)
(2)
[t]
1+ BA
VOA[t] =
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.
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.
Rev. C | Page 14 of 24
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